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Basic Overview

Fluid is an open-source .NET template engine based on the Liquid template language. It is a secure template language that is also very accessible for non-programmer audiences.

The following content is based on the 2.0.0-beta version, which is the recommended version, even though some of its API might vary significantly. To see the corresponding content for v1.0, use this version


Tutorials

Deane Barker wrote a very comprehensive tutorial on how to write Liquid templates with Fluid. For a high-level overview, read The Four Levels of Fluid Development, which describes different stages of using Fluid.


Features


Contents


Source

<ul id="products">
  {% for product in products %}
    <li>
      <h2>{{product.name}}</h2>
      Only {{product.price | price }}

      {{product.description | prettyprint | paragraph }}
    </li>
  {% endfor %}
</ul>

Result

<ul id="products">
    <li>
      <h2>Apple</h2>
      $329

      Flat-out fun.
    </li>
    <li>
      <h2>Orange</h2>
      $25

      Colorful. 
    </li>
    <li>
      <h2>Banana</h2>
      $99

      Peel it.
    </li>
</ul>

Notice


Using Fluid in your project

You can directly reference the NuGet package.

The code samples in this document assume you have registered the Fluid namespace with using Fluid;.

Hello World

Source

var parser = new FluidParser();

var model = new { Firstname = "Bill", Lastname = "Gates" };
var source = "Hello {{ Firstname }} {{ Lastname }}";

if (parser.TryParse(source, out var template, out var error))
{   
    var context = new TemplateContext(model);

    Console.WriteLine(template.Render(context));
}
else
{
    Console.WriteLine($"Error: {error}");
}

Result

Hello Bill Gates

Model security

Fluid templates can read public properties and fields from the model and from objects reachable through it. This is by design; a model passed to TemplateContext should be treated as the template's readable data boundary.

When rendering an untrusted template, pass a dedicated model that contains only the data the template is allowed to read. Do not pass domain entities, service objects, configuration objects, or other object graphs that may expose sensitive data through public members.

Thread-safety

A FluidParser instance is thread-safe and should be shared by the whole application. A common pattern is to declare the parser in a local static variable:

    private static readonly FluidParser _parser = new FluidParser();

An IFluidTemplate instance is thread-safe and can be cached and reused by multiple threads concurrently.

A TemplateContext instance is not thread-safe, and a new instance should be created every time an IFluidTemplate instance is used.

Values registered in TemplateOptions.GlobalValues are shared by every context created from those options. Values set directly on a TemplateContext belong to that rendering. Custom tags that need temporary values should use a scope lease:

using var scope = context.EnterScope(ScopeBehavior.Local);
context.SetValue("temporary", value);

Local scopes inherit values and keep assignments local. WriteThrough scopes keep values assigned with LocalScope.SetOwnValue temporary while normal assignments update the caller, which matches include and loop behavior. Isolated scopes can only read the context's initial values and GlobalValues, which matches the render tag.


NativeAOT and trimming

Fluid works when targeting NativeAOT and trimmed deployments.

  1. Reuse TemplateOptions instances (for example, at app startup).
  2. If you use runtime MemberAccessStrategy.Register<T...> calls, execute them during application startup before rendering templates.
  3. Prefer [FluidRegister] on a custom TemplateOptions subclass for model types known at compile time.
  4. Validate your app with AOT/trim publish settings:
dotnet publish -c Release -r <RID> -p:PublishAot=true

Source generation (optional)

When the Fluid.SourceGenerator analyzer is enabled, Fluid can generate strongly-typed member accessors for types declared with FluidRegisterAttribute.

The recommended pattern is to declare a custom TemplateOptions subclass and add one FluidRegisterAttribute per model type:

using Fluid;

[FluidRegister(typeof(Person))]
[FluidRegister(typeof(Address))]
public partial class PublicTemplateOptions : TemplateOptions
{
}

Use the generated options type like any other TemplateOptions instance:

var options = new PublicTemplateOptions();

The generated registrations are instance-scoped and are applied automatically to each PublicTemplateOptions instance. Runtime registrations still work and can be added normally:

options.MemberAccessStrategy.Register<Product, object>((product, name) => product.Name);

Custom member accessors

Custom accessors derive from MemberAccessor and return a ValueTask<FluidValue> directly.

private sealed class ProductDisplayNameAccessor : MemberAccessor
{
    public override ValueTask<FluidValue> GetAsync(
        object obj,
        string name,
        TemplateContext context)
    {
        return CreateValueTask(((Product)obj).Name, context);
    }
}

options.MemberAccessStrategy.Register<Product>(
    "display_name",
    new ProductDisplayNameAccessor());

The protected CreateValueTask overloads convert synchronous, Task<T>, and ValueTask<T> results using the TemplateOptions.ValueConverters configured for the current context. Return NilValue.Instance for a Liquid nil value; a null FluidValue is reserved as the accessor's not-handled result.

Alternatively, explicit profile methods can apply generated registrations to any TemplateOptions instance:

public static partial class FluidProfiles
{
    [FluidRegister(typeof(Person))]
    [FluidRegister(typeof(Address))]
    public static partial void ApplyPublic(TemplateOptions options);
}

Use it with any options instance:

var options = new TemplateOptions();
FluidProfiles.ApplyPublic(options);

Source generator

Fluid includes a Roslyn source generator (project: Fluid.SourceGenerator) that can compile Liquid templates at build time and expose them as strongly-typed static properties.

This is useful when you want:

MSBuild setup

  1. Reference the generator as an analyzer.

If you use a project reference:

<ItemGroup>
  <ProjectReference Include="../Fluid.SourceGenerator/Fluid.SourceGenerator.csproj"
                    OutputItemType="Analyzer"
                    ReferenceOutputAssembly="false" />
</ItemGroup>
  1. Provide templates to the generator using AdditionalFiles.
<ItemGroup>
  <AdditionalFiles Include="Templates/**/*.liquid" />
</ItemGroup>
  1. (Optional) Set the templates root folder.

When set, the generator:

<PropertyGroup>
  <FluidTemplatesFolder>Templates</FluidTemplatesFolder>
</PropertyGroup>

Usage

Annotate a partial class with FluidTemplates, using include and optional exclude glob patterns. Each matching file generates a dedicated static property returning an IFluidTemplate.

using Fluid.SourceGenerator;

namespace MyApp;

[FluidTemplates("**/*.liquid", Exclude = new[] { "_*.liquid" })]
public static partial class Templates
{
}

With templates like:

The generator will create:

Glob patterns

The generator supports a simple glob syntax:

Multiple include patterns are supported via the attribute constructor:

[FluidTemplates("**/*.liquid", "**/*.txt")]

Notes


Adding custom filters

Filters can be async or not. They are defined as a delegate that accepts an input, a set of arguments and the current context of the rendering process.

Here is the downcase filter as defined in Fluid.

Source

public static ValueTask<FluidValue> Downcase(FluidValue input, FilterArguments arguments, TemplateContext context)
{
    return new StringValue(input.ToStringValue().ToLower());
}

Registration

Filters are registered in an instance of TemplateOptions. This options object can be reused every time a template is rendered.

var options = new TemplateOptions();
options.Filters.AddFilter('downcase', Downcase);

var context = new TemplateContext(options);

Altering exposed .NET properties

Converting object types

Use the ValueConverters property to return different values than those provided by the model classes and properties:

var options = new TemplateOptions();
options.ValueConverters.Add(o => o is DateTime d ? new StringValue($"This is a date time: {d}") : null);

The previous example will return a custom value instead of the actual DateTime. When no conversion should be applied, null is returned.

Customizing object properties

A common scenario is to access named properties on an object that do not exist in the source class, or should return a different result.

In this case, the ValueConverters can be used to return a specific wrapper/proxy FluidValue instance. In practice, you can inherit from ObjectValueBase as it implements how most objects should behave.

The following example shows how to provide a custom transformation for any Person object:

private class PersonValue : ObjectValueBase
{
    public PersonValue(Person value) : base(value)
    {
    }

    public override ValueTask<FluidValue> GetValueAsync(string name, TemplateContext context)
    {
        if (name == "Bingo")
        {
          return new StringValue("Hello, World!");
        }
    }
}

This custom type can be used with a converter so that any time a Person is used, it is wrapped as a PersonValue.

var options = new TemplateOptions();
options.ValueConverters.Add(o => o is Person p ? new PersonValue(p) : null);

Invoking the member Bingo on a Person instance will then return the string "Hello, World!":

{{ myPerson.Bingo }}

Note: This technique can also be used to substitute existing properties with other values or even computed data.


Handling undefined values

Fluid evaluates members lazily, so undefined identifiers can be detected precisely when they are consumed. By default, undefined values render as empty strings without raising errors.

Tracking undefined values

To track missing values during template rendering, assign a delegate to TemplateOptions.Undefined or TemplateContext.Undefined. This delegate is called each time an undefined variable is accessed and receives the variable path and parent object type. The type argument is null when there is no target object (for example, when a global value is missing).

var missingVariables = new List<string>();

var context = new TemplateContext();
context.Undefined = (name, type) =>
    {
        missingVariables.Add(name);
        return ValueTask.FromResult<FluidValue>(NilValue.Instance);
    }
};

var template = FluidTemplate.Parse("Hello {{ user.name }} in {{ city }}!");

await template.RenderAsync(context);

### Strict variables

If you prefer templates to fail fast when they reference a variable that does not exist, enable strict variable mode by setting `TemplateOptions.StrictVariables` to `true`. When `StrictVariables` is `true`, any attempt to access an undefined variable throws a `FluidException` containing the variable name. This makes missing data issues visible immediately instead of silently rendering as an empty string.

```csharp
var options = new TemplateOptions { StrictVariables = true };
var context = new TemplateContext(options);

// Parsing a template that references an undefined variable
var template = FluidTemplate.Parse("Hello {{ user.name }}!");

// Throws FluidException: Undefined variable 'user'
await template.RenderAsync(context);

When StrictVariables is disabled (the default), you can still track missing variables using the Undefined delegate described above, or provide fallback values by returning a custom FluidValue.

// missingVariables now contains ["user.name", "city"]


### Strict filters

By default, applying an unknown filter simply returns the input value unchanged:

```liquid
{{ 'hello' | unknown }}  => hello

If you would rather fail fast when a template references a filter that has not been registered, enable strict filter mode by setting TemplateOptions.StrictFilters to true:

var options = new TemplateOptions { StrictFilters = true };
var context = new TemplateContext(options);

var template = FluidTemplate.Parse("{{ 'hello' | unknown }}");
// Throws FluidException: Undefined filter 'unknown'
await template.RenderAsync(context);

Known filters continue to work normally when StrictFilters is enabled:

{{ 'hello' | upcase }}  => HELLO

Use StrictFilters together with StrictVariables to enforce both variable and filter correctness during authoring.

Returning custom values for undefined values

The Undefined delegate can return a custom FluidValue to provide fallback values or error messages for missing values:

var options = new TemplateOptions
{
    Undefined = (name, type) =>
    {
        // Return a custom default value for undefined variables
        return ValueTask.FromResult<FluidValue>(new StringValue($"[{name} not found]"));
    }
};

var template = FluidTemplate.Parse("Hello {{ user.name }} in {{ city }}!");
var context = new TemplateContext(options);

var result = await template.RenderAsync(context);
// Outputs: "Hello [user.name not found] in [city not found]!"

Logging undefined accesses

You can use the Undefined delegate to log missing values for debugging or monitoring:

var options = new TemplateOptions
{
    Undefined = (path, type) =>
    {
        Console.WriteLine($"Missing variable: {path}, parent type: {type?.Name ?? "<none>"}");
        return ValueTask.FromResult<FluidValue>(NilValue.Instance);
    }
};

var template = FluidTemplate.Parse("{{ first }} {{ second }}");
var context = new TemplateContext(options);
await template.RenderAsync(context);
// Logs: "Missing variable: first"
// Logs: "Missing variable: second"

Object members casing

By default, the properties of a registered object are case-sensitive and registered as they are in their source code. For instance, the property FirstName would be accessed using the {{ p.FirstName }} tag.

However, you can register these properties with different cases, like camelCase (firstName), snake_case (first_name), or even make them case-insensitive. The ModelNamesComparer option accepts an instance of System.StringComparer.

The following example configures the templates to use camel casing.

var options = new TemplateOptions() 
{ 
    ModelNamesComparer = StringComparers.CamelCase
}

With this setting, both model properties and context properties are accessible using camel-casing:

{{ firstName }} {{ lastName }}

Loading templates asynchronously

TemplateOptions.FileProvider uses the asynchronous ITemplateFileProvider contract. Templates stored in a remote service can be loaded without blocking:

var options = new TemplateOptions
{
    FileProvider = new DelegateTemplateFileProvider(async (path, context, cancellationToken) =>
    {
        var metadata = await templateStore.GetMetadataAsync(path, cancellationToken);
        if (metadata is null)
        {
            return null;
        }

        return new TemplateSourceInfo(
            metadata.LastModified,
            async cancellationToken => await templateStore.OpenReadAsync(path, cancellationToken),
            cacheKey: $"{tenantId}:{path}");
    })
};

var context = new TemplateContext(options)
{
    CancellationToken = requestAborted
};
var result = await template.RenderAsync(context);

The provider returns null when a path does not exist. Fluid tries the requested path first, then appends DefaultFileExtension when necessary.

Fluid calls the provider to obtain the source version before checking its parsed-template cache. The stream is opened only on a cache miss. LastModified must advance whenever the content changes; remote implementations can cache metadata themselves if checking it requires a network request. When a provider can return different content for the same path in different contexts, set TemplateSourceInfo.CacheKey to a stable value that includes the tenant or other source identity.

Existing Microsoft.Extensions.FileProviders.IFileProvider implementations can be adapted:

options.FileProvider = new FileProviderTemplateFileProvider(existingFileProvider);

FluidViewEngineOptions.ViewsFileProvider and PartialsFileProvider use the same asynchronous contract for views, layouts, _ViewStart files, and partials. ASP.NET Core MVC view-name discovery remains synchronous because IViewEngine.FindView has no asynchronous contract; configure FluidMvcViewOptions.ViewLocationFileProvider with an IFileProvider for that lookup.

Always use RenderAsync with an asynchronous provider. The synchronous Render APIs must block if the provider suspends.

TemplateContext.CancellationToken is also checked at render entry, between statements, on loop iterations and built-in array enumeration, and before buffered output is flushed. Custom filters and FluidValue implementations receive the context and should check the token during long-running work.


Execution limits

Limiting template recursion

When invoking {% include 'sub-template' %} statements, it is possible that some templates create an infinite recursion that could block the server. To prevent this, the TemplateOptions class defines a default MaxRecursion = 100 that prevents templates from having a depth greater than 100.

Limiting template execution

A template can inadvertently perform enough work to block the server. MaxSteps limits statements, loop iterations, range construction, and built-in collection enumeration. MaxOutputSize limits cumulative rendered output, captured blocks, macro results, and amplified string operations. MaxCollectionSize limits ranges and collections materialized by built-in operations.

MaxSteps, MaxOutputSize, and MaxCollectionSize are unlimited by default to preserve compatibility. Set all of them when rendering untrusted templates. TemplateContext.CancellationToken complements these limits and should also be set.

Rendering user-provided templates safely

Fluid is a template engine, not a security sandbox. An application that accepts templates from users should apply all of the following controls:

  1. Limit the template source length before parsing. Render limits do not limit parsing.
  2. Pass a dedicated model containing only data the user is allowed to read. Templates can read public properties and fields from every object reachable through the model. Do not pass domain entities, services, dependency-injection containers, configuration, or secrets.
  3. Configure recursion, work, output, and collection limits. The appropriate values depend on the templates and capacity of the application; start conservatively and adjust using representative load tests.
  4. Set a per-render cancellation deadline in addition to the request cancellation token.
  5. Keep the default null file provider unless user templates need include, render, or from. If they do, use a tenant-scoped or allow-listed provider that cannot access application files or another tenant's templates.
  6. Expose only trusted custom filters, tags, values, and file providers. Extension code executes with the permissions of the application and must honor cancellation and equivalent resource limits.
  7. Apply normal service protections such as request-size limits, authentication, rate limits, bounded concurrency, and memory or process isolation where the threat model requires it.

The following is an example starting point. The numerical limits are illustrative and should be tuned for the application:

private const int MaxTemplateLength = 100_000;

private static readonly FluidParser Parser = new FluidParser();

private static readonly TemplateOptions UserTemplateOptions = new TemplateOptions
{
    MaxRecursion = 20,
    MaxSteps = 10_000,
    MaxOutputSize = 1_000_000,
    MaxCollectionSize = 10_000
    // FileProvider retains its default NullFileProvider.
};

public static async ValueTask<string> RenderUserTemplateAsync(
    string source,
    SafeTemplateModel model,
    CancellationToken requestAborted)
{
    if (source.Length > MaxTemplateLength)
    {
        throw new ArgumentException("The template is too large.", nameof(source));
    }

    if (!Parser.TryParse(source, out var template, out var error))
    {
        throw new ArgumentException(error, nameof(source));
    }

    using var timeout = CancellationTokenSource.CreateLinkedTokenSource(requestAborted);
    timeout.CancelAfter(TimeSpan.FromSeconds(2));

    var context = new TemplateContext(model, UserTemplateOptions)
    {
        CancellationToken = timeout.Token
    };

    return await template.RenderAsync(context);
}

TemplateOptions, FluidParser, and parsed templates can be shared. Create a new TemplateContext for every render because it is not thread-safe and owns the execution counters and cancellation token.

When a file provider can return different content for the same path in different tenants or security scopes, set TemplateSourceInfo.CacheKey to a stable value that includes that scope. This prevents a cached parsed template from crossing the same boundary enforced by the provider.

MaxOutputSize counts UTF-16 characters written through Fluid's output abstraction. If the rendered result is encoded to a response with a separate byte limit, enforce that transport limit as well. Custom filters, values, tags, and output implementations are responsible for enforcing equivalent limits for work they perform outside Fluid's built-in operations.


Converting CLR types

Whenever an object is manipulated in a template, it is converted to a specific FluidValue instance that provides a dynamic type system somewhat similar to the one in JavaScript.

In Liquid, they can be Number, String, Boolean, Array, Dictionary, or Object. Fluid will automatically convert the CLR types to the corresponding Liquid ones, and also provides specialized ones.

To customize this conversion, you can add value converters.

Adding a value converter

When the conversion logic is not directly inferred from the type of an object, a value converter can be used.

Value converters can return:

The following example shows how to convert any instance implementing an interface to a custom string value:

var options = new TemplateOptions();

options.ValueConverters.Add((value) => value is IUser user ? user.Name : null);

Note: Type mappings are defined globally for the application.


Encoding

By default, Fluid doesn't encode the output. Encoders can be specified when calling Render() or RenderAsync() on the template.

HTML encoding

To render a template with HTML encoding, use the System.Text.Encodings.Web.HtmlEncoder.Default instance.

This encoder is used by default for the MVC View engine.

Disabling encoding contextually

When an encoder is defined, you can use a special raw filter or {% raw %} ... {% endraw %} tag to prevent a value from being encoded, for instance if you know that the content is HTML and is safe.

Source

{% assign html = '<em>This is some html</em>' %}

Encoded: {{ html }}
Not encoded: {{ html | raw }

Result

&lt;em%gt;This is some html&lt;/em%gt;
<em>This is some html</em>

Captured blocks are not double-encoded

When using capture blocks, the inner content is flagged as pre-encoded and won't be double-encoded if used in a {{ }} tag.

Customizing JSON output

The json filter uses System.Text.Json.JsonSerializerOptions to control the JSON output format. You can customize these options through TemplateOptions.JsonSerializerOptions or TemplateContext.JsonSerializerOptions.

Example: Indented JSON output

var options = new TemplateOptions
{
    JsonSerializerOptions = new JsonSerializerOptions
    {
        WriteIndented = true
    }
};

var context = new TemplateContext(options);
context.SetValue("data", new { name = "John", age = 30 });

{{ data | json }}

Result

{
  "name": "John",
  "age": 30
}

You can also set JsonSerializerOptions per TemplateContext, but it is recommended to reuse JsonSerializerOptions instances and define them in a TemplateOptions instance that can be reused across TemplateContext instances.

JSON encoding

By default, all JSON strings are encoded using the default JavaScriptEncoder instance. This can be changed by setting the JsonSerializerOptions.JavaScriptEncoder property to JavaScriptEncoder.UnsafeRelaxedJsonEscaping.

{{ "你好，这是一条短信" | json" }}

With the default JSON encoder:

"\u4F60\u597D\uFF0C\u8FD9\u662F\u4E00\u6761\u77ED\u4FE1"

Using the relaxed JSON encoding:

// This variable should be static and reused for all template contexts
var options = new TemplateOptions
{
    JsonSerializerOptions = new JsonSerializerOptions
    {
        JavaScriptEncoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping
    }
    
};

var context = new TemplateContext(options);

Result:

"你好，这是一条短信"

Localization

By default, templates are rendered using an invariant culture so that the results are consistent across systems. This is important, for instance, when rendering dates, times, and numbers.

However, you can define a specific culture to use when rendering a template using the TemplateContext.CultureInfo property.

Source

var options = new TemplateOptions();
options.CultureInfo = new CultureInfo("en-US");
var context = new TemplateContext(options);
var result = template.Render(context);

{{ 1234.56 }}
{{ "now" | date: "%v" }}

Result

1234.56
Tuesday, August 1, 2017

Money filters

Fluid implements the Shopify money filters. They are not registered by default, add them to the filters of the TemplateOptions instance.

var options = new TemplateOptions();
options.Filters.WithMoneyFilters();
Filter Source Result
money {{ 1134.65 | money }} $1,134.65
money_with_currency {{ 1134.65 | money_with_currency }} $1,134.65 USD
money_without_currency {{ 1134.65 | money_without_currency }} 1,134.65
money_without_trailing_zeros {{ 10.00 | money_without_trailing_zeros }} $10

Amounts are rounded away from zero, so 10.005 is rendered as $10.01.

Cultures and currencies

By default, the currency and the way amounts are formatted are derived from TemplateOptions.CultureInfo. The default culture is the invariant one, which has no currency, in which case USD is used.

options.CultureInfo = new CultureInfo("de-DE");

{{ 1134.65 | money_with_currency }}

1.134,65 € EUR

A specific currency can be set with MoneyOptions.Currency, using its ISO 4217 code. It is also accepted as an argument of every money filter, which is useful when a single template renders multiple currencies.

options.MoneyOptions.Currency = "EUR";

{{ 10 | money }}
{{ 10 | money: 'GBP' }}
{{ 10 | money: currency: 'JPY' }}

€10.00
£10.00
¥10

The symbol and the number of decimal digits of the most common currencies are known to Fluid. Others can be added, or replaced, in MoneyOptions.Currencies. A currency that is not registered is rendered using its code as the symbol.

options.MoneyOptions.Currencies["BTC"] = new MoneyCurrency("BTC", "₿", decimalDigits: 8);

Amounts stored in cents

Shopify stores prices as integers representing cents. Set MoneyOptions.AmountsInCents to divide the input of the money filters by 100, which makes it possible to reuse Shopify templates as-is.

options.MoneyOptions.AmountsInCents = true;

{{ 1450 | money }}

$14.50

Custom formats

MoneyOptions.MoneyFormat and MoneyOptions.MoneyWithCurrencyFormat override the culture based formatting, using the same placeholders as the Shopify currency formatting settings. These placeholders are culture independent.

Placeholder Result for 1134.65
{{amount}} 1,134.65
{{amount_no_decimals}} 1,135
{{amount_with_comma_separator}} 1.134,65
{{amount_no_decimals_with_comma_separator}} 1.135
{{amount_with_apostrophe_separator}} 1'134.65
{{amount_no_decimals_with_space_separator}} 1 135
{{amount_with_space_separator}} 1 134,65
{{amount_with_period_and_space_separator}} 1 134.65
{{currency}} the ISO 4217 code of the currency, e.g. USD
{{currency_symbol}} the symbol of the currency, e.g. $

{{currency}} and {{currency_symbol}} are specific to Fluid, and let a single format be used with the currency that is resolved when the template is rendered. Unknown placeholders are rendered verbatim.

options.MoneyOptions.MoneyFormat = "{{amount_with_comma_separator}} kr";

{{ 1134.65 | money }}

1.134,65 kr

money_without_currency always renders the amount on its own and ignores these formats. When MoneyWithCurrencyFormat is not set, money_with_currency uses MoneyFormat followed by the currency code.

Rendering a different currency per request

MoneyOptions is application-wide configuration and is expected to be configured once. To use a different currency for a single rendering, assign a MoneyOptions instance on the TemplateContext.

var context = new TemplateContext(options)
{
    MoneyOptions = new MoneyOptions { Currency = "EUR" }
};

Time zones

📖 For a comprehensive guide on working with time zones in Fluid, see TimeZones.md

System time zone

TemplateOptions and TemplateContext provide a property to define a default time zone to use when parsing dates and times. The default value is the current system's time zone. Setting a custom one can also prevent different environments (data centers) from generating different results.

Important: The TimeZone property is used for parsing date strings, not for automatically converting dates during rendering. To convert dates to a specific timezone for display, use the time_zone filter.

Note: The date filter conforms to the Ruby date and time formats https://ruby-doc.org/core-3.0.0/Time.html#method-i-strftime. To use the .NET standard date formats, use the format_date filter.

Source

var context = new TemplateContext { TimeZone = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time") } ;
var result = template.Render(context);

{{ '1970-01-01 00:00:00' | date: '%c' }}

Result

Wed Dec 31 19:00:00 -08:00 1969

Converting time zones

Dates and times can be converted to specific time zones using the time_zone: <iana> filter.

Example

var context = new TemplateContext();
context.SetValue("published", DateTime.UtcNow);

{{ published | time_zone: 'America/New_York' | date: '%+' }}

Result

Tue Aug  1 17:04:36 -05:00 2017

Customizing tags and blocks

Fluid's grammar can be modified to accept any new tags and blocks with any custom parameters. The parser is based on Parlot which makes it completely extensible.

Unlike blocks, tags don't have a closing element (e.g., cycle, increment). A closing element will match the name of the opening tag with an end suffix, like endfor. Blocks are useful when manipulating a section of a template as a set of statements.

Fluid provides helper methods to register common tags and blocks. All tags and blocks always start with an identifier that is the tag name.

Each custom tag needs to provide a delegate that is evaluated when the tag is matched. Each delegate will be able to use these properties:

Registering a custom tag

Here are some examples:

Source

parser.RegisterIdentifierTag("hello", (identifier, writer, encoder, context) =>
{
    writer.Write("Hello ");
    writer.Write(identifier);
});

{% hello you %}

Result

Hello you

Registering a custom block

Blocks are created the same way as tags, and the lambda expression can then access the list of statements inside the block.

Source


parser.RegisterExpressionBlock("repeat", async (value, statements, writer, encoder, context) =>
{
    var fluidValue = await value.EvaluateAsync(context);

    for (var i = 0; i < fluidValue.ToNumberValue(); i++)
    {
        await statements.RenderStatementsAsync(writer, encoder, context);
    }

    return Completion.Normal;
});

{% repeat 1 | plus: 2 %}Hi! {% endrepeat %}

Result

Hi! Hi! Hi!

Custom parsers

If identifier, empty and expression parsers are not sufficient, the methods RegisterParserBlock and RegisterParserTag accept any custom parser construct. These can be the standard ones defined in the FluidParser class, like Primary, or any other composition of them.

For instance, RegisterParseTag(Primary.AndSkip(Comma).And(Primary), ...) will expect two Primary elements separated by a comma. The delegate will then be invoked with a ValueTuple<Expression, Expression> representing the two Primary expressions.

Registering a custom operator

Operators are used to compare values, like > or contains. Custom operators can be defined if special comparisons need to be provided.

Source

The following example creates a custom xor operator that will evaluate to true if only one of the left and right expressions is true when converted to booleans.

XorBinaryExpression.cs

using Fluid.Ast;
using Fluid.Values;
using System.Threading.Tasks;

namespace Fluid.Tests.Extensibility
{
    public class XorBinaryExpression : BinaryExpression
    {
        public XorBinaryExpression(Expression left, Expression right) : base(left, right)
        {
        }

        public override async ValueTask<FluidValue> EvaluateAsync(TemplateContext context)
        {
            var leftValue = await Left.EvaluateAsync(context);
            var rightValue = await Right.EvaluateAsync(context);

            return BooleanValue.Create(leftValue.ToBooleanValue() ^ rightValue.ToBooleanValue());
        }
    }
}

Parser configuration

parser.RegisteredOperators["xor"] = (a, b) => new XorBinaryExpression(a, b);

Usage

{% if true xor false %}Hello{% endif %}

Result

Hello

Accessing the concrete syntax tree

The syntax tree is accessible by casting the template to its concrete FluidTemplate type and using the Statements property.

Source

var template = (FluidTemplate)iTemplate;
var statements = template.Statements;

ASP.NET MVC View Engine

The package Fluid.MvcViewEngine provides a convenient way to use Liquid as a replacement for, or in combination with, Razor in ASP.NET MVC.

Configuration

Registering the view engine

  1. Reference the Fluid.MvcViewEngine NuGet package
  2. Add a using statement on Fluid.MvcViewEngine
  3. Call AddFluid() in your Startup.cs.

Sample

using Fluid.MvcViewEngine;

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc().AddFluid();
    }
}

Registering view models

Because the Liquid language only allows known members to be accessed, the View Model classes need to be registered in Fluid, usually from a static constructor so that the code is run only once for the application.

View Model registration

View models are automatically registered and available as the root object in liquid templates. Custom model registrations can be added when calling AddFluid().

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc().AddFluid(o => o.TemplateOptions.Register<Person>());
    }
}

More ways to register types and members can be found in the Allow-listing object members section.

Registering custom tags

When using the MVC View engine, custom tags can still be added to the parser. Refer to this section on how to create custom tags.

It is recommended to create a custom class inheriting from FluidViewParser, and to customize the tags in the constructor of this new class. This class can then be registered as the default parser for the MVC view engine.

using Fluid.Ast;
using Fluid.MvcViewEngine;

namespace Fluid.MvcSample
{
    public class CustomFluidViewParser : FluidViewParser
    {
        public CustomFluidViewParser()
        {
            RegisterEmptyTag("mytag", static async (s, w, e, c) =>
            {
                await w.WriteAsync("Hello from MyTag");

                return Completion.Normal;
            });
        }
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.Configure<MvcViewOptions>(options =>
        {
            options.Parser = new CustomFluidViewParser();
        });

        services.AddMvc().AddFluid();
    }
}

Layouts

Index.liquid

{% layout '_layout.liquid' %}

This is the home page

The {% layout [template] %} tag accepts one argument which can be any expression that returns the relative location of a Liquid template that will be used as the master template.

The layout tag is optional in a view. It can also be defined multiple times or conditionally.

From a layout template, the {% renderbody %} tag is used to depict the location of the view's content inside the layout itself.

Layout.liquid

<html>
  <body>
    <div class="menu"></div>
    
    <div class="content">
      {% renderbody %}
    </div>
    
    <div class="footer"></div>
  </body>
</html>

Sections

Sections are defined in a layout for views to render content in specific locations. For instance, a view can render some content in a menu or a footer section.

Rendering content in a section

{% layout '_layout.liquid' %}

This is the home page

{% section menu %}
  <a href="h#">This link goes in the menu</a>
{% endsection %}

{% section footer %}
  This text will go in the footer
{% endsection %}

Rendering the content of a section

<html>
  <body>
    <div class="menu">
      {% rendersection menu %}
    </div>
    
    <div class="content">
      {% renderbody %}
    </div>
    
    <div class="footer">
      {% rendersection footer %}
    </div>
  </body>
</html>

ViewStart files

Defining the layout template in each view might be cumbersome and make it difficult to change it globally. To prevent that, it can be defined in a _ViewStart.liquid file.

When a view is rendered, all _ViewStart.liquid files from its current and parent directories are executed beforehand. This means multiple files can be defined to set settings for a group of views.

_ViewStart.liquid

{% layout '_layout.liquid' %}
{% assign background = 'ffffff' }

You can also define other variables or render some content.

Custom views locations

It is possible to add custom file locations containing views by adding them to FluidMvcViewOptions.ViewsLocationFormats.

The default ones are:

Where {0} is the view name, and {1} is the controller name.

For partials, the list is defined in FluidMvcViewOptions.PartialsLocationFormats:

Layouts will be searched in the same locations as Views.

Execution

The content of a view is parsed once and kept in memory until the file or one of its dependencies changes. Once parsed, the tags are executed every time the view is called. In comparison, Razor views are first compiled and then instantiated every time they are rendered. This means that on startup or when the view is changed, views with Fluid will run faster than those in Razor, unless you are using precompiled Razor views. In all cases, Razor views will be faster on subsequent calls as they are compiled directly to C#.

This difference makes Fluid very suitable for rapid development cycles where the views can be deployed and updated frequently. And because the Liquid language is secure, developers can give access to them with more confidence.


View Engine

The Fluid ASP.NET MVC View Engine is based on an MVC-agnostic view engine provided in the Fluid.ViewEngine package. The same options and features are available, but without requiring ASP.NET MVC. This is useful to provide the same experience when building templates using layouts and sections.

Usage

Use the class FluidViewRenderer : IFluidViewRenderer and FluidViewEngineOptions.

Whitespace control

Liquid follows strict rules with regard to whitespace support. By default, all spaces and new lines are preserved from the template. The Liquid syntax and some Fluid options allow you to customize this behavior.

Hyphens

For example:

{%  assign name = "Bill" %}
{{ name }}

There is a new line after the assign tag which will be preserved.

Outputs:


Bill

Tags and values can use hyphens to strip whitespace.

Example:

{%  assign name = "Bill" -%}
{{ name }}

Outputs:

Bill

The -%} strips the whitespace from the right side of the assign tag.

Template Options

Fluid provides the TemplateOptions.Trimming property that can be set with predefined preferences for when whitespace should be stripped automatically, even if hyphens are not present in tags and output values.

Greedy Mode

When greedy mode is disabled in TemplateOptions.Greedy, only the spaces before the first new line are stripped. Greedy mode is enabled by default since this is the standard behavior of the Liquid language.


Custom filters

Some non-standard filters are provided by default:

format_date

Formats dates and times using standard .NET date and time formats. It uses the current culture of the system.

Input

"now" | format_date: "G"

Output

6/15/2009 1:45:30 PM

Documentation: https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-date-and-time-format-strings

format_number

Formats numbers using standard .NET number formats.

Input

123 | format_number: "N"

Output

123.00

Documentation: https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings

format_string

Formats custom strings using standard .NET format strings.

Input

"hello {0} {1:C}" | format_string: "world" 123

Output

hello world $123.00

Documentation: https://docs.microsoft.com/en-us/dotnet/api/system.string.format


Functions

Fluid provides optional support for functions, which is not part of the standard Liquid templating language. As such, it is not enabled by default.

Enabling functions

When instantiating a FluidParser, set the FluidParserOptions.AllowFunctions property to true.

var parser = new FluidParser(new FluidParserOptions { AllowFunctions = true });

When functions are used while the feature is not enabled, a parse error will be returned.

Declaring local functions with the macro tag

macro allows you to define reusable chunks of content to invoke with a local function.

{% macro field(name, value='', type='text') %}
<div class="field">
  <input type="{{ type }}" name="{{ name }}"
         value="{{ value }}" />
</div>
{% endmacro %}

Now field is available as a local property of the template and can be invoked as a function.

{{ field('user') }}
{{ field('pass', type='password') }}

Macros need to be defined before they are used, as they are discovered as the template is executed.

Importing functions from external templates

Macros defined in an external template must be imported before they can be invoked.

Omit the import list to import every macro defined by the template:

{% from 'forms' %}

{{ field('user') }}
{{ field('pass', type='password') }}

Use an explicit import list to import only selected macros:

{% from 'forms' import field %}

{{ field('user') }}
{{ field('pass', type='password') }}

The import-all form is the equivalent of a wildcard import; no wildcard token is required. Both forms evaluate the external template without rendering its output and copy only macros, not its assigned variables. An imported macro replaces a same-named value in the current scope, so prefer selective imports when name collisions are possible. Use include to render external template content and from to load its macros.

Extensibility

Functions are FluidValue instances implementing the InvokeAsync method. This allows any template to be provided custom function values as part of the model, the TemplateContext, or globally with options.

A FunctionValue type is also available to provide out-of-the-box functions. It takes a delegate that returns a ValueTask<FluidValue> as the result.

var lowercase = new FunctionValue((args, context) => 
{
  var firstArg = args.At(0).ToStringValue();
  var lower = firstArg.ToLowerCase();
  return new StringValue(lower);
});

var context = new TemplateContext();
context.SetValue("tolower", lowercase);

var parser = new FluidParser(new FluidParserOptions { AllowFunctions = true });
parser.TryParse("{{ tolower('HELLO') }}", out var template, out var error);
template.Render(context);

Order of execution

With tags containing more than one and or or operator, operators are evaluated in order from right to left. You cannot change the order of operations using parentheses. This is the same for filters, which are executed from left to right. However, Fluid provides an option to support grouping expressions with parentheses.

Enabling parentheses

When instantiating a FluidParser, set the FluidParserOptions.AllowParentheses property to true.

var parser = new FluidParser(new FluidParserOptions { AllowParentheses = true });

When parentheses are used while the feature is not enabled, a parse error will be returned (except for ranges like (1..4)).

At that point a template like the following will work:

{{ 1 | plus : (2 | times: 3) }}

Visiting and altering a template

Fluid provides a Visitor pattern that allows you to analyze what a template is made of, and also to alter it. This can be used, for instance, to check if a specific identifier is used, replace some filters with others, or remove any expression that might not be authorized.

Visiting a template

The Fluid.Ast.AstVisitor class can be used to create a custom visitor.

Here is an example of a visitor class that records if an identifier is accessed anywhere in a template:

  public class IdentifierIsAccessedVisitor : AstVisitor
  {
      private readonly string _identifier;

      public IdentifierIsAccessedVisitor(string identifier)
      {
          _identifier = identifier;
      }

      public bool IsAccessed { get; private set; }

      public override IFluidTemplate VisitTemplate(IFluidTemplate template)
      {
          // Initialize the result each time a template is visited with the same visitor instance

          IsAccessed = false;
          return base.VisitTemplate(template);
      }

      protected override Expression VisitMemberExpression(MemberExpression memberExpression)
      {
          var firstSegment = memberExpression.Segments.FirstOrDefault() as IdentifierSegment;

          if (firstSegment != null)
          {
              IsAccessed |= firstSegment.Identifier == _identifier;
          }

          return base.VisitMemberExpression(memberExpression);
      }
  }

And its usage:

var template = new FluidParser().Parse("{{ a.b | plus: 1}}");

var visitor = new IdentifierIsAccessedVisitor("a");
visitor.VisitTemplate(template);

Console.WriteLine(visitor.IsAccessed); // writes True

Rewriting a template

The Fluid.Ast.AstRewriter class can be used to create a custom rewriter.

Here is an example of a visitor class that replaces any plus filter with a minus one:

  public class ReplacePlusFiltersVisitor : AstRewriter
  {
      protected override Expression VisitFilterExpression(FilterExpression filterExpression)
      {
          if (filterExpression.Name == "plus")
          {
              return new FilterExpression(filterExpression.Input, "minus", filterExpression.Parameters);
          }

          return filterExpression;
      }
  }

And its usage:


var template = new FluidParser().Parse("{{ 1 | plus: 2 }}");

var visitor = new ReplacePlusFiltersVisitor();
var changed = visitor.VisitTemplate(template);

var result = changed.Render();

Console.WriteLine(result); // writes -1

Visiting templates parsed during rendering

You can apply visitors and rewriters to templates that are parsed before they are cached by using the TemplateParsed callback on TemplateOptions. This works for all template parsing scenarios including the ViewEngine, include and render statements.

var options = new TemplateOptions
{
    FileProvider = new FileProviderTemplateFileProvider(fileProvider)
};
options.TemplateParsed = (path, template) =>
{
    var visitor = new MyCustomVisitor();
    return visitor.VisitTemplate(template);
};

The TemplateParsed callback is invoked after a template is parsed but before it is cached. This means:

Custom parsers

The custom statements and expressions can also be visited by using one of these methods:

They all expose a TagName property and, optionally, Statements and Value properties when applicable.

Performance

Fluid is fast, but only if you follow these best practices:

Cache IFluidTemplate instances

It is common for the same templates to be rendered over time. In this case, it is beneficial to cache the resulting IFluidTemplate instance from the FluidParser.Parse() method. You can use the template name with a timestamp or its content as the cache key. If your templates can evolve, ensure that the cache is not unbounded and entries eventually get evicted. The recommended approach is to use a singleton IMemoryCache that can be configured with size limits and eviction time.

IFluidTemplate instances are thread-safe for read access and can be shared by multiple concurrent threads.

Reuse the TemplateOptions instance

These instances are meant to be reused. This is why there is a separation between TemplateContext, which is per rendering, and TemplateOptions, which contains state that is shared across all renderings, such as property resolutions and lambdas. A convenient approach is to declare them as static, though you should adapt this to your needs.

TemplateOptions instances are thread-safe for read access and can be shared by multiple concurrent threads.

Reuse the FluidParser instance

Instantiating a FluidParser instance is expensive, do it once and reuse the instance. This can be registered as a singleton if you use dependency injection, but in most cases a static instance makes sense since it's rare to customize these.

Render directly to UTF-8

On .NET 8 and later, Utf8FluidOutput writes directly to an IBufferWriter<byte> without accumulating the rendered response as UTF-16 or adding an ASP.NET Core dependency to Fluid:

await template.RenderAsync(response.BodyWriter, HtmlEncoder.Default, context);

// The destination remains caller-owned.
await response.BodyWriter.FlushAsync(requestAborted);

The RenderAsync overload creates and disposes Utf8FluidOutput, ensuring its UTF-8 encoder state is finalized. The destination remains caller-owned and is not flushed or completed. Create Utf8FluidOutput directly when multiple templates need to share one character stream. Its FlushAsync method transcodes buffered characters while preserving encoder state so surrogate pairs remain valid across intermediate template flushes. Because IFluidOutput writes synchronously, transport flushing is not attempted in the middle of a render. MaxOutputSize continues to count UTF-16 characters, not encoded bytes.

Benchmarks

A benchmark application is provided in the source code to compare Fluid, Scriban, DotLiquid, Liquid.NET, and Handlebars.NET. Run it locally to analyze the time it takes to execute specific templates.

TL;DR — Fluid is faster and allocates less memory than all other well-known .NET Liquid parsers.

Results

Parse: Parses a simple HTML template containing filters and properties

Scriban takes 46% longer to parse and allocates over 3 times as much memory as Fluid.

Parse benchmark comparison

ParseBig: Parses a Blog Post template

Scriban takes 66% longer to parse and allocates almost 4 times as much memory as Fluid.

Large template parse benchmark comparison

Render: Renders a simple HTML template containing filters and properties, with 100 elements

DotLiquid takes over 10 times as long to render and allocates 18 times as much memory as Fluid. The second best, Handlebars (Mustache), takes 33% longer and allocates over 3 times as much memory.

Render benchmark comparison

Tested on 8/18/2026 with

Benchmark.NET data 
BenchmarkDotNet v0.15.8, macOS Sequoia 15.7.9 (24G830) [Darwin 24.6.0]
Apple M4 Pro, 1 CPU, 14 logical and 14 physical cores
.NET SDK 10.0.400
  [Host]   : .NET 10.0.11 (10.0.11, 10.0.1126.37416), Arm64 RyuJIT armv8.0-a
  ShortRun : .NET 10.0.11 (10.0.11, 10.0.1126.37416), Arm64 RyuJIT armv8.0-a

Job=ShortRun  IterationCount=3  LaunchCount=1
WarmupCount=3

| Method                       | Mean       | Error         | StdDev     | Ratio | RatioSD | Gen0    | Gen1    | Allocated | Alloc Ratio |
|----------------------------- |-----------:|--------------:|-----------:|------:|--------:|--------:|--------:|----------:|------------:|
| Fluid_Parse                  |   1.685 us |     0.1182 us |  0.0065 us |  1.00 |    0.00 |  0.3910 |  0.0019 |    3.2 KB |        1.00 |
| Scriban_Parse                |   2.464 us |     0.2919 us |  0.0160 us |  1.46 |    0.01 |  1.2245 |  0.0648 |  10.01 KB |        3.13 |
| DotLiquid_Parse              |   4.054 us |     0.3527 us |  0.0193 us |  2.41 |    0.01 |  1.9608 |  0.0229 |  16.05 KB |        5.02 |
| Handlebars_Parse             | 452.116 us |   433.5420 us | 23.7639 us | 268.40 |   12.25 | 15.6250 |  7.8125 | 133.62 KB |       41.82 |
|                              |            |               |            |       |         |         |         |           |             |
| Fluid_ParseBig               |   8.393 us |     0.5638 us |  0.0309 us |  1.00 |    0.00 |  1.5717 |  0.0458 |  12.86 KB |        1.00 |
| Scriban_ParseBig             |  13.928 us |     1.0365 us |  0.0568 us |  1.66 |    0.01 |  5.8746 |  1.1749 |  48.07 KB |        3.74 |
| DotLiquid_ParseBig           |  20.154 us |     2.4570 us |  0.1347 us |  2.40 |    0.02 | 11.4136 |  0.5188 |  93.46 KB |        7.27 |
| Handlebars_ParseBig          | 356.951 us | 1,331.1202 us | 72.9632 us | 42.53 |    7.53 | 16.6016 |  7.8125 | 139.87 KB |       10.88 |
|                              |            |               |            |       |         |         |         |           |             |
| Fluid_Render                 |  16.426 us |     1.5309 us |  0.0839 us |  1.00 |    0.01 |  4.4250 |  0.0610 |  36.26 KB |        1.00 |
| Fluid_SourceGenerated_Render |  30.694 us |     6.0248 us |  0.3302 us |  1.87 |    0.02 |  5.8594 |  0.0610 |  47.98 KB |        1.32 |
| Scriban_Render               | 127.750 us |     7.0525 us |  0.3866 us |  7.78 |    0.04 | 43.7012 |  8.0566 | 358.12 KB |        9.88 |
| DotLiquid_Render             | 166.841 us |     6.6613 us |  0.3651 us | 10.16 |    0.05 | 80.5664 | 15.3809 | 659.18 KB |       18.18 |
| Handlebars_Render            |  21.926 us |     1.8049 us |  0.0989 us |  1.33 |    0.01 | 14.5874 |       - | 119.31 KB |        3.29 |

Used by

Fluid is known to be used in the following projects:

Please create a pull request to be listed here.

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