🧊BlazorMemoire 🧊

A Blazor <Memo> component, similar to React.memo, that freezes its child subtree until explicit dependency keys change, preventing unnecessary re-renders and making lifecycle methods fire only when there's been a true change.

Why memoisation?

Blazor re-renders a child whenever its parent renders, but not always. ComponentBase has a built-in optimisation: if a component's parameters are all primitive, immutable types (string, int, bool, Guid, DateTime, etc - see all) and none of their values changed, Blazor skips the re-render for you. That's why simple components often feel "free": the framework is quietly detecting that nothing changed.

That optimisation only covers primitives though. As soon as a component takes a complex parameter, for example an object, a List<int>, a string[], a record, or Action / Func<T>, Blazor can no longer prove it's unchanged, so it plays it safe and re-renders every time the parent does. This often catches people out because a component that rendered efficiently for weeks suddenly starts re-rendering on every parent update, and the only thing that changed was adding a non-primitive parameter. Nothing looks obviously wrong, and there's no warning, the change detection just stopped applying.

When a subtree does heavy work or makes network calls and database queries in response to parameter changes, or on render, those redundant renders can really add up, slowing things down for users and increasing system load.

Why <Memo>?

BlazorMemoire's <Memo> lets you wrap any subtree and provide a set of dependency keys. The subtree renders once, then stays frozen until one or more keys change. No new parameters flow in, no lifecycle methods fire, and nothing downstream re-renders unless you've declared that it should.

• Skip expensive work: Freeze subtrees that would otherwise re-run costly logic, queries, or network requests on every parent render.
• Control from the call site: Decide when a subtree updates where you use it, not inside the component.
• Works with any component: Wrap third-party or shared components you can't (or don't want to) modify.
• One declaration, whole subtree: Freeze a component and all its descendants together, without touching their source.
• Minimal performance overhead, often a substantial gain: <Memo> is fast, and consumes negligible amounts of memory in nearly all cases. The performance cost of comparing the dependency keys is low, which means <Memo> only needs to skips a little rendering work to be a net positive. When used where it will skip a lot of rendering work - the savings can outweigh the cost many times over.

Why not ShouldRender?

Blazor's built-in ShouldRender override lets a component decide internally whether to re-render. This works, but has limitations:

• ShouldRender doesn't prevent parameter diffing. Even with a ShouldRender override, Blazor still calls SetParametersAsync then OnParametersSet/OnParametersSetAsync which can mean redundant execution of expensive network requests or database queries. <Memo> short-circuits before that happens so child components receive no new parameters at all when keys haven't changed.
• Declarative subtree freezing at the call site. You can skip a subtree by returning false from a parent's ShouldRender, but that couples the decision to the parent and stops the parent re-rendering too. <Memo> freezes just the wrapped subtree in one declaration, leaving the surrounding component free to render normally.
• The component controls its own re-render policy. If you want the same component to re-render on different conditions in different places, you're stuck as ShouldRender is baked into the component itself. <Memo> moves that decision to the call site, so the parent chooses when the subtree updates.
• It requires modifying the component. Third-party or shared components can't have ShouldRender added from the outside. Wrapping them in <Memo> gives you render control without touching their source.

ShouldRender is still the simpler choice when a component only needs to skip renders based on its own state, and you control its source.

<Memo> earns its place when you need to control rendering at the point of use instead of in the component, cut out rendering work for a whole subtree, or can't easily modify the component.

Installation

dotnet add package BlazorMemoire

Usage

Wrap any subtree in a <Memo> component and provide dependency keys. The child content only re-renders when a key value changes.

@using BlazorMemoire.Components

<Memo Keys="@([user.Id, selectedTab])" Deep="@false">
    <ExpensiveChildComponent User="@user" Tab="@selectedTab" />
</Memo>

Keys behaviour and comparison options

Comparison modes

Shallow comparison (default with Deep="false")

By default (with Deep="false"), each incoming key element is compared against the corresponding element from the previous render's snapshot, position by position.

The comparison is a null-safe call to the existing element's object.Equals method (i.e. existing.Equals(incoming)). two nulls are equal and otherwise the stored element's own equality decides. This is allocation-free and involves no reflection.

• Primitives, strings, records, enums, DateTime, TimeOnly, etc.: compared by value, because those types override Equals to compare by value.
• Reference-type collections (List<T>, Dictionary<K,V>, arrays, HashSet<T>, etc.): compared by reference. Two distinct instances with identical contents are treated as changed.

@* Default mode: a fresh List each render is a new reference, so the child DOES re-render. *@
<Memo Keys="@([new List<int> { 1, 2, 3 }])">
    <ChildComponent />
</Memo>

@* To freeze on content in default/shallow mode, reuse the same instance across renders: *@
<Memo Keys="@([_stableList])">
    <ChildComponent />
</Memo>

[!NOTE] Wrapping a collection in a record does not give you content comparison. A record's generated Equals compares each field with EqualityComparer<T>.Default, and for a List<T> field that is reference equality, so two records holding equal-content but distinct lists are still unequal. Use Deep="true" (or a type that implements structural Equals itself) when you need content comparison.

Deep structural equality (opt-in with Deep="true")

Set Deep="true" to compare key elements by deep value equality instead:

• Collections (arrays, lists, dictionaries, sets) are compared element-wise
• Dictionaries are compared by key/value pairs; sets compared unordered
• Nested collections compared recursively (up to a depth limit of 32, which falls back to always rendering if exceeded)
• Lazy enumerables (LINQ queries, yield return generators) are materialised when the snapshot is created, so the comparison captures their current values

@* Deep mode: equal-content lists compare equal, so the child does NOT re-render. *@
<Memo Keys="@([new List<int> { 1, 2, 3 }])" Deep="@true">
    <ChildComponent />
</Memo>

Deep is expected to be constant for a given <Memo> instance; changing it between renders is treated as a key change and forces a re-render.

Choosing a mode

Modes

• Default mode is the most performant option with a single O(n) pass over the keys, calling each element's object.Equals method. No reflection, no boxing beyond what the object? keys already carry, and the snapshot reuses its backing buffer across renders (references copied, nothing enumerated or materialised). Prefer this mode with stable references or primitive keys on hot render paths.
• Deep mode trades work for convenience, but is still highly performant. It walks collections element-wise, matches dictionaries by key and sets in an order-insensitive way, and recurses into nested enumerables/collections with a max depth of 32. Lazy enumerables are fully enumerated and allocated into arrays on snapshot so their values are stable (this is the main extra cost) so avoid Deep="true" with IEnumerable keys for very large or expensive-to-enumerate objects unless a simpler memo key for it cannot be derived. Primitive-element arrays and lists still take an internal Span fast-path.

Performance & Benchmarks

The repository includes a BenchmarkDotNet project (Components.Benchmarks) that drives the real Blazor render pipeline through a minimal renderer. The numbers below are illustrative (Apple M1 Max, .NET 10). Run them yourself with dotnet run -c Release --project Components.Benchmarks -- --filter "*".

Memoisation pays off in proportion to the work it skips. With stable keys, a <Memo> freezes its child so the child's render work never runs. Measuring a parent re-render where the child does a varying amount of work:

The wrapper adds negligible time overhead per-render (nanoseconds, within measurement noise), and allocates 0 bytes, with dictionaries being the sole exception due to boxing.

A trivial child has almost no work to skip, so the net performance gain is roughly zero; the benefit grows quickly as the wrapped subtree gets more expensive, however.

Shallow and deep mode perform almost identically with stable keys, because a frozen child skips its render work in either mode; the only difference between them is key-comparison cost, which is negligible for simple keys.

When keys change every render, <Memo> can't help. It pays for the key comparison and then re-renders anyway, costing roughly 2× the un-memoised baseline, so reserve <Memo> for subtrees that are actually stable most of the time.

The overhead of Deep="true" versus Deep="false" is paid on every render as <Memo> must compare the keys to decide whether to freeze the child, even when they're unchanged. That cost is negligible when rendering work is skipped, but it's pure overhead when keys change and rendering work happens anyway. The default per-key object.Equals comparison is effectively free either way.

The performance cost of deep comparison scales with the size and shape of the keys: primitive arrays and lists stay in the nanoseconds via a Span fast-path, records and sets grow linearly. Dictionaries are the most expensive case, but for a small set of keys and dictionaries with a small number of items, as is typically the case for parameters, the savings by eliminating work easily outweigh the performance overhead.

In short: prefer the default of Deep="false", and keep keys small when using Deep="true". Use <Memo> in a targeted fashion where it is of most benefit instead of applying it by default.

Requirements

• .NET 9.0 or .NET 10.0
• ASP.NET Core (Blazor Server, WebAssembly, or Auto)

License

MIT