Add contravariance to RenderFragment<TValue>

[Copilot]

Add contravariance to RenderFragment

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Mark TValue as contravariant in RenderFragment

Description

Marking TValue as contravariant with the in modifier enables passing render fragments that accept base types where derived types are expected, eliminating complex reflection-based adapters in generic component composition.

Core Change:

// Before
public delegate RenderFragment RenderFragment<TValue>(TValue value);

// After
public delegate RenderFragment RenderFragment<in TValue>(TValue value);

Enables:

// Non-generic fragment handling base type
RenderFragment<IList<Product>> baseTemplate = (IList<Product> items) => builder => { /* ... */ };

// Can now be assigned where specific type is expected
RenderFragment<List<Product>> specificTemplate = baseTemplate; // ✅ Works with contravariance

// DynamicComponent scenario now works directly
var parameters = new Dictionary<string, object>
{
    ["ItemsTemplate"] = baseTemplate, // ✅ No adapter needed
};
<DynamicComponent Type="typeof(Pager<Product>)" Parameters="parameters" />

Testing:

• 7 new tests covering contravariance scenarios (base class, interface, method parameters, DynamicComponent use cases)
• All 1,191 existing tests pass unchanged
• Backward compatible (contravariance relaxes type restrictions)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>RenderFragment contravariance on TValue</issue_title>
<issue_description>## Background and Motivation

RenderFragment is currently invariant, which makes passing render fragments that accept a base class awkward when composing generic components. Marking TValue as contravariant enables passing a fragment that handles a base type to places expecting a fragment for a derived type (or a nongeneric to a generic wrapper) without manual adapters.

@code {
	// Non-generic fragment that renders from the base list type
	RenderFragment<IList> ItemsTemplate = (IList models) => @<div>
		@foreach (var item in models.Items) { <span>@item.ToString()</span> }
	</div>;

	// We dynamically pick a T at runtime (e.g., via reflection/type factory)
	var itemType = actualType; // e.g., typeof(Product)
	var componentType = typeof(Pager<>).MakeGenericType(itemType);

	// But we can’t assign Found where RenderFragment<List<T>> is expected
	var parameters = new Dictionary<string, object?>
	{
		["ItemsTemplate"] = ItemsTemplate, // ❌ invariant delegate blocks this
	};

	<DynamicComponent Type="@componentType" Parameters="parameters" />
}

Proposed API

Add contravariance to TValue of RenderFragment.

public delegate RenderFragment RenderFragment<in TValue>(TValue value);

Usage Examples

The above mentioned code would run without exceptions.

Alternative Designs

The alternative is to make this adapter

  // Adapter: IList -> List<T> signature
static object CreateTypedTemplate(Type itemType, RenderFragment<IList> baseTemplate)
{
	// Make a delegate: RenderFragment<List<T>> (List<T> list) => baseTemplate(list)
	var listType = typeof(List<>).MakeGenericType(itemType);

	// Build a strongly-typed lambda via reflection
	var param = System.Linq.Expressions.Expression.Parameter(listType, "list");

	// Convert List<T> to IList for the base template call
	var convert = System.Linq.Expressions.Expression.Convert(param, typeof(System.Collections.IList));

	// baseTemplate(converted)
	var invoke = System.Linq.Expressions.Expression.Invoke(
	  System.Linq.Expressions.Expression.Constant(baseTemplate),
	  convert
	);

	var lambdaType = typeof(RenderFragment<>).MakeGenericType(listType);
	var lambda = System.Linq.Expressions.Expression.Lambda(lambdaType, invoke, param);

	return lambda.Compile();
}

Risks

Breaking assumed invariance via reflection in code/libraries that assume it.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes RenderFragment<TValue> contravariance on TValue #63961

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[dotnet-policy-service]

Greetings! You've submitted a PR that modifies code that is shared with https://github.com/dotnet/runtime . Please make sure you synchronize this code with the changes in that repo!

[javiercn]

Tests are only for demonstration purposes. We will remove them as this is just a compiler restriction that we are relaxing

[ilonatommy]

Isn't this public API change? Unshipped file does not need an update? With *REMOVED* etc

[javiercn]

Yes, but the analyzer might not pick this up. To be clear, it's a public API change, but I don't believe this is breaking.

[Copilot]

Pull request overview

This pull request adds contravariance to the RenderFragment<TValue> delegate by marking the TValue type parameter with the in modifier. This enables passing render fragments that accept base types where derived types are expected, simplifying generic component composition and eliminating the need for complex reflection-based adapters when working with dynamic components.

Key Changes

• Modified RenderFragment<TValue> delegate signature to make TValue contravariant
• Added 7 comprehensive unit tests covering various contravariance scenarios (base classes, interfaces, method parameters, DynamicComponent usage)
• Tests validate both the core functionality and the specific issue scenario described in the GitHub issue

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Components/Components/src/RenderFragment.cs	Added in modifier to TValue type parameter, making the delegate contravariant
src/Components/Components/test/RenderFragmentContravarianceTest.cs	Added 4 tests covering contravariance with base classes, interfaces, method parameters, and object type
src/Components/Components/test/RenderFragmentIssueScenarioTest.cs	Added 3 tests validating the exact GitHub issue scenario with DynamicComponent and demonstrating elimination of adapter code

[Copilot]

The XML documentation for the RenderFragment delegate should be updated to mention that TValue is contravariant. This is important for API documentation because contravariance affects how developers can use the delegate. Consider adding a remark or updating the typeparam description to note that TValue is contravariant, allowing render fragments that accept base types to be used where derived types are expected.

Suggested change

	/// <typeparam name="TValue">The type of object.</typeparam>
	/// <param name="value">The value used to build the content.</param>
	/// <typeparam name="TValue">
	/// The type of object. This type parameter is contravariant, so a <see cref="RenderFragment{TValue}"/>
	/// that accepts a base type can be used where a fragment for a derived type is expected.
	/// </typeparam>
	/// <param name="value">The value used to build the content.</param>
	/// <remarks>
	/// Because <typeparamref name="TValue"/> is contravariant, you can, for example, use a
	/// <see cref="RenderFragment{TValue}"/> declared as <c>RenderFragment&lt;object&gt;</c> in place of
	/// a <c>RenderFragment&lt;string&gt;</c>, since <c>string</c> derives from <c>object</c>.
	/// </remarks>