[API Proposal]: Add FieldInfo.CreateDelegate

[Neme12]

Background and motivation

MethodInfo.CreateDelegate is a really useful method to get a fast method invoke function for methods obtained by reflection. I miss the same method on FieldInfo. On FieldInfo, the delegate would have to return a ref to the field to be valid.

API Proposal

namespace System.Reflection;

// Current
// public abstract class MethodInfo : MethodBase
// {
// 	public virtual Delegate CreateDelegate(Type delegateType);
// 	public virtual Delegate CreateDelegate(Type delegateType, object? target);
// 	public T CreateDelegate<T>() where T : Delegate;
// 	public T CreateDelegate<T>(object? target) where T : Delegate;
// }

// Proposed
public abstract partial class FieldInfo : MemberInfo
{
	public virtual Delegate CreateDelegate(Type delegateType);
	public virtual Delegate CreateDelegate(Type delegateType, object? target);
	public T CreateDelegate<T>() where T : Delegate;
	public T CreateDelegate<T>(object? target) where T : Delegate;
}

API Usage

static class B
{
    private string _foo;
}

static class C
{
	private static FieldInfo _fieldInfo = typeof(B).GetField("_foo", BindingFlags.NonPublic | BindingFlags.Instance)!;
	private static FooDelegate _fieldAccessor = _fieldInfo.CreateDelegate<FooDelegate>();

	// Use _fieldAccessor repeatedly to access the field value (or set it to a new value)
	// without the overhead of reflection after the initial delegate creation.

	private delegate ref string FooDelegate(B b);
}

Alternative Designs

I don't know

Risks

I don't know any

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[tannergooding]

Why not just use UnsafeAccessor?

There's an example for both instance and static fields here: https://learn.microsoft.com/en-us/dotnet/api/system.runtime.compilerservices.unsafeaccessorattribute?view=net-10.0#examples

[Neme12]

@tannergooding I do use that whenever I can. But sometimes you don't know statically what the field is.

[huoyaoyuan]

MethodBase.CreateDelegate can be implement because there is a method body for the delegate. For field, there's no dedicated code body for accessing the field, but usually inserted into the calling method.

[Neme12]

@huoyaoyuan But it can be generated, either via expression trees or Reflection.Emit. I have my own working implementation for this.

[tannergooding]

Things working with NAOT is an important consideration, especially for new API surface.

There are ways to make even this proposal work there, but it also gets complicated and I'm not sure its worth the complexity. There are a number of APIs to build your own helper here if you are truly in a dynamic scenario and don't want to use source generators and don't care to support AOT.

[MichalPetryka]

#94976 and #28001 together would be a more AOT friendly alternative here.