API proposal: Activator.CreateFactory and ConstructorInfo.CreateDelegate

[GrabYourPitchforks]

(See also #32520.)

API proposal

namespace System
{
    public static class Activator
    {
      // EXISTING APIs (abridged):
      public static T CreateInstance<T>();
      public static object? CreateInstance(Type type);
      public static object? CreateInstance(Type type, bool nonPublic);

      // NEW PROPOSED APIs:
      public static Func<T> CreateFactory<T>(); // no new() constraint, matches existing API
      public static Func<object?> CreateFactory(Type type);
      public static Func<object?> CreateFactory(Type type, bool nonPublic);
    }
}

// STRETCH GOAL APIs (see end of proposal):
namespace System.Reflection
{
    public class ConstructorInfo
    {
      public static TDelegate CreateDelegate<TDelegate>();
      public static Delegate CreateDelegate(Type delegateType);
    }
}

// REMOVED FROM PROPOSAL
/*
namespace System.Runtime.CompilerServices
{
    public static class RuntimeHelpers
    {
        // EXISTING API:
        public static object GetUninitializedObject(Type type);

        // NEW PROPOSED API:
        public static Func<object> GetUninitializedObjectFactory(Type type);
    }
}
*/

Discussion

Instantiating objects whose types are not known until runtime is fairly common practice among frameworks and libraries. Deserializers perform this task frequently. Web frameworks and DI frameworks might create per-request instances of objects, then destroy those objects at the end of the request.

APIs like Activator.CreateInstance and the System.Reflection surface area can help with this to a large extent. However, those are sometimes seen as heavyweight solutions. We've built some caching mechanisms into the framework to suppose these use cases. But it's still fairly common for high-performance frameworks to bypass Activator and the reflection stack and to go straight to manual codegen. See below for some examples.

See below for some concrete samples.

System.Text.Json

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/ReflectionEmitMemberAccessor.cs

Lines 39 to 51 in 9d85b82

	if (realMethod == null)
	{
	LocalBuilder local = generator.DeclareLocal(type);
	generator.Emit(OpCodes.Ldloca_S, local);
	generator.Emit(OpCodes.Initobj, type);
	generator.Emit(OpCodes.Ldloc, local);
	generator.Emit(OpCodes.Box, type);
	}
	else
	{
	generator.Emit(OpCodes.Newobj, realMethod);
	}

ASP.NET Core

dotnet/aspnetcore#14615 (though they're using TypeBuilder to work around this right now)

Other runtime + libraries

runtime/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/MetadataViewGenerator.cs

Lines 371 to 380 in 9d85b82

	// Implemet the static factory
	// public object Create(IDictionary<string, object>)
	// {
	// return new <ProxyClass>(dictionary);
	// }
	MethodBuilder factoryMethodBuilder = proxyTypeBuilder.DefineMethod(MetadataViewGenerator.MetadataViewFactoryName, MethodAttributes.Public | MethodAttributes.Static, typeof(object), CtorArgumentTypes);
	ILGenerator factoryIL = factoryMethodBuilder.GetILGenerator();
	factoryIL.Emit(OpCodes.Ldarg_0);
	factoryIL.Emit(OpCodes.Newobj, proxyCtor);
	factoryIL.Emit(OpCodes.Ret);

Ref emit incurs a substantial upfront perf hit, but it does generally provide a win amortized over the lifetime of the cached method as it's invoked over and over again. However, this comes with its own set of problems. It's difficult for developers to get the exact IL correct across the myriad edge cases that might exist. It's not very memory-efficient. And as runtimes that don't allow codegen become more commonplace, it complicates the callers' code to have to decide the best course of action to take for any given runtime.

These proposed APIs attempt to solve the problem of creating a basic object factory using the best mechanism applicable to the current runtime. The exact mechanism used can vary based on runtime: perhaps it's codegen, perhaps it's reflection, perhaps it's something else. But the idea is that the performance of these APIs should rival the best hand-rolled implementations that library authors can create.

Shortcomings, not solved here

This API is not a panacea to address all performance concerns developers have with the reflection stack. For example, this won't change the perf characteristics of MethodInfo.Invoke. But it could be used to speed up the existing Activator.CreateInstance<T> APIs and to make other targeted improvements. In general, this API provides an alternative pattern that developers can use so that they don't have to roll solutions themselves.

It also does not fully address the concern of calls to parameterized ctors, such as you might find in DI systems. The API RuntimeHelpers.GetUninitializedObjectFactory does help with this to some extent. The caller can cache that factory to instantiate "blank" objects quickly, then use whatever existing fast mechanism they wish to call the object's real ctor over the newly allocated instance.

I hope to have a better solution for this specific scenario in a future issue.

Stretch goal APIs

The APIs ConstructorInfo.CreateDelegate<TDelegate>() and friends are meant to allow invoking parameterless or parameterful ctors. These are primarily useful when the objects you're constructing have a common signature in their constructors, but you don't know the actual type of the object at compile time.

Consider:

public class MyService
{
    public MyService(IServiceProvider serviceProvider) { }
}

public class MyOtherService
{
    public MyOtherService(IServiceProvider serviceProvider) { }
}

In these cases, the caller would get the ConstructorInfo they care about, then call constructorInfo.CreateDelegate<Func<IServiceProvider, object>>(). Depending on which ConstructorInfo was provided, the returned delegate will create either a MyService or a MyOtherService, calling the appropriate ctor with the caller-provided IServiceProvider.

I say "stretch goal" because the parameter shuffling involved here would involve spinning up the JIT. We can keep this overhead to a minimum because within the runtime we can take shortcuts that non-runtime components can't take with the typical DynamicMethod-based way of doing things. So while there's less JIT cost compared to a standard DynamicMethod-based solution, there's still some JIT cost, so it doesn't fully eliminate startup overhead. (This overhead isn't much different than the overhead the runtime already incurs when it sees code like Func<string, bool> func = string.IsNullOrEmpty;, since the runtime already needs to create a parameter shuffling thunk for this scenario.)

[davidfowl]

Related

runtime/src/libraries/Common/src/Extensions/ActivatorUtilities/ActivatorUtilities.cs

Line 106 in 23e13a3

public static ObjectFactory CreateFactory(Type instanceType, Type[] argumentTypes)

. DI enabled, find me a constructor to create a factory for.

Here's the ObjectFactory definition

runtime/src/libraries/Common/src/Extensions/ActivatorUtilities/ObjectFactory.cs

Line 25 in e3ffd34

delegate object ObjectFactory(IServiceProvider serviceProvider, object[] arguments);

.

Example usage:
https://github.com/dotnet/aspnetcore/blob/04c3192d3cec396a9f2343fb5407d6777349b9df/src/SignalR/server/Core/src/Internal/DefaultHubActivator.cs#L13

[GrabYourPitchforks]

Yeah, that's the part I'm having trouble with. What you're talking about is the more general case of fast method invocation. I'm blanking right now on a good, reliable, high-performance way to do that without involving codegen. Right now MethodInfo.Invoke is the best tool to suit that need due to how extremely flexible it is. But of course this flexibility comes at a price, and many callers don't want to pay the price for the flexibility they don't need. Aspnet is a good case study on this. :)

Let's assume for the moment that you had a good way of invoking arbitrary instance methods in a high-performance fashion. The APIs proposed here would allow you to extend that same concept to constructors of arbitrary signature. To do this, you'd use the proposed RuntimeHelpers.GetUninitializedObjectFactory API to get a zero-inited instance of the target object, where no ctors have been run. Then you'd invoke the ctor as you would any other instance method.

Edit: As an example of what I said above, assume that the pattern you wanted to support instantiating was .ctor(IServiceProvider). To do that on top of this API, your factory helper would look like the below.

public class MyFactory
{
    private Func<object> _allocator;
    private Action<object, IServiceProvider> _ctor;

    public MyFactory(Type targetType)
    {
        if (targetType.IsValueType) { /* throw */ }
        _allocator = RuntimeHelpers.GetUninitializedObjectFactory(targetType);
        MethodInfo mi = targetType.GetMethod(".ctor", new Type[] { typeof(IServiceProvider) });
        _ctor = (Action<object, IServiceProvider>)mi.CreateDelegate(typeof(Action<object, IServiceProvider>), null /* instance */);
    }

    public object CreateInstance(IServiceProvider serviceProvider)
    {
        object instance = _allocator(); // these two calls together are basically 'newobj'
        _ctor(instance, serviceProvider);
        return instance;
    }
}

In this scenario there's zero codegen involved. If you wanted to support value types it gets a bit more complicated. Maybe that's something we could help with here.

[stephentoub]

public static Func<T> CreateFactory<T>();

I'm trying to understand the benefit of this over Activator.CreateInstance<T>(). If the above yields significant improvements, why wouldn't Activator.CreateInstance<T>() be changed to the equivalent of:

private static class Cache<T> { public static readonly Func<T> Func = CreateFactory<T>(); }
public static T CreateInstance<T>() => Cache<T>.Func();

?

I'm sure I'm just missing something.

[GrabYourPitchforks]

I do plan on adding some of the existing Activator code paths to take advantage of this. But there's still considerable indirection while the activator tries to look up the appropriate factory method. In my experimentation this indirection alone was twice the cost of instantiating the target object.

For applications which maintain their own caching mechanisms we can return to them a factory which bypasses all of the built in indirection. This is the most desirable from the perspective of giving them bare metal performance so that there's no need for them to use their own codegen.

Edit: activator doesn't even initialize the cache until the second call to the constructor. The reason is that we want to optimize for the case where the type is instantiated only once, perhaps as part of app startup.

[jkotas]

CreateFactory(Type type, bool nonPublic)

Do we also need an overload without nonPublic argument?

(Also, we need to be thinking about the linker annotations when introducing new reflection APIs.)

I'm trying to understand the benefit of this over Activator.CreateInstance()

For the generic overload, the fundamental advantage of the factory method compared to Activator.CreateInstance<T> is that Activator.CreateInstance<T> wraps exceptions in TargetInvocationException that adds an extra frame with EH. The factory method avoids this overhead. To get rid of this overhead, we would need to do a breaking change to stop wrapping exceptions in TargetInvocationException.

For the non-generic overload, the additional fundamental advantage of the factory method compared to Activator.CreateInstance are the indirections from the caching policy. The factory method avoids this overhead by moving the caching policy to the application. To get rid of this overhead, we would need to do a non-pay-for-play caching policy that would make all System.RuntimeType instances more expensive.

[GrabYourPitchforks]

Do we also need an overload without nonPublic argument?

We probably should, for convenience and to help lessen confusion.

Also, we need to be thinking about the linker annotations when introducing new reflection APIs.

Do you have pointers on this topic? I don't see any annotations on the existing Activator.CreateInstance API, so I guess the linker itself is aware of the Activator APIs and special-cases calls to them?

[jkotas]

Do you have pointers on this topic?

#36229 . It is work in progress. These annotations were discussed during the API review meetings a few weeks ago.

[stephentoub]

Activator.CreateInstance wraps exceptions in TargetInvocationException that adds an extra frame with EH. The factory method avoids this overhead

I see. It's unfortunate we pay that overhead, in particular in the : new() constraint case, where the developer is just writing new T() and so a TargetInvocationException is even more unexpected. Would we recommend the C# compiler change its code gen to use the new APIs? Presumably not, for the same breaking change concern?

[jkotas]

Presumably not, for the same breaking change concern?

Right. Second reason for not recommending this API to C# compiler is that Roslyn would need to generate a boilerplate to cache the delegate.

The factory method adds an extra frame as well compared to just calling new MyType(). It does not have EH, but it is still an extra frame. And the factory method pays an extra overhead for delegate invocation. I guess that the generic factory method may have no performance advantage compared to optimized Activator.CreateInstance<T>.

[GrabYourPitchforks]

Using Activator.CreateInstance(Type, ...) is slightly faster than Activator.CreateInstance<T>() for reference types, but I think the strongly typed overload is still worth the convenience.

Activator.CreateInstance<T>() (where T is a value type with an explicit ctor) allocates then unboxes. Activator.CreateFactory<T>() is allocation free for this scenario. But we might be able to perform whatever contortions we need to behind the scenes to make CreateInstance allocation free here as well.

[stephentoub]

Using Activator.CreateInstance(Type, ...) is slightly faster than Activator.CreateInstance<T>() for reference types

Only in specific situations, such as using the generic overload from another generic context? Otherwise I don't see that.. I just tried this:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    [Benchmark] public Program A1() => Activator.CreateInstance<Program>();
    [Benchmark] public object A2() => Activator.CreateInstance(typeof(Program));
    [Benchmark] public Program A3() => (Program)Activator.CreateInstance(typeof(Program));
}

and got this:

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
A1	28.89 ns	0.246 ns	0.205 ns	0.0038	-	-	24 B
A2	30.03 ns	0.512 ns	0.479 ns	0.0038	-	-	24 B
A3	30.17 ns	0.257 ns	0.241 ns	0.0038	-	-	24 B

Activator.CreateInstance<T>() (where T is a value type with an explicit ctor) allocates then unboxes.

Since you can't currently write such a T in C#, I'm not very concerned about optimizing for that case. It also seems like something we should be able to fix if it was really meaningful.

[benaadams]

Using Activator.CreateInstance(Type, ...) is slightly faster than Activator.CreateInstance<T>() for reference types, but I think the strongly typed overload is still worth the convenience.

In practice should the comparison be with (T)Activator.CreateInstance(typeof(T), ...); since you would likely immediately cast object to something usable