Async/Await support for .NET 4.0

async.cs

// Copyright (c) 2012 Daniel Grunwald
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy of this
// software and associated documentation files (the "Software"), to deal in the Software
// without restriction, including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
// to whom the Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Security;
using System.Threading.Tasks;

// This is a minimal implementation of the stuff needed by the C# 5 Beta
// compiler for 'async'/'await' feature.
// Simply add this file to a .NET 4.0 project, and 'async'/'await' will work
// in your code without requiring your users to install .NET 4.5.
// However, you still need the C# 5 compiler for compiling your code.
// Caveats:
//  - Stack trace is lost when rethrowing exceptions
//  - Unhandled Exceptions in 'async void' methods are re-thrown immediately
//    and not posted to the synchronization context
//  - Flowing ExecutionContext is not supported
//  --> do not use this in security critical code (APTCA)
//  - The code is not optimized. Tasks are always created, repeated boxing may occur.

namespace System.Threading.Tasks
{
	internal static class TaskEx
	{
		public static TaskAwaiter GetAwaiter(this Task task)
		{
			return new TaskAwaiter(task);
		}
		
		public static TaskAwaiter<T> GetAwaiter<T>(this Task<T> task)
		{
			return new TaskAwaiter<T>(task);
		}
	}
	
	internal struct TaskAwaiter : INotifyCompletion
	{
		readonly Task task;
		
		internal TaskAwaiter(Task task)
		{
			this.task = task;
		}
		
		internal static TaskScheduler TaskScheduler {
			get {
				if (SynchronizationContext.Current == null)
					return TaskScheduler.Default;
				else
					return TaskScheduler.FromCurrentSynchronizationContext();
			}
		}
		
		public bool IsCompleted {
			get { return task.IsCompleted; }
		}
		
		public void OnCompleted(Action continuation)
		{
			this.task.ContinueWith(
				delegate (Task task) {
					continuation();
				}, TaskAwaiter.TaskScheduler);
		}
		
		public void GetResult()
		{
			try {
				task.Wait();
			} catch (AggregateException ex) {
				throw ex.InnerExceptions[0];
			}
		}
	}
	
	internal struct TaskAwaiter<T> : INotifyCompletion
	{
		readonly Task<T> task;
		
		internal TaskAwaiter(Task<T> task)
		{
			this.task = task;
		}
		
		public bool IsCompleted {
			get { return task.IsCompleted; }
		}
		
		public void OnCompleted(Action continuation)
		{
			this.task.ContinueWith(
				delegate (Task<T> task) {
					continuation();
				}, TaskAwaiter.TaskScheduler);
		}
		
		public T GetResult()
		{
			try {
				return task.Result;
			} catch (AggregateException ex) {
				throw ex.InnerExceptions[0];
			}
		}
	}
}

namespace System.Runtime.CompilerServices
{
	internal interface INotifyCompletion
	{
		void OnCompleted(Action continuation);
	}
	
	internal interface ICriticalNotifyCompletion : INotifyCompletion
	{
		[SecurityCritical]
		void UnsafeOnCompleted(Action continuation);
	}
	
	internal interface IAsyncStateMachine
	{
		void MoveNext();
		void SetStateMachine(IAsyncStateMachine stateMachine);
	}
	
	internal struct AsyncVoidMethodBuilder
	{
		public static AsyncVoidMethodBuilder Create()
		{
			return new AsyncVoidMethodBuilder();
		}
		
		public void SetException(Exception exception)
		{
			throw exception;
		}
		
		public void SetResult()
		{
		}
		
		public void SetStateMachine(IAsyncStateMachine stateMachine)
		{
			// Should not get called as we don't implement the optimization that this method is used for.
			throw new NotImplementedException();
		}
		
		public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
		{
			stateMachine.MoveNext();
		}
		
		public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : INotifyCompletion where TStateMachine : IAsyncStateMachine
		{
			awaiter.OnCompleted(stateMachine.MoveNext);
		}
		
		public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine
		{
			awaiter.OnCompleted(stateMachine.MoveNext);
		}
	}
	
	internal struct AsyncTaskMethodBuilder
	{
		TaskCompletionSource<object> tcs;
		
		public Task Task { get { return tcs.Task; } }
		
		public static AsyncTaskMethodBuilder Create()
		{
			AsyncTaskMethodBuilder b;
			b.tcs = new TaskCompletionSource<object>();
			return b;
		}
		
		public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
		{
			stateMachine.MoveNext();
		}
		
		public void SetStateMachine(IAsyncStateMachine stateMachine)
		{
			// Should not get called as we don't implement the optimization that this method is used for.
			throw new NotImplementedException();
		}
		
		public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : INotifyCompletion where TStateMachine : IAsyncStateMachine
		{
			awaiter.OnCompleted(stateMachine.MoveNext);
		}
		
		public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine
		{
			awaiter.OnCompleted(stateMachine.MoveNext);
		}
		
		public void SetResult()
		{
			tcs.SetResult(null);
		}
		
		public void SetException(Exception exception)
		{
			tcs.SetException(exception);
		}
	}
	
	internal struct AsyncTaskMethodBuilder<T>
	{
		TaskCompletionSource<T> tcs;
		
		public Task<T> Task { get { return tcs.Task; } }
		
		public static AsyncTaskMethodBuilder<T> Create()
		{
			AsyncTaskMethodBuilder<T> b;
			b.tcs = new TaskCompletionSource<T>();
			return b;
		}
		
		public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
		{
			stateMachine.MoveNext();
		}
		
		public void SetStateMachine(IAsyncStateMachine stateMachine)
		{
			// Should not get called as we don't implement the optimization that this method is used for.
			throw new NotImplementedException();
		}
		
		public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : INotifyCompletion where TStateMachine : IAsyncStateMachine
		{
			awaiter.OnCompleted(stateMachine.MoveNext);
		}
		
		public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine
		{
			AwaitOnCompleted(ref awaiter, ref stateMachine);
		}
		
		public void SetResult(T result)
		{
			tcs.SetResult(result);
		}
		
		public void SetException(Exception exception)
		{
			tcs.SetException(exception);
		}
	}
}

Will this work in net 3.5 projects too?

No, but take a look at AsyncBridge.