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See the Async attribute in the examples folder included with boo.

Below is an updated version of this attribute that could be included with Boo.Lang.Useful:

#region license
// Copyright (c) 2005, Sorin Ionescu
// All rights reserved.
// 
// Redistribution and use in source and binary forms, with or without modification,
// are permitted provided that the following conditions are met:
// 
//     * Redistributions of source code must retain the above copyright notice,
//     this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above copyright notice,
//     this list of conditions and the following disclaimer in the documentation
//     and/or other materials provided with the distribution.
//     * Neither the name of Sorin Ionescu nor the names of its
//     contributors may be used to endorse or promote products derived from this
//     software without specific prior written permission.
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
// SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
// OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
// THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#endregion

namespace Boo.Lang.Useful.Attributes

import System;
import System.Runtime.Remoting.Messaging
import Boo.Lang.Compiler
import Boo.Lang.Compiler.Ast
import Boo.Lang.Compiler.Steps

class AsyncMethodAttribute(AbstractAstAttribute):
"""
Adds the asynchronous helpers Begin<Method> and 
End<Method> for a method.
"""	
	_method as Method
	_accessModifiers as TypeMemberModifiers
	_disposed as ReferenceExpression
	
	def constructor():
		pass
		
	def constructor(disposed as ReferenceExpression):
		_disposed = disposed
	
	override def Apply(node as Node):	
		assert node isa Method
		
		_method = node
		
		# Set the wrapper methods to the same accessibility
		# as the wrappedmethod.
		if _method.IsProtected and _method.IsInternal:
			_accessModifiers = TypeMemberModifiers.Protected
			_accessModifiers = _accessModifiers | TypeMemberModifiers.Internal
			
		elif _method.IsPublic:
			_accessModifiers = TypeMemberModifiers.Public
			
		elif _method.IsProtected:
			_accessModifiers = TypeMemberModifiers.Protected
			
		elif _method.IsInternal:
			_accessModifiers = TypeMemberModifiers.Internal
			
		elif _method.IsPrivate:
			_accessModifiers = TypeMemberModifiers.Private
		
		CreateBeginMethod()
		CreateEndMethod()
	
	private def CreateBeginMethod():
		beginMethod = Method(
			LexicalInfo: LexicalInfo,
			Name: "Begin${_method.Name}",
			Modifiers: _accessModifiers,
			ReturnType: SimpleTypeReference(LexicalInfo, "System.IAsyncResult"))
		
		# Copy the BeginInvoke parameters:
		beginMethod.Parameters.ExtendWithClones(_method.Parameters)
		beginMethod.Parameters.Add(ParameterDeclaration(
			LexicalInfo: LexicalInfo,
			"callback",
			SimpleTypeReference(LexicalInfo, "System.AsyncCallback")))
			
		beginMethod.Parameters.Add(
			ParameterDeclaration(
				LexicalInfo: LexicalInfo,
				"state",
				SimpleTypeReference(LexicalInfo, "object")))
							 
		# Creates the delegate that will call the wrapped method asynchronously.
		asyncInvocation = MethodInvocationExpression(
			LexicalInfo: LexicalInfo,
			Target: AstUtil.CreateReferenceExpression(
				"${_method.Name}.BeginInvoke"))
		
		for parameter in beginMethod.Parameters:
			asyncInvocation.Arguments.Add(
				ReferenceExpression(LexicalInfo, parameter.Name))
		
		if _disposed is not null:
			CreateDisposedObjectCheck(beginMethod)
			
		beginMethod.Body.Add(
			ReturnStatement(
				LexicalInfo: LexicalInfo,
				Expression: asyncInvocation))
		
		_method.DeclaringType.Members.Add(beginMethod)
		
	private def CreateEndMethod():
		endMethod = Method(
			LexicalInfo: LexicalInfo,
			Name: "End${_method.Name}",
			Modifiers: _accessModifiers)
		
		# Copy the EndInvoke parameters: EndMethod(result as IAsyncResult).
		endMethod.Parameters.Add(
			ParameterDeclaration(
				LexicalInfo: LexicalInfo,
				"result",
				SimpleTypeReference(LexicalInfo, "System.IAsyncResult")))
		
		# Creates the delegate that will call the wrapped method asynchronously.
		asyncInvocation = MethodInvocationExpression(
			LexicalInfo: LexicalInfo,
			Target: AstUtil.CreateReferenceExpression(
				"${_method.Name}.EndInvoke"))
		
		asyncInvocation.Arguments.Add(
			ReferenceExpression(LexicalInfo, "result"))
		
		if _disposed is not null:
			CreateDisposedObjectCheck(endMethod)
		
		endMethod.Body.Add(
			ReturnStatement(
				LexicalInfo: LexicalInfo,
				Expression: asyncInvocation))
		
		_method.DeclaringType.Members.Add(endMethod)
		
		# Cache the voidType reference because the CompilerContext
		# will be lost after this method returns.
		# See AbstractCompilerComponent.Dispose().
		voidType = Context.TypeSystemServices.VoidType
		
		Context.Parameters.Pipeline.AfterStep += def (
			sender as object,
			e as CompilerStepEventArgs):
				
			if e.Step isa ProcessMethodBodies:
				if _method.ReturnType.Entity is voidType:
					returnStatement = endMethod.Body.Statements[-1] as ReturnStatement
					
					endMethod.Body.Statements.Replace(
						returnStatement,
						ExpressionStatement(
							LexicalInfo: LexicalInfo,
							Expression: returnStatement.Expression))
							
	private def CreateDisposedObjectCheck(method as Method):
		exceptionCreation = MethodInvocationExpression(
			LexicalInfo: LexicalInfo,
			Target: AstUtil.CreateReferenceExpression(
				"System.ObjectDisposedException"))
		
		exceptionCreation.Arguments.Add(
			StringLiteralExpression(LexicalInfo, _method.DeclaringType.Name))
		
		trueBlock = Block()
		trueBlock.Add(
			RaiseStatement(
				LexicalInfo: LexicalInfo,
				Exception: exceptionCreation))
		
		method.Body.Add(
			IfStatement(LexicalInfo, _disposed.CloneNode(), trueBlock, null))

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