异步委托
异步委托提供以异步方式调用同步方法的能力。当同步调用一个委托时，调用方法直接对当前线程调用目标方法。如果编译器支持异步委托，则它将生成该调用方法以及 BeginInvoke 和 EndInvoke 方法。如果调用 BeginInvoke 方法，则公共语言运行库将对请求进行排队并立即返回到调用方。将对来自线程池的线程调用该目标方法。提交请求的原始线程自由地继续与目标方法并行执行，该目标方法是对线程池线程运行的。如果已经对 BeginInvoke 指定了回调，当目标方法返回时将调用它。在回调中，使用 EndInvoke 方法来获取返回值和输入/输出参数。如果没有对 BeginInvoke 指定回调，则可以在提交请求的原始线程上使用 EndInvoke。
使用委托
对于异步编程，如果调用方使用一个委托，则该调用方在调用一个方法时必须定义该委托。在以下代码示例中，首先定义该委托，接着创建该委托的实例，然后调用它。下面的示例说明为异步调用 Factorize 方法定义一个模式的调用方：
using System;
using System.Runtime.Remoting;
public delegate bool FactorizingCallback(
int factorizableNum,
ref int primefactor1,
ref int primefactor2);
// This is a class that receives a callback when the the results are available.
public class ProcessFactorizedNumber
{
private int _ulNumber;
public ProcessFactorizedNumber(int number)
{
_ulNumber = number;
}
// Note the qualifier one-way.
[OneWayAttribute()]
public void FactorizedResults(IAsyncResult ar)
{
int factor1=0, factor2=0;
// Extract the delegate from the AsyncResult.
FactorizingCallback fd =
(FactorizingCallback) ((AsyncResult)ar).AsyncDelegate;
// Obtain the result.
fd.EndInvoke(ref factor1, ref factor2, ar);
// Output results.
Console.WriteLine("On CallBack: Factors of {0} : {1} {2}",
_ulNumber, factor1, factor2);
}
}
Asynchronous Variation 1 – call
// The Asynchronous Variation 1 call, calls
// the ProcessFactorizedNumber.FactorizedResults callback
// when the call completes.
public void FactorizeNumber1()
{
// The following is the Client code.
PrimeFactorizer pf = new PrimeFactorizer();
FactorizingCallback fd = new FactorizingCallback(pf.Factorize);
// Asynchronous Variation 1
int factorizableNum = 1000589023, temp=0;
// Create an instance of the class which is going
// to be called when the call completes.
ProcessFactorizedNumber fc = new ProcessFactorizedNumber(factorizableNum);
// Define the AsyncCallback delegate.
AsyncCallback cb = new AsyncCallback(fc.FactorizedResults);
// You can stuff any object as the state object.
Object state = new Object();
// Asynchronously invoke the Factorize method on pf.
IAsyncResult ar = fd.BeginInvoke(
factorizableNum,
ref temp,
ref temp,
cb,
state);
//
// Do some other useful work.
//. . .
}
Asynchronous Variation 2
// Asynchronous Variation 2
// Waits for the result.
public void FactorizeNumber2()
{
// The following is the Client code.
PrimeFactorizer pf = new PrimeFactorizer();
FactorizingCallback fd = new FactorizingCallback(pf.Factorize);
// Asynchronous Variation 1
int factorizableNum = 1000589023, temp=0;
// Create an instance of the class which is going
// to called when the call completes.
ProcessFactorizedNumber fc = new ProcessFactorizedNumber(factorizableNum);
// Define the AsyncCallback delegate.
AsyncCallback cb =
new AsyncCallback(fc.FactorizedResults);
// Can stuff any object as the state object.
Object state = new Object();
// Asynchronously invoke the Factorize method on pf.
IAsyncResult ar = fd.BeginInvoke(
factorizableNum,
ref temp,
ref temp,
null,
null);
ar.AsyncWaitHandle.WaitOne(10000, false);
if (ar.IsCompleted)
{
int factor1=0, factor2=0;
// Obtain the result.
fd.EndInvoke(ref factor1, ref factor2, ar);
// Output results.
Console.WriteLine("Sequencial : Factors of {0} : {1} {2}",
factorizableNum, factor1, factor2);
}
}
编译器和公共语言运行库支持
当编译器发出 FactorizingCallback 委托类时（在按如下所示分析其定义后），它将使用异步方法签名以及 Invoke 方法生成 BeginInvoke 和 EndInvoke 方法，如以下代码中所示：
public class FactorizingCallback : delegate
{
public bool Invoke(
int factorizableNum,
ref int primefactor1,
ref int primefactor2);
// The following code was supplied by the compiler.
public IAsyncResult BeginInvoke(
int factorizableNum,
ref int primefactor1,
ref int primefactor2,
AsyncCallback cb,
Object AsyncState
);
// The following code was supplied by the compiler.
public bool EndInvoke(
ref int primefactor1,
ref int primefactor2,
IAsyncResult ar);
}
编译器提供的委托 BeginInvoke 和 EndInvoke 方法
使用用户指定的委托签名，编译器应发出具有 Invoke、BeginInvoke 和 EndInvoke 方法的委托类。BeginInvoke 和 EndInvoke 方法应被修饰为本机的。因为这些方法被标记为本机的，所以公共语言运行库在类加载时自动提供该实现。加载程序确保它们未被重写。
异步委托编程示例
以下代码通过求解某些数字因子的简单类阐释如何使用 .NET 异步编程。
using System;
using System.Threading;
using System.Runtime.Remoting;
// Create an asynchronous delegate.
public delegate bool FactorizingCallback(
int factorizableNum,
ref int primefactor1,
ref int primefactor2);
// Create a class the factorizers the number.
public class PrimeFactorizer.
{
public bool Factorize(
int factorizableNum,
ref int primefactor1,
ref int primefactor2)
{
primefactor1 = 1;
primefactor2 = factorizableNum;
// Factorize using a low-tech approach.
for (int i=2;i<factorizableNum;i++)
{
if (0 == (factorizableNum % i))
{
primefactor1 = i;
primefactor2 = factorizableNum / i;
break;
}
}
if (1 == primefactor1 )
return false;
else
return true ;
}
}
// Class that receives a callback when the the results are available.
public class ProcessFactorizedNumber
{
private int _ulNumber;
public ProcessFactorizedNumber(int number)
{
_ulNumber = number;
}
// Note the qualifier is one-way.
[OneWayAttribute()]
public void FactorizedResults(IAsyncResult ar)
{
int factor1=0, factor2=0;
// Extract the delegate from the AsyncResult.
FactorizingCallback fd = (FactorizingCallback)((AsyncResult)ar).AsyncDelegate;
// Obtain the result.
fd.EndInvoke(ref factor1, ref factor2, ar);
// Output results.
Console.WriteLine("On CallBack: Factors of {0} : {1} {2}",
_ulNumber, factor1, factor2);
}
}
// Class that shows variations of using Asynchronous
public class Simple.
{
// The following demonstrates the Asynchronous Pattern using a callback.
public void FactorizeNumber1()
{
// The following is the client code.
PrimeFactorizer pf = new PrimeFactorizer();
FactorizingCallback fd = new FactorizingCallback(pf.Factorize);
int factorizableNum = 1000589023, temp=0;
// Create an instance of the class which is going
// to be called when the call completes.
ProcessFactorizedNumber fc = new ProcessFactorizedNumber(factorizableNum);
// Define the AsyncCallback delegate.
AsyncCallback cb = new AsyncCallback(fc.FactorizedResults);
// You can stuff any object as the state object
Object state = new Object();
// Asynchronously invoke the Factorize method on pf.
IAsyncResult ar = fd.BeginInvoke(
factorizableNum,
ref temp,
ref temp,
cb,
state);
//
// Do some other useful work.
//. . .
}
// The following demonstrates the Asynchronous Pattern using a BeginInvoke, followed by waiting with a timeout.
public void FactorizeNumber2()
{
// The following is the client code.
PrimeFactorizer pf = new PrimeFactorizer();
FactorizingCallback fd = new FactorizingCallback(pf.Factorize);
int factorizableNum = 1000589023, temp=0;
// Create an instance of the class which is going
// to be called when the call completes.
ProcessFactorizedNumber fc = new ProcessFactorizedNumber(factorizableNum);
// Define the AsyncCallback delegate.
AsyncCallback cb =
new AsyncCallback(fc.FactorizedResults);
// You can stuff any object as the state object.
Object state = new Object();
// Asynchronously invoke the Factorize method on pf.
IAsyncResult ar = fd.BeginInvoke(
factorizableNum,
ref temp,
ref temp,
null,
null);
ar.AsyncWaitHandle.WaitOne(10000, false);
if (ar.IsCompleted)
{
int factor1=0, factor2=0;
// Obtain the result.
fd.EndInvoke(ref factor1, ref factor2, ar);
// Output results.
Console.WriteLine("Sequencial : Factors of {0} : {1} {2}",
factorizableNum, factor1, factor2);
}
}
public static void Main(String[] args)
{
Simple simple = new Simple();
simple.FactorizeNumber1();
simple.FactorizeNumber2();
}
}
总结
上面是VS.NET中.NET异步编程方面的基本概念和示例代码，整理出来给大家参考一下。有任何建议请MAIL我 paulni@citiz.net。