WaitCallback 委托
表示线程池线程要执行的回调方法。
**命名空间:**System.Threading
**程序集:**mscorlib（在 mscorlib.dll 中）
语法
声明
<ComVisibleAttribute(True)> _
Public Delegate Sub WaitCallback ( _
state As Object _
)
用法
Dim instance As New WaitCallback(AddressOf HandlerMethod)
[ComVisibleAttribute(true)]
public delegate void WaitCallback (
Object state
)
[ComVisibleAttribute(true)]
public delegate void WaitCallback (
Object^ state
)
/** @delegate */
/** @attribute ComVisibleAttribute(true) */
public delegate void WaitCallback (
Object state
)
JScript 支持使用委托，但不支持进行新的声明。
参数
- state
包含回调方法要使用的信息的对象。
备注
WaitCallback 表示要在 ThreadPool 线程上执行的回调方法。创建委托，方法是将回调方法传递给 WaitCallback 构造函数。您的方法必须具有此处所显示的签名。
通过将 WaitCallback 委托传递给 ThreadPool.QueueUserWorkItem 来将任务排入队列以便执行。您的回调方法将在某个线程池线程可用时执行。
提示
Visual Basic 用户可以省略 WaitCallback 构造函数，在向 QueueUserWorkItem 传递回调方法时只需使用 AddressOf 运算符即可。Visual Basic 将自动调用正确的委托构造函数。
如果要将信息传递给回调方法，请创建包含所需信息的对象，并在将任务排入队列以便执行时将它传递给 QueueUserWorkItem。每次执行您的回调方法时，state 参数都包含此对象。
有关如何使用回调同步线程池线程的更多信息，请参见 托管线程池。
示例
Imports System
Imports System.Threading
Public Class Example
<MTAThread> _
Public Shared Sub Main()
' Queue the task.
ThreadPool.QueueUserWorkItem( _
New WaitCallback(AddressOf ThreadProc) _
)
' Note that you do not have to create the WaitCallback delegate
' explicitly in Visual Basic. The following line also queues
' the task:
'ThreadPool.QueueUserWorkItem(AddressOf ThreadProc)
Console.WriteLine("Main thread does some work, then sleeps.")
' If you comment out the Sleep, the main thread exits before
' the thread pool task runs. The thread pool uses background
' threads, which do not keep the application running. (This
' is a simple example of a race condition.)
Thread.Sleep(1000)
Console.WriteLine("Main thread exits.")
End Sub
' This thread procedure performs the task.
Shared Sub ThreadProc(stateInfo As Object)
' No state object was passed to QueueUserWorkItem, so
' stateInfo is null.
Console.WriteLine("Hello from the thread pool.")
End Sub
End Class
using System;
using System.Threading;
public class Example {
public static void Main() {
// Queue the task.
ThreadPool.QueueUserWorkItem(new WaitCallback(ThreadProc));
Console.WriteLine("Main thread does some work, then sleeps.");
// If you comment out the Sleep, the main thread exits before
// the thread pool task runs. The thread pool uses background
// threads, which do not keep the application running. (This
// is a simple example of a race condition.)
Thread.Sleep(1000);
Console.WriteLine("Main thread exits.");
}
// This thread procedure performs the task.
static void ThreadProc(Object stateInfo) {
// No state object was passed to QueueUserWorkItem, so
// stateInfo is null.
Console.WriteLine("Hello from the thread pool.");
}
}
using namespace System;
using namespace System::Threading;
ref class Example
{
public:
// This thread procedure performs the task.
static void ThreadProc( Object^ stateInfo )
{
// No state object was passed to QueueUserWorkItem, so
// stateInfo is 0.
Console::WriteLine( "Hello from the thread pool." );
}
};
int main()
{
// Queue the task.
ThreadPool::QueueUserWorkItem( gcnew WaitCallback( Example::ThreadProc ) );
Console::WriteLine( "Main thread does some work, then sleeps." );
// If you comment out the Sleep, the main thread exits before
// the thread pool task runs. The thread pool uses background
// threads, which do not keep the application running. (This
// is a simple example of a race condition.)
Thread::Sleep( 1000 );
Console::WriteLine( "Main thread exits." );
return 0;
}
import System.*;
import System.Threading.*;
import System.Threading.Thread;
public class Example
{
public static void main(String[] args)
{
// Queue the task.
ThreadPool.QueueUserWorkItem(new WaitCallback(ThreadProc));
Console.WriteLine("Main thread does some work, then sleeps.");
// If you comment out the Sleep, the main thread exits before
// the thread pool task runs. The thread pool uses background
// threads, which do not keep the application running. (This
// is a simple example of a race condition.)
Thread.Sleep(1000);
Console.WriteLine("Main thread exits.");
} //main
// This thread procedure performs the task.
static void ThreadProc(Object stateInfo)
{
// No state object was passed to QueueUserWorkItem, so
// stateInfo is null.
Console.WriteLine("Hello from the thread pool.");
} //ThreadProc
} //Example
平台
Windows 98、Windows 2000 SP4、Windows CE、Windows Millennium Edition、Windows Mobile for Pocket PC、Windows Mobile for Smartphone、Windows Server 2003、Windows XP Media Center Edition、Windows XP Professional x64 Edition、Windows XP SP2、Windows XP Starter Edition
.NET Framework 并不是对每个平台的所有版本都提供支持。有关受支持版本的列表，请参见系统要求。
版本信息
.NET Framework
受以下版本支持：2.0、1.1、1.0
.NET Compact Framework
受以下版本支持：2.0、1.0
请参见
参考
System.Threading 命名空间
ThreadPool 类