vb.net异步操作示例

IAsyncResult 接口由包含可异步操作的方法的类实现。

它是启动异步操作的方法的返回类型，如 FileStream.BeginRead¹，

也是结束异步操作的方法的第三个参数的类型，如 FileStream.EndRead²。

当异步操作完成时，IAsyncResult 对象也将传递给由 AsyncCallback³ 委托调用的方法。

支持 IAsyncResult 接口的对象存储异步操作的状态信息，并提供同步对象以允许线程在操作完成时终止。

有关如何使用 IAsyncResult 接口的详细说明，请参见“使用异步方式调用同步方法⁴”主题。

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下面的示例说明如何使用 IAsyncResult 来获取异步操作的返回值。

Imports System
Imports System.Threading
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Contexts
Imports System.Runtime.Remoting.Messaging

'' Context-Bound type with Synchronization Context Attribute' <Synchronization()>
Public Class SampleSyncronized
    Inherits ContextBoundObject
    ' A method that does some work - returns the square of the given number
    Public Function Square(ByVal i As Integer) As Integer
        Console.Write("SampleSyncronized.Square called. ")
        Console.WriteLine("The hash of the current thread is: {0}",Thread.CurrentThread.GetHashCode())
        Return i * i
    End Function
    'Square
End Class
'SampleSyncronized'

' Async delegate used to call a method with this signature asynchronously'
Delegate Function SampSyncSqrDelegate(ByVal i As Integer) As Integer
'Main sample class
Public Class AsyncResultSample
    Public Shared Sub Main()
        Dim callParameter As Integer = 0
        Dim callResult As Integer = 0
        'Create an instance of a context-bound type SampleSynchronized'Because SampleSynchronized is context-bound,the object    sampSyncObj 'is a transparent proxy
         ‘定义

        Dim sampSyncObj As New SampleSyncronized()
        'call the method synchronously 
        Console.Write("Making a synchronous call on the object. ")
        Console.WriteLine("The hash of the current thread is: {0}",Thread.CurrentThread.GetHashCode())
        callParameter = 10
        callResult = sampSyncObj.Square(callParameter)
        ‘同异操作方法

        Console.WriteLine("Result of calling sampSyncObj.Square with {0} is {1}.",callParameter,callResult)
        Console.WriteLine("")
        Console.WriteLine("") 'call the method asynchronously 
        Console.Write("Making an asynchronous call on the object. ")
        Console.WriteLine("The hash of the current thread is: {0}",Thread.CurrentThread.GetHashCode())
        ’异步操作方法

        Dim sampleDelegate As New SampSyncSqrDelegate(AddressOf sampSyncObj.Square)
        callParameter = 17
        Dim aResult As IAsyncResult = sampleDelegate.BeginInvoke(callParameter,Nothing,Nothing)
        'Wait for the call to complete 
        aResult.AsyncWaitHandle.WaitOne()
        callResult = sampleDelegate.EndInvoke(aResult)
        Console.WriteLine("Result of calling sampSyncObj.Square with {0} is {1}.",callResult)

        Console.ReadLine()

    End Sub
    'Main
End Class 'AsyncResultSample