AsyncCompletedEventArgs.UserState 属性

定义

获取异步任务的唯一标识符。Gets the unique identifier for the asynchronous task.

public:
 property System::Object ^ UserState { System::Object ^ get(); };
public object UserState { get; }
member this.UserState : obj
Public ReadOnly Property UserState As Object

属性值

唯一标识异步任务的对象引用;如果未设置任何值,则为 nullAn object reference that uniquely identifies the asynchronous task; otherwise, null if no value has been set.

示例

下面的代码示例演示如何使用 AsyncOperation 跟踪异步操作的生存期。The following code example demonstrates using an AsyncOperation to track the lifetime of asynchronous operations. 此代码示例是为 System.ComponentModel.AsyncOperationManager 类提供的更大示例的一部分。This code example is part of a larger example provided for the System.ComponentModel.AsyncOperationManager class.

using System;
using System.Collections;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Globalization;
using System.Threading;
using System.Windows.Forms;
Imports System.Collections
Imports System.Collections.Specialized
Imports System.ComponentModel
Imports System.Drawing
Imports System.Globalization
Imports System.Threading
Imports System.Windows.Forms
// This event handler updates the ListView control when the
// PrimeNumberCalculator raises the CalculatePrimeCompleted
// event. The ListView item is updated with the appropriate
// outcome of the calculation: Canceled, Error, or result.
private void primeNumberCalculator1_CalculatePrimeCompleted(
    object sender, 
    CalculatePrimeCompletedEventArgs e)
{
    Guid taskId = (Guid)e.UserState;

    if (e.Cancelled)
    {   
        string result = "Canceled";

        ListViewItem lvi = UpdateListViewItem(taskId, result);

        if (lvi != null)
        {
            lvi.BackColor = Color.Pink;
            lvi.Tag = null;
        }
    }
    else if (e.Error != null)
    {
        string result = "Error";

        ListViewItem lvi = UpdateListViewItem(taskId, result);

        if (lvi != null)
        {
            lvi.BackColor = Color.Red;
            lvi.ForeColor = Color.White;
            lvi.Tag = null;
        }
    }
    else
    {   
        bool result = e.IsPrime;

        ListViewItem lvi = UpdateListViewItem(
            taskId, 
            result, 
            e.FirstDivisor);

        if (lvi != null)
        {
            lvi.BackColor = Color.LightGray;
            lvi.Tag = null;
        }
    }
}
' This event handler updates the ListView control when the
' PrimeNumberCalculator raises the CalculatePrimeCompleted
' event. The ListView item is updated with the appropriate
' outcome of the calculation: Canceled, Error, or result.
Private Sub primeNumberCalculator1_CalculatePrimeCompleted( _
    ByVal sender As Object, _
    ByVal e As CalculatePrimeCompletedEventArgs) _
    Handles primeNumberCalculator1.CalculatePrimeCompleted

    Dim taskId As Guid = CType(e.UserState, Guid)

    If e.Cancelled Then
        Dim result As String = "Canceled"

        Dim lvi As ListViewItem = UpdateListViewItem( _
            taskId, _
            result)

        If (lvi IsNot Nothing) Then
            lvi.BackColor = Color.Pink
            lvi.Tag = Nothing
        End If

    ElseIf e.Error IsNot Nothing Then

        Dim result As String = "Error"

        Dim lvi As ListViewItem = UpdateListViewItem( _
            taskId, result)

        If (lvi IsNot Nothing) Then
            lvi.BackColor = Color.Red
            lvi.ForeColor = Color.White
            lvi.Tag = Nothing
        End If
    Else
        Dim result As Boolean = e.IsPrime

        Dim lvi As ListViewItem = UpdateListViewItem( _
            taskId, _
            result, _
            e.FirstDivisor)

        If (lvi IsNot Nothing) Then
            lvi.BackColor = Color.LightGray
            lvi.Tag = Nothing
        End If
    End If

End Sub

注解

如果类支持多个异步方法或多次调用单个方法,则可以通过检查 UserState 属性的值来确定哪些任务引发了方法名称Completed 事件。If a class supports multiple asynchronous methods, or multiple invocations of a single method, you can determine which task raised the MethodNameCompleted event by checking the value of the UserState property. 你的代码需要跟踪这些标记(称为任务 Id),因为它们对应的异步任务会开始和完成。Your code will need track these tokens, known as task IDs, as their corresponding asynchronous tasks start and complete.

此属性的值是在对启动任务的异步方法的原始调用期间设置的。The value of this property is set during the original call to the asynchronous method that started the task.

适用于

另请参阅