如何：用 Web 服务方法链接异步调用
本主题专门介绍一项旧有技术。现在应通过使用以下链接来创建 XML Web 服务和 XML Web 服务客户端： Windows Communication Foundation.
下面的代码示例演示在 Web 服务方法进行多次异步调用且这些调用必须按顺序执行时如何链接异步调用。BeginGetAuthorRoyalties 方法进行异步调用，以确定传入的作者是否有效并设置名为 AuthorRoyaltiesCallback 的中间回调，以接收结果。然后，将异步调用该中间回调，以获取该作者的版税（如果该作者有效）。
示例
using System.Web.Services;
using System.Data;
using System;
// This imports the proxy class for the Web services
// that the sample communicates with.
using AsyncWS.localhost;
namespace AsyncWS
{
[WebService(Namespace="https://www.contoso.com/")]
public class MyService : System.Web.Services.WebService
{
public RemoteService remoteService;
public MyService()
{
remoteService = new RemoteService();
}
[WebMethod]
public IAsyncResult BeginGetAuthorRoyalties(String Author,
AsyncCallback callback, Object asyncState)
{
// Saves the current state for the call that gets the author's
// royalties.
AsyncStateChain state = new AsyncStateChain();
state.originalState = asyncState;
state.Author = Author;
state.originalCallback = callback;
// Creates an intermediary callback.
AsyncCallback chainedCallback = new
AsyncCallback(AuthorRoyaltiesCallback);
return remoteService.BeginGetAuthors(chainedCallback,state);
}
// Intermediate method to handle chaining the
// asynchronous calls.
public void AuthorRoyaltiesCallback(IAsyncResult ar)
{
AsyncStateChain state = (AsyncStateChain)ar.AsyncState;
RemoteService rs = new RemoteService();
// Gets the result from the call to GetAuthors.
Authors allAuthors = rs.EndGetAuthors(ar);
Boolean found = false;
// Verifies that the requested author is valid.
int i = 0;
DataRow row;
while (i < allAuthors.authors.Rows.Count && !found)
{
row = allAuthors.authors.Rows[i];
if (row["au_lname"].ToString() == state.Author)
{
found = true;
}
i++;
}
if (found)
{
AsyncCallback cb = state.originalCallback;
// Calls the second Web service, because the author is
// valid.
rs.BeginReturnedStronglyTypedDS(state.Author,cb,state);
}
else
{
// Cannot throw the exception in this function or the XML Web
// service will hang. So, set the state argument to the
// exception and let the End method of the chained XML Web
// service check for it.
ArgumentException ex = new ArgumentException(
"Author does not exist.","Author");
AsyncCallback cb = state.originalCallback;
// Call the second Web service, setting the state to an
// exception.
rs.BeginReturnedStronglyTypedDS(state.Author,cb,ex);
}
}
[WebMethod]
public AuthorRoyalties EndGetAuthorRoyalties(IAsyncResult
asyncResult)
{
// Check whehter the first Web service threw an exception.
if (asyncResult.AsyncState is ArgumentException)
throw (ArgumentException) asyncResult.AsyncState;
else
return remoteService.EndReturnedStronglyTypedDS(asyncResult);
}
}
// Class to wrap the callback and state for the intermediate
// asynchronous operation.
public class AsyncStateChain
{
public AsyncCallback originalCallback;
public Object originalState;
public String Author;
}
}
Imports System.Web.Services
Imports System.Data
Imports System
' This imports the proxy class for the Web services
' that the sample communicates with.
Imports AsyncWS_VB.localhost
Namespace AsyncWs
<WebService(Namespace:="https://www.contoso.com/")> _
Public Class MyService
Inherits WebService
Public remoteService As remoteService
Public Sub New()
MyBase.New()
remoteService = New localhost.RemoteService()
End Sub
' Defines the Begin method.
<WebMethod()> _
Public Function BeginGetAuthorRoyalties(ByVal Author As String, _
ByVal callback As AsyncCallback, ByVal asyncState As Object) _
As IAsyncResult
' Saves the current state for the call that gets the author's
' royalties.
Dim state As AsyncStateChain = New AsyncStateChain()
state.originalState = asyncState
state.Author = Author
state.originalCallback = callback
' Creates an intermediary callback.
Dim chainedCallback As AsyncCallback = New AsyncCallback( _
AddressOf AuthorRoyaltiesCallback)
' Begin asynchronous communictation with a different XML Web
' service.
Return remoteService.BeginGetAuthors(chainedCallback, state)
End Function
' Intermediate method to handle chaining the asynchronous calls.
Public Sub AuthorRoyaltiesCallback(ByVal ar As IAsyncResult)
Dim state As AsyncStateChain = CType(ar.AsyncState, _
AsyncStateChain)
Dim rs As RemoteService = New RemoteService()
' Gets the result from the call to GetAuthors.
Dim allAuthors As Authors = rs.EndGetAuthors(ar)
Dim found As Boolean = False
' Verifies that the requested author is valid.
Dim i As Integer = 0
Dim row As DataRow
While (i < allAuthors.authors.Rows.Count And (Not found))
row = allAuthors.authors.Rows(i)
If (row("au_lname").ToString() = state.Author) Then
found = True
End If
i = i + 1
End While
If (found) Then
Dim cb As AsyncCallback = state.originalCallback
' Calls the second Web service, because the author is
' valid.
rs.BeginReturnedStronglyTypedDS(state.Author, cb, state)
Else
' Cannot throw the exception in this function or the XML Web
' service will hang. So, set the state argument to the
' exception and let the End method of the chained XML Web
' service check for it.
Dim ex As ArgumentException = New ArgumentException( _
"Author does not exist.", "Author")
Dim cb As AsyncCallback = state.originalCallback
' Call the second Web service, setting the state to an
' exception.
rs.BeginReturnedStronglyTypedDS(state.Author, cb, ex)
End If
End Sub
' Define the End method.
<WebMethod()> _
Public Function EndGetAuthorRoyalties(ByVal asyncResult As _
IAsyncResult) As localhost.AuthorRoyalties
' Return the asynchronous result from the other Web service.
Return remoteService.EndReturnedStronglyTypedDS(asyncResult)
End Function
End Class
' Class to wrap the callback and state for the intermediate asynchronous
' operation.
Public Class AsyncStateChain
Public originalCallback As AsyncCallback
Public originalState As Object
Public Author As String
End Class
End Namespace
另请参见
任务
概念
异步 XML Web services 方法
与 XML Web services 进行异步通信