How to: Chain Asynchronous Calls with a Web Service Method

This topic is specific to a legacy technology. XML Web services and XML Web service clients should now be created using Windows Communication Foundation.

The following code example demonstrates how to chain asynchronous calls when a Web service method makes more than one asynchronous call and the calls must execute sequentially. The BeginGetAuthorRoyalties method makes an asynchronous call to determine whether the author passed in is valid and sets up an intermediate callback named AuthorRoyaltiesCallback to receive the results. That intermediate callback then asynchronously calls to get the royalties for that author, if the author is valid.

Example

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="http://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:="http://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

See Also

Tasks

How to: Create Asynchronous Web Service Methods

Concepts

Asynchronous XML Web Service Methods
Communicating with XML Web Services Asynchronously

Other Resources

XML Web Services Using ASP.NET