If WebClientAsyncResult.CompletedSynchronously is set to true within the AsyncCallback delegate, it is probable that the thread that called SoapHttpClientProtocol.BeginInvoke(string, Object[], AsyncCallback, object) is the current thread. In this case, you should be careful about making another asynchronous Begin call from the callback method, since it can cause the stack to grow without bound if the calls continue to complete synchronously. Instead, the call should be completed at the original call site, outside the callback method.

Namespace: System.Web.Services.Protocols
Assembly: System.Web.Services (in System.Web.Services.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0