调用Web服务时异步/等待不起作用

Question

For some reason, I can't get the generated [Service Method Name]Async version of any method to compile in a web service I create in VB.NET.

I get the same format of error message, no matter whether I'm using the actual service I've created or even the default service with its HelloWorld and HelloWorldAsync methods. So I'm obviously not doing something key that's required.

The format of the error message in the VS 2012 code window is [Service MethodName]Response cannot be converted to [Type that will be returned]. So in the case of the default HelloWorld method, this code snippet : Imports StackOverflowClient.TestServiceReference

Public Class Form1 Dim SOClient As New TestWebServiceSoapClient

Private Async Sub SayHelloAsync()
    Label1.Text = Await SOClient.HelloWorldAsync()
End Sub

End Class

(Where StackOverflowClient is the name of the project,TestServiceReference is the Namespace assigned to the Service, and SOClient is the proxy class) will error out with the message: HelloWorldResponse cannot be converted to String. (In my real application, the message will be GetUserResponse cannot be converted to the User type I've created in the service.

I've also sometimes had an error message of "Expression does not produce a value" when trying to do this with classes from the service, eg: lblUser.Text = StaffInfoService.GetNextStaffMemberAsync()

I have the 'Allow generation of asynchronous operations' checkbox checked in the Service Reference Settings.

I can't see what I've missed. I don't want to have to use the older Begin/Completed/Callback approach of pre VS2010 unless I absolutely have to. Any pointers to using Async/Await for this task will be really appreciated.

Answer 1

That sounds like HelloWorldAsync is returning a type of Task<HelloWorldResponse> . When you Await it, the task is "unwrapped" and the type of the value is then HelloWorldResponse . As the error message states, you can't convert this to a string. Perhaps you are looking for a particular property of that type?