Unable to debug WCF service message

In my case the problem turned out to be a mismatch between security settings on client and server. I was using a custom binding like this:

<customBinding>
    <binding name="AuthorisedBinaryHttpsBinding" receiveTimeout="00:03:00" sendTimeout="00:03:00">
      <!-- this next element caused the problem: -->
      <security authenticationMode="UserNameOverTransport">
      </security>
      <binaryMessageEncoding>
        <readerQuotas maxDepth="100" maxStringContentLength="1000000"
          maxArrayLength="655360000" />
      </binaryMessageEncoding>
      <httpsTransport />
    </binding>
  </customBinding>

When I removed the security element that I've highlighted above the problem with the "Unable to automatically debug" message went away.

To solve the problem I first turned on WCF tracing. This showed me that WCF was throwing a MessageSecurityException:

Security processor was unable to find a security header in the message. This might be because the message is an unsecured fault or because there is a binding mismatch between the communicating parties. This can occur if the service is configured for security and the client is not using security.

That pointed me to look at the Binding settings on the client side. It turned out that I hadn't added the required security element to my custom binding there. Since I was doing this through code, I needed the following (note the 3rd line):

  var binding = new CustomBinding(
      binaryEncoding,
      SecurityBindingElement.CreateUserNameOverTransportBindingElement(),
      new HttpsTransportBindingElement { MaxReceivedMessageSize = MaxMessageSize, });

As to why Visual Studio was showing that error, I've no idea - looks to me to be a bug.

Automatically attaching to a service has the following limitations:

• The service must be part of the Visual Studio solution you are debugging.

• The service must be hosted. It may be part of a Web Site Project (File System and HTTP), Web Application Project (File System and HTTP), or WCF Service Library project. WCF Service Library projects can be either Service Libraries or Workflow Service Libraries.

• The service must be invoked from a WCF client.

• Debugging must be enabled with the following code in the app.config or Web.config file:

  <system.web>
    <compilation debug="true" />
  </system.web>
  

See Limitations on WCF Debugging

In addition, if both projects (client and service) are in the same solution but will run in different processes (for example if you are using your local IIS Server for development and are running your web application on a different application pool than the service it consumes), you may need to enable "Multiple startup projects" for the solution (on Solution Properties -> Startup Project) so that the debugger can attach to both.

To avoid the service browser window to show up every time you debug, you may set the "Start Action" (on service project properties) to "Don't open a page. Wait for request from external application."

This is from personal experience and may help others.

I was fighting with this exact same error for over an hour and low and behold I restarted VS2008 and it magically fixed itself. Give it a try as it might save you some time.