Transporting my-model-type in WCF service call

See more:

C#

WCF

Silverlight

Hi,

Think this one is simple - at least simple to ask: I have a working WCF service exposed on a IIS-7 hosted website and a Silverlight 4 consuming app to call it. Then I have a service method like:

public ObservableCollection<Headline> GetHeadlines()
{...}

Where

Heading

is part of a model-project referenced by both the WS and the SL app.

Now, when I publish this WS and browse it in IE it fails with:

The server was unable to process the request due to an internal error. For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the <serviceDebug> configuration behavior) on the server in order to send the exception information back to the client, or turn on tracing as per the Microsoft .NET Framework 3.0 SDK documentation and inspect the server trace logs.

However if I change the method signature to:

public ObservableCollection<string> GetHeadlines()
{...}

it all works fine.

Seems to work for all build-in types like string and List<string> etc. but not for my model classes. I don't understand why. Is it some streaming issue perhaps?

Model class is really simple - no base, on interface - just a placeholder with two members.