Error handling with Silverlight and WCF

I was recently working on a big Silverlight 3 project. While trying to figure out how to get error messages to the user, I found out that simply throwing an exception won't return to the client as expected.

The error looks like this and contains non of the exception information you would expect:

The remote server returned an error: NotFound.

That's not very helpful. I need a better way to handle errors and provide a useful message back to the user. My first thought was adding error properties to each return object. That wouldn't work though if I just wanted to return a simple boolean or return nothing at all.

The best solution I see is to use an out parameter. I put together an error class to handle this:

 1: [DataContract]
 2:  public class WCFError
 3:  {
 4:      public WCFError(string Message, string Type)
 5:      {
 6:          this.Message = Message;
 7:          this.Type = Type;
 8:      }
 9:
10:      [DataMember]
11:      public string Message { get; set; }
12:
13:      [DataMember]
14:      public string Type { get; set; }
15:
16:      [DataMember]
17:      public string UserMessage
18:      {
19:          get
20:          {
21:              return "The following error has occured: " + Message;
22:          }
23:          set { }
24:      }
25:
26:  }

Now that I have the error class, I can use it in my service. All I have to do is add an out parameter of type WCFError and it will be available in Silverlight.

 1: [OperationContract]
 2:   public string DoWork(out WCFError error)
 3:   {
 4:       error = null;
 5:
 6:       try
 7:       {
 8:           //do some data work
 9:           //if error in database
10:           error = new WCFError("Table not found", "Database");
11:           return null;
12:       }
13:       catch (Exception ex)
14:       {
15:           error = new WCFError(ex.Message, "Application");
16:           return null;
17:       }
18:
19:   }

 1: void clnt_DoWorkCompleted(object sender,
         SilverlightApplication1.ServiceReference1.DoWorkCompletedEventArgs e)
 2: {
 3:     if (e.error != null)
 4:     {
 5:         MessageBox.Show(e.error.UserMessage);
 6:         return;
 7:     }
 8:     if (e.Result != null)
 9:     {
10:         string t = e.Result;
11:     }
12: }

So now, through the completed event args, I have access to the out parameter. I should note that, by default, there is an Error parameter added by WCF, so error isn’t a great choice for the parameter name like I did in this example. You could name your out parameter Fault so it is less confusing.

By using an out parameter, I was able to add error handling to my service with little work. For the time being, I feel this is the best option you have to get users a meaningful message in the event of an exception or any other error in the service.