WCF error handling anyone : The Official Microsoft Silverlight Site

06-23-2008 7:23 AM

WCF error handling anyone

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Hi,

I've understood that the SOAP error handling mechanism with fault contracts doesn't work in beta 2 and possibly will not work in the RTM because of browser limitations.

Does anybody have a pattern of handling WCF errorrs in Silverlight without hacking too heavily ?

Tim

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Joined on 05-18-2007
Posts 64

06-23-2008 8:18 AM

Re: WCF error handling anyone

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Please try to refer to sladapter's post here

http://silverlight.net/forums/p/18852/64012.aspx#64012

if it still cant help, you can ask here with more details

Regards,
SteveWong (HongKong)
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Client App Dev

SteveWong

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Joined on 03-27-2008
Hong Kong
Posts 1,220

06-23-2008 9:26 AM

Re: WCF error handling anyone

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Hi,

I think sladapters post is for a slightly different case where there is a 404 error but the cause is unclear.

My problem is that when I rise an custom error in the service it ends up as a 404 in my client no matter what I try to do. Creating a FaultContract for the error type does not work and I got the impression it might not work in the final release so I am looking for a clean solution to propagate server/service errors to a Silverlight client.

Tim

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06-23-2008 12:10 PM

Re: WCF error handling anyone

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I ended up rolling my own custom error object that I pass to the client as an out parameter. It means I have to do some extra coding, but I decided it was worth it, even if it might require some refactoring when (if?) proper soap exceptions can be caught on the client side.

hwsoderlund

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Joined on 11-09-2005
Posts 112

06-23-2008 12:34 PM

Re: WCF error handling anyone

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You can read this thread and I posted how I handle exception in WCF and pass it to Silverlight:

http://silverlight.net/forums/p/17944/60019.aspx#60019

sladapter
Software Engineer
Aprimo, Inc

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sladapter

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06-23-2008 2:32 PM

Re: WCF error handling anyone

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hwsoderlund:

I ended up rolling my own custom error object that I pass to the client as an out parameter. It means I have to do some extra coding, but I decided it was worth it, even if it might require some refactoring when (if?) proper soap exceptions can be caught on the client side.

Hi,

Sounds like a pretty nice solution, I didn't know out parameters were allowed in service calls. Do you have any example of your solution ?

Tim

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06-24-2008 3:01 AM

Re: WCF error handling anyone

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The code is somewhat modified for clarity.

The error object:

[DataContract]
public class GLSErrorObject
{
    [DataMember]
    public string ErrorID { get; set; }
    [DataMember]
    public string ErrorMessage { get; set; }
    [DataMember]
    public DateTime ErrorTime { get; set; }
}

On the server side:

public bool Login(string userName, string password, out GLSErrorObject GLSError)
{
    GLSError = null;
    try
    {
        bool success = AuthenticateUser(userName, password);
 
        return success;
    }
    catch (Exception ex)
    {
        GLSError = new GLSErrorObject();
        GLSError.ErrorMessage = ex.Message;
        GLSError.ErrorTime = DateTime.Now;
        //etc...
    }
    return false;
}

And on the client side the callback method will look something like this:

void AuthenticationClient_LoginCompleted(object sender, LoginCompletedEventArgs e)
{
    if (e.Error != null)
    {
        throw new Exception(e.Error.Message);
        return;
    }
    else if (e.GLSError != null)
    {
        //Handle our custom error
        throw new Exception(e.GLSError.ErrorMessage);
    }
    else
    {
        //All is well
    }
    //etc...
 

/Henrik

hwsoderlund

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Joined on 11-09-2005
Posts 112

06-24-2008 3:57 PM

Re: WCF error handling anyone

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Hi Henrik and thanks for the example code, I guess I will implement something similar.

Btw, interesting how one can access out parameters as part of the EventArgs.

Tim

Tim Favour

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Posts 64

07-10-2008 5:54 PM

Re: WCF error handling anyone

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I implemented a slightly different approach by creating a generic class to wrap a fault message string along with the actual webservice operation result. This ends up being pretty clean due to a bit of relection magic on the Silverlight client side. Here's what this would look like on the service side:

[ServiceContract]
public interface IMyService {

    [OperationContract]
    GenericResult<double> Div(double p1,double p2);
}public class MyService : IMyService {

    public GenericResult<double> Div(double p1,double p2) {

        try {

            return GenericResult<double>(p1/p2);
        }
        catch (DivideByZeroException e) {

            return GenericResult<double>(e);
        }
    }
}

Here’s what the Silverlight client side call to the generated WCF proxy would look like:

void Div() {

    MyServiceClient client = new MyServiceClient();

    client.DivCompleted += new EventHandler<DivCompletedEventArgs>(OnDivCompleted);
    client.DivAsync(10,20);
}

void OnDivCompleted(object sender,DivCompletedEventArgs args) {

    double result;

    try {

        result = GenericResult.GetOrThrow(args.Result);
    }
    catch (ServiceException e) {

        // Handle exceptions thrown by the service
    }
    catch (Exception e) {

        // Handle exceptions thrown by Silverlight or WCF
    }
}

The code for this is pretty straight forward. The only realy trick necessary is the reflection to access the Result and Fault properties of the proxy class generated for the GenericResult<T>. Here's the code: public class ServiceException : Exception {

    public ServiceException(string message)
        : base(message) {

    }
} #if !SILVERLIGHT
[DataContract]#endif
public class GenericResult<TResult> {

    public static TResult GetOrThrow(object wrappedResult) {

        if (wrappedResult == null)
            throw new ArgumentNullException("wrappedResult");

        System.Type     resultType = wrappedResult.GetType();
        PropertyInfo    property;
        string          fault;

        property = resultType.GetProperty("Fault");
        if (property != null && property.PropertyType == typeof(string)) {

            fault = (string) property.GetValue(wrappedResult,null);
            if (fault != null)
                throw new ServiceException(fault);
        }

        property = resultType.GetProperty("Result");
        if (property == null)
            throw new InvalidOperationException("GenericResult: Result object passed does not expose a [Result] property.");

        if (!object.ReferenceEquals(typeof(TResult),property.PropertyType))
            throw new InvalidOperationException(
                          String.Format("GenericResult: Result object has type [{0}] rather than the expected [{1}] type.",
                                        property.PropertyType.FullName,typeof(TResult).FullName));

        return (TResult) property.GetValue(wrappedResult,null);
    }

    public GenericResult() {

        this.Result = default(TResult);
        this.Fault = null;
    }

    public GenericResult(TResult result) {

        this.Result = result;
        this.Fault = null;
    }

    public GenericResult(Exception e) {

        this.Result = default(TResult);
        this.Fault = e.Message;
    }

#if !SILVERLIGHT
    [DataMember]
#endif
    public TResult Result { get; set; }

#if !SILVERLIGHT
    [DataMember]
#endif
    public string Fault { get; set; }
}

Note that these classes are designed to compile in Silverlight as well as normal .NET projects.

--------------
Jeff Lill
LillTek, LLC

jslill