Generic return types with interface type params in WCF

Question

How can I return interface types in generic type parameters from the OperationContracts in my WCF REST service? More specifically, it works for one operation but not when I add a second operation with a generic T that is an interface.

I'm using JSON as the request and response format, feeding a non-WCF client that parses the JSON responses for the data it needs. I am not using SOAP, or the WSDL generated by the service.

My service interface:

[ServiceContract]
[ServiceKnownType("GetServiceKnownTypes", typeof(ServiceKnownTypesHelper))]
public interface IMyService
{
    [WebGet(UriTemplate="count")]
    [OperationContract]
    IServiceResult<int> GetCount();

    [WebGet(UriTemplate="desc")]
    [OperationContract]
    IServiceResult<string> GetDescription();

    [WebGet(UriTemplate="foo")]
    [OperationContract]
    IServiceResult<IFooData> GetFooData();

    // Fails when I invoke either method if I uncomment this operation.
    //[WebGet(UriTemplate="bar")]
    //[OperationContract]
    //IServiceResult<IBarData> GetBarData();
}

I left GetCount() and GetDescription() in the example to point out that those two generic results work fine, but obviously they are concrete types. And even GetFooData() works fine until I add a second method of IServiceResult<T> where T is an interface.

The return types of GetFooData() and GetBarData() are not the same, nor are the concrete classes that implement them.

You may imagine that I've reduced the implementation to a skeleton as I don't think the implementation is the heart of the problem:

#region My service implementation
public class MyService : IMyService
{
    public IServiceResult<int> GetCount()
    {
        return new ServiceResult<int>(42);
    }
    public IServiceResult<string> GetDescription()
    {
        return new ServiceResult<string>("Muffins");
    }
    public IServiceResult<IFooData> GetFooData()
    {
        return new ServiceResult<IFooData>(new FooData() { Foo = 99 });
    }
    public IServiceResult<IBarData> GetBarData()
    {
        return new ServiceResult<IBarData>(new BarData() { Bar = "Elvis was here" });
    }
}
#endregion

#region ServiceKnownTypesHelper.GetServiceKnownTypes():
public static class ServiceKnownTypesHelper
{
    private static IList<Type> serviceKnownTypes = new List<Type>()
        {
            typeof(FooData),
            typeof(BarData),
            typeof(ServiceResult<int>),
            typeof(ServiceResult<string>),
            typeof(ServiceResult<IFooData>),
            typeof(ServiceResult<IBarData>),
        };

    public static IEnumerable<Type> GetServiceKnownTypes(ICustomAttributeProvider paramIgnored)
    {
        return serviceKnownTypes;
    }
}
#endregion

#region IServiceResult<T> and its concrete implementation:
public interface IServiceResult<T>
{
    IList<string> Errors { get; }
    T Value { get; set; }
}

[DataContract]
public class ServiceResult<T> : IServiceResult<T>
{
    public ServiceResult(T value)
    {
        this.Value = value;
    }

    private IList<string> errors = new List<string>();

    [DataMember]
    public IList<string> Errors
    {
        get
        {
            return this.errors;
        }
    }

    [DataMember]
    public T Value { get; set; }
}
#endregion

#region IFooData and its concrete implementation:
public interface IFooData
{
    int Foo { get; set; }
}

[DataContract]
public class FooData: IFooData
{
    [DataMember]
    public int Foo { get; set; }
}
#endregion

#region IBarData and its concrete implementation:
public interface IBarData
{
    string Bar { get; set; }
}

[DataContract]
public class BarData: IBarData
{
    [DataMember]
    public string Bar { get; set; }
}
#endregion

And the error message when I invoke GetBarData() from the browser:

Type 'ServiceResult`1[IBarData]' cannot be added to list of known types since
another type 'ServiceResult`1[IFooData]' with the same data contract name
'http://schemas.datacontract.org/2004/07/ServiceResultOfanyType' is
already present. 

The rest of the error message is a red herring about colliding collection types List<Test> and Test[], which isn't the case here.

Clearly, IFooData and IBarData aren't the same, nor are the classes that implement them.

So why do ServiceResult<IFooData> and ServiceResult<IBarData> both resolve to ServiceResultOfanyType?

Am I missing something, or is there no way to fix this?

Answer 1

After much trial and error, I've finally got this working with minimal changes:

• My service operations now return ServiceResult<T> instead of IServiceResult<T>. In fact, IServiceResult<T> is now gone entirely.

• GetServiceKnownTypes() no longer returns all the variants of ServiceResult<T>. I only return the DataContracts used as T.

  #region My service interface
  [ServiceContract]
  [ServiceKnownType("GetServiceKnownTypes", typeof(ServiceKnownTypesHelper))]
  public interface IMyService
  {
      [WebGet(UriTemplate="count")]
      [OperationContract]
      ServiceResult<int> GetCount();
  
      [WebGet(UriTemplate="desc")]
      [OperationContract]
      ServiceResult<string> GetDescription();
  
      [WebGet(UriTemplate="foo")]
      [OperationContract]
      ServiceResult<IFooData> GetFooData();
  
      [WebGet(UriTemplate="bar")]
      [OperationContract]
      ServiceResult<IBarData> GetBarData();
  }
  #endregion
  #region My service implementation (minus bodies)
  public class MyService : IMyService
  {
      public ServiceResult<int> GetCount() {}
      public ServiceResult<string> GetDescription() {}
      public ServiceResult<IFooData> GetFooData() {}
      public ServiceResult<IBarData> GetBarData() {}
  }
  #endregion
  #region ServiceKnownTypes
  // My list of ServiceKnownTypes is now much shorter and simpler. I was feeding the service too much information
  public static class ServiceKnownTypesHelper
  {
      private static IList<Type> serviceKnownTypes = new List<Type>()
          {
              typeof(FooData),
              typeof(BarData),
              //typeof(ServiceResult<int>),
              //typeof(ServiceResult<string>),
              //typeof(ServiceResult<IFooData>),
              //typeof(ServiceResult<IBarData>),
          };
  
      // Remaining implementation is the same as before
  }
  #endregion
  #region ServiceResult<T> with no interface (it's not used or needed)
  [DataContract]
  public class ServiceResult<T> //: IServiceResult<T>
  {
      // implementation is the same as before
  }
  #endregion
  

I can now invoke all of those methods and get back lists of generic types referred to by their interfaces in the ServiceContract.