Question

I’m trying to specify a known type in my config, but I’m having problems with the fact that it derives from Object. I can make it work specifying the known type via attribute. But in this case I need to make it work from the config.

Here’s an example. The following works fine:

[ServiceContract]
[ServiceKnownType(typeof(MyData))]
public interface IContract
{
    [OperationContract]
    void Send(object data);
}

[DataContract]
public class MyData
{
    [DataMember]
    public string Message { get; set; }
}

But if I remove the ServiceKnownType attribute and put the following in the config:

<system.runtime.serialization>
  <dataContractSerializer>
    <declaredTypes>
      <add type="System.Object, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
        <knownType type="WpfApplication1.MyData, WpfApplication1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
      </add>
    </declaredTypes>
  </dataContractSerializer>
</system.runtime.serialization>

I get a ConfigurationErrorsException with the message “The value for the property ‘type’ is not valid. The error is: The type System.Object cannot be used as a declared type in config.”

Is there anyway to make this work via config?

Answer 1

The answer turns out to be it’s not possible to do what I want to do in the config file alone. The config above corresponds to the [KnownType] attribute used on DataContracts. There appears to be no way to implement [ServiceKnownType] in the config.

An alternate approach is to use [ServiceKnownType(methodName, type)] attribute with a custom configuration section. The new config looks like this:

<configuration>
  <configSections>
    <section
      name="serviceKnownTypes"
      type="WpfApplication1.ServiceKnownTypesSection, WpfApplication1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
  </configSections>
  <serviceKnownTypes>
    <declaredServices>
      <serviceContract type="WpfApplication1.IContract, WpfApplication1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null">
        <knownTypes>
          <knownType type="WpfApplication1.MyData, WpfApplication1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
        </knownTypes>
      </serviceContract>
    </declaredServices>
  </serviceKnownTypes>
</configuration>

The contracts:

[ServiceContract]
[ServiceKnownType("GetServiceKnownTypes", typeof(KnownTypeHelper))]
public interface IContract
{
    [OperationContract]
    void Send(object data);
}

[DataContract]
public class MyData
{
    [DataMember]
    public string Message { get; set; }
}

The helper class that contains the callback that returns the list of known types

public static class KnownTypeHelper
{
    public static IEnumerable<Type> GetServiceKnownTypes(ICustomAttributeProvider provider)
    {
        List<Type> result = new List<Type>();

        ServiceKnownTypesSection serviceKnownTypes = (ServiceKnownTypesSection)ConfigurationManager.GetSection("serviceKnownTypes");
        DeclaredServiceElement service = serviceKnownTypes.Services[((Type)(provider)).AssemblyQualifiedName];

        foreach (ServiceKnownTypeElement knownType in service.KnownTypes)
        {
            result.Add(knownType.Type);
        }

        return result;
    }
}

Information on creating custom config sections can be found here:

http://msdn.microsoft.com/en-us/library/2tw134k3.aspx

http://msdn.microsoft.com/en-us/library/system.configuration.configurationcollectionattribute.aspx