In Wcf how to use message class for streamed transfer?

Question

In .NET WCF I want to use TransferMode.Streamed. Thus I need to have a Message object to transfer more than one parameter. To avoid a lot of message classes for any combination of parameters to transfer I tried it with a template class. Example for two parameters:

[MessageContract]
public class StreamMessage<TA,TB>
{
    [MessageHeader(MustUnderstand = true)]
    public TA Value1;

    [MessageBodyMember(Order = 1)]
    public TB Value2;
}

If I use the template class to transfer an FileStream object, the client receives the stream always closed. Server:

public StreamMessage<String,FileStream> DownloadFromServer()
{
    Console.WriteLine("DownloadFromServer()");
    const string filename = @"c:\The\File\Name";
    var result = new StreamMessage<String,FileStream>();
    result.Value1 = filename;
    result.Value2 = File.OpenRead(filename);
    return result;
}

This does not happen without usage of message template. Any suggestions?

Answer 1

FileSteam can not be serialized. Try using the Steam class:

public StreamMessage<String, Stream> DownloadFromServer()
{
    Console.WriteLine("DownloadFromServer()");
    const string filename = @"c:\The\File\Name";
    var result = new StreamMessage<String, Stream>();
    result.Value1 = filename;
    result.Value2 = File.OpenRead(filename);
    return result;
}

Answer 2

Based on Florians answer the following is possible, eg for two values and one stream value:

[MessageContract]
public class StreamMessage<T1,T2>
{
    [MessageHeader(MustUnderstand = true)]
    public T1 Val1;

    [MessageHeader(MustUnderstand = true)]
    public T2 Val2;

    [MessageBodyMember(Order = 1)]
    public Stream Stream;
}

In case of type Stream, WCF restrict this to only one object in body. Therefore I decided two have to templates: Message and StreamMessage.