Rest Service Encoding problem

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C#

REST

Hi experts,
When I enter some Unicode (UTF-8) characters in my query string, it will change in my program.

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single,
             ConcurrencyMode = ConcurrencyMode.Single, IncludeExceptionDetailInFaults = true)]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class RestServices : IRESTServices
{
    public Stream GetClientNameById(string id, string message)
    {
        string ReturnString = string.Format("id={0} and message ={1}", id, message);
        return GenerateStreamFromString(ReturnString);
    }

private Stream GenerateStreamFromString(string s)
{
    MemoryStream stream = new MemoryStream();
    StreamWriter writer = new StreamWriter(stream, Encoding.GetEncoding("utf-8"));
    writer.Write(s);
    writer.Flush();
    stream.Position = 0;
    return stream;
}

}

the message will change, if I enter the address below:
http://localhost:8000/DEMOService/subscribe?userID=14953828&message=مرحبا

It will change to ÓáÇã

How can I use utf-8?

This is the interface:

[ServiceContract(Name = "RESTDemoServices")]
[XmlSerializerFormat]
public interface IRESTServices
{
    [OperationContract]
    [WebGet(UriTemplate = Routing.GetClientRoute, BodyStyle = WebMessageBodyStyle.Wrapped, RequestFormat=WebMessageFormat.Json, ResponseFormat=WebMessageFormat.Json)]
    Stream GetClientNameById(string id, string message);
}

And:

public static class Routing
{
    public const string GetClientRoute = "/subscribe?userID={id}&message={message}";
}

Thank you