Message contract in WCF

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C#

WCF

I added a message contract class like below

[MessageContract]
    public class Credential
    {
        private string key = String.Empty;
        private string val = String.Empty;

        [MessageBodyMember(
          Name = "test",
          Namespace = "http://www.examples.com"
        )]
        public string Key
        {
            get { return key; }
            set { this.key = value; }
        }

        [MessageHeader(
          Name = "testval",
          Namespace = "http://www.examples.com"
        )]
        public string Val
        {
            get { return val; }
            set { this.val = value; }
        }
    }

Added a Service contract like below

[ServiceContract]
    public interface IService1
    {

        [OperationContract]
        Hashtable GetHeader();

        Credential obj
        {
            [OperationContract]
            get;
            [OperationContract]
            set;
        }
        // TODO: Add your service operations here
    }

Now in the WCF service i am implementing the property

public class Service1 : IService1
    {
        private Credential _obj;
        public Credential obj
        {
            get { return _obj; }
            set { _obj = value; }
        }

        public Hashtable GetHeader()
        {
            Hashtable ht = new Hashtable();
            foreach (MessageHeaderInfo h in OperationContext.Current.IncomingMessageHeaders)
            {
                ht.Add(h.Name, h.ToString());
            }
            return ht;
        }
    }

Now in the client i am trying to set the message contract object like below.

localhost.Service1 obj = new localhost.Service1();
            localhost.Credential objCredential = new localhost.Credential();
            objCredential.test = "test";
            obj.set_obj(objCredential);
            obj.GetHeader();

But the soap header never seems to have the value i assigned to " objCredential.test".Why is it so?