这听起来像你想要的东西像SOAP的Web服务的头部。我不是一个WCFMaven，但这个looks like the WCF equivalent。

实际上这并不难，我能想到的最好方法是编写一个IClientMessageInspector，它在其BeforeSendRequest方法中将SOAP头添加到Message.Headers中。
参见例如http://weblogs.asp.net/paolopia/archive/2007/08/23/writing-a-wcf-message-inspector.aspx

你不能把这件事做得这么简单。这需要花些功夫。
SOAP头确实是向服务传递带外数据和/或从服务传递带外数据的最佳方式，但是您已经定义了契约，添加头将改变契约。
我认为您必须开始使用消息契约。
原件：

[DataContract]
public class ComplexObject
{
    [DataMember(Name = "Id")] 
    public int Id;

    [DataMember] 
    public string Name;
}

[ServiceContract()]
public interface IMyContract
{
    void MyOperation(ComplexObject co);
}

public class MyService : IMyContract
{
    #region Implementation of IMyContract

    public void MyOperation(ComplexObject co)
    {
        // use co.*
    }

    #endregion
}

使用消息协定：

[DataContract]
public class ComplexObject
{
    [DataMember(Name = "Id")] 
    public int Id;

    [DataMember] 
    public string Name;
}

[DataContract]
public class MyHeader
{
    [DataMember]
    public string UserName;

    [DataMember]
    public string Password;
}

[DataContract]
public class OutputHeader
{
    [DataMember]
    public string Token;
}

[MessageContract]
public class MyOperationRequest
{
    [MessageHeader]
    public MyHeader Authentication;

    [MessageBodyMember]
    public ComplexObject TheObject;
}

[MessageContract]
public class MyOperationResponse
{
    [MessageHeader] 
    public OutputHeader OutputHeader;
}

[ServiceContract()]
public interface IMyContract
{
    MyOperationResponse MyOperation(MyOperationRequest request);
}

public class MyService : IMyContract
{
    public MyOperationResponse MyOperation(MyOperationRequest request)
    {
        // use request.TheObject.*
        // Can also read request.Authentication.*

        return new MyOperationResponse 
               { OutputHeader = new OutputHeader { Token = "someToken" } };
    }
}