如何在不修改生成的类的情况下将SoapExtension属性添加到生成的Web服务客户端代理？

using System;
using System.IO;
using System.Web.Services.Protocols;

public class MySoapExtension : SoapExtension
{
    private Stream _originalStream;
    private Stream _newStream;

    public override Stream ChainStream(Stream stream)
    {
        _originalStream = stream;
        _newStream = new MemoryStream();
        return _newStream;
    }

    public override void ProcessMessage(SoapMessage message)
    {
        switch (message.Stage)
        {
            case SoapMessageStage.BeforeSerialize:
                break;
            case SoapMessageStage.AfterSerialize:
                _newStream.Position = 0;
                var reader = new StreamReader(_newStream);
                var xml = reader.ReadToEnd();
                xml = AddSoapHeader(xml);
                var writer = new StreamWriter(_originalStream);
                writer.Write(xml);
                writer.Flush();
                break;
            case SoapMessageStage.BeforeDeserialize:
                break;
            case SoapMessageStage.AfterDeserialize:
                break;
        }
    }

    private string AddSoapHeader(string xml)
    {
        // 在这里添加自定义的SOAP头部信息
        return xml;
    }
}

 <system.web>
    <webServices>
      <soapExtensionTypes>
        <add type="MySoapExtension, MyAssembly" priority="1" group="0" />
      </soapExtensionTypes>
    </webServices>
  </system.web>
</configuration>