使用CookieContainer

代码如下：
web server服务端：

 [WebMethod(true)]
    public void WriteSession(string text)
    {
        Session["a"] = text;
    }

    [WebMethod(true)]
    public string ReadSession()
    {
        return Session["a"].ToString();
    }

客户端：

System.Net.CookieContainer cc = new System.Net.CookieContainer();

 localhost.WebService ws = new localhost.WebService();
 ws.CookieContainer = cc;
 ws.WriteSession(TextBox1.Text);

 TextBox2.Text = ws.ReadSession();