asp.net中使用callback

①由用户触发

1. 在button的 onclick 中写入：
   onclick="<%= ClientScript.GetCallbackEventReference(this,"document.getElementById('txtUserName').value","onCallBack",null) %>" 
2. 在 xxx.aspx.cs 中继承 ICallbackEventHandler  并实现其方法。

   public partial class webPage_callBackBtn : System.Web.UI.Page, ICallbackEventHandler
   {
       protected string strUserInfo;
       protected void Page_Load(object sender, EventArgs e)
       {
       }
   
       #region ICallbackEventHandler 成员
   
       public string GetCallbackResult()
       {
           return strUserInfo;
       }
   
       public void RaiseCallbackEvent(string eventArgument)
       {
           if (eventArgument == "") return;
           System.Data.SqlClient.SqlConnection conn = new System.Data.SqlClient.SqlConnection();
           conn.ConnectionString = System.Web.Configuration.WebConfigurationManager.ConnectionStrings["nowthWindConnectionString"].ConnectionString;
   
           SqlCommand cmd = new SqlCommand();
           cmd.CommandType = CommandType.Text;
           cmd.Parameters.Add("@FirstName", SqlDbType.NVarChar, 10)
               .Value = eventArgument;
           cmd.CommandText = "SELECT EmployeeID, LastName FROM Employees WHERE FirstName=@FirstName";
           cmd.Connection = conn;
   
           SqlDataReader reader;
           ConnectionState previousConnectionState = conn.State;
   
           try
           {
               if (conn.State == ConnectionState.Closed)
               {
                   conn.Open();
               }
   
               reader = cmd.ExecuteReader();
   
               using (reader)
               {
                   while (reader.Read())
                   {
                       // Process SprocResults datareader here.
                       strUserInfo += reader[0];
                   }
               }
               strUserInfo += "###";
           }
           finally
           {
               if (previousConnectionState == ConnectionState.Closed)
               {
                   conn.Close();
               }
           }
       }
   
       #endregion
   }