datagrid删除数据时出现异常
我的代码是这么写的：
using   System;
using   System.Collections;
using   System.ComponentModel;
using   System.Data;
using   System.Drawing;
using   System.Web;
using   System.Web.SessionState;
using   System.Web.UI;
using   System.Web.UI.WebControls;
using   System.Web.UI.HtmlControls;
using   System.Data.SqlClient;
namespace   sbumit.admin
{
///   <summary>
///   degree_m   的摘要说明。
///   </summary>
public   class   degree_m   :   System.Web.UI.Page
{
protected   System.Web.UI.WebControls.Label   Label1;
protected   System.Web.UI.WebControls.Button   btndegree;
protected   System.Web.UI.WebControls.RequiredFieldValidator   RequiredFieldValidator1;
protected   System.Web.UI.WebControls.TextBox   tbxdegree;
protected   System.Web.UI.WebControls.DataGrid   DataGrid1;

private   void   Page_Load(object   sender,   System.EventArgs   e)
{
if(Session[ "usertype "]==null   ||   Session[ "usertype "].GetHashCode()   <   1   ||   Session[ "usertype "].GetHashCode()   >   1)
{
//没有登录或身份不是管理员
Response.Redirect( "login.aspx ") ;
}
else
{
this.bindtodatagrid();
}
//   在此处放置用户代码以初始化页面
}
public   void   bindtodatagrid()
{
//绑定数据库
SqlConnection   con=db.dbcon();
con.Open();
SqlDataAdapter   sda_d=new   SqlDataAdapter();//数据适配器
sda_d.SelectCommand=new   SqlCommand( "select   *   from   degree ",con);//检索出现有的学位
DataSet   set_d=new   DataSet();
sda_d.Fill(set_d, "degree ");
con.Close();
this.DataGrid1.DataSource=set_d.Tables[ "degree "];
this.DataGrid1.DataBind();
}


#region   Web   窗体设计器生成的代码
override   protected   void   OnInit(EventArgs   e)
{
//
//   CODEGEN:   该调用是   ASP.NET   Web   窗体设计器所必需的。
//
InitializeComponent();
base.OnInit(e);
}

///   <summary>
///   设计器支持所需的方法   -   不要使用代码编辑器修改
///   此方法的内容。
///   </summary>
private   void   InitializeComponent()
{        
this.DataGrid1.DeleteCommand   +=   new   System.Web.UI.WebControls.DataGridCommandEventHandler(this.DataGrid1_DeleteCommand_1);
this.btndegree.Click   +=   new   System.EventHandler(this.btndegree_Click);
this.Load   +=   new   System.EventHandler(this.Page_Load);

}
#endregion


private   void   btndegree_Click(object   sender,   System.EventArgs   e)
{
SqlConnection   con=db.dbcon();
con.Open();
SqlCommand   addp=new   SqlCommand( "insert   into   degree(degree)   values   ( ' "+this.tbxdegree.Text+ " ')   ",con);
try
{
addp.ExecuteNonQuery();
this.Label1.Text=message.Show( "学位添加成功！ ");
}
catch
{
this.Label1.Text=message.Show( "学位添加失败！ ");
}
}

private   void   DataGrid1_DeleteCom