继续求救啊。。。。。。。。。

谁能帮我看看，我的代码那错了，，，，为什么不能执行更新语句啊。。。。。
 CollectSelected();

        this.TextBox6.Text = string.Empty;
        foreach (object tmp in this.SelectedItems)
            this.TextBox6.Text += tmp.ToString() + ",";
        string a = this.TextBox6.Text;        
        a = a.TrimEnd(',');
        SqlConnection con = new SqlConnection("server=localhost;database=User;user=grid;password=grid");
        con.Open();
        SqlCommand com = new SqlCommand("（update Message set huiyuanjiezhishijian='" + this.Text1.Value + "' where ID in(" + a +"))", con);
        SqlDataAdapter da = new SqlDataAdapter(com);
        DataSet ds = new DataSet();
        Response.Write(a+"-----------------");
        Response.Write("update Message set huiyuanjiezhishijian='" + this.Text1.Value + "' where ID in(" + a +")"  + "-----------------");
        con.Close();