问题在代码中间! 谢谢帮忙! 

 private void Button1_Click(object sender, System.EventArgs e)
  {
   Label3.Text="";
   Label4.Text="";
   Label5.Text="";
   Label6.Text="";

   if(TextBox1.Text=="")
   {
    Label3.Text="用户名不能为空";
    return;
   }
   if(TextBox2.Text=="")
   {
    Label4.Text="密码不能为空";
    return;
   }
   else
   {
    SqlConnection conn = new SqlConnection("server=(local);database=user_data;pwd=sa;uid=sa");
    conn.Open();
    string SQL="select * from user1 where user1='"+this.TextBox1.Text+"'";
    SqlCommand cmd = new SqlCommand(SQL,conn);
    SqlDataReader ds;
    ds = cmd.ExecuteReader();
    while(ds.Read())
    {
     if(ds[1].ToString()=="" || ds[1].ToString()==null)

{
      Label5.Text="用户名不正确";     ---------------问题:"用户名不正确"为什么不能执行

      ds.Close();
      conn.Close();
      return;
     }     
     else
     {
      ds.Close();
      SqlDataReader dr;
      dr = cmd.ExecuteReader();
      while(dr.Read())
      {
       if(dr["pwd"].ToString() == this.TextBox2.Text)
       {
        dr.Close();
        conn.Close();
        Response.Redirect("usersud.aspx");
       }
       else
       {
        Label6.Text="密码不正确";
        dr.Close();
        conn.Close();
        return;
       }

      }

          }
    }
   }
   
           
  }