sql 执行结果为什么总是返回-1 - .NET技术 / ASP.NET

//点击登录按钮进行登录，为什么总是执行 Response.Write("<script> alert('真的错了！')</script>");


protected void Button1_Click(object sender, EventArgs e)
  {
  SqlConnection con = new SqlConnection("Server=.;database=Best;uid=sa;");
  con.Open();
  string sql = "select * from Userlogin where username='" + this.TextBox1.Text + "'and Pwd='" + this.TextBox2.Text + "'";
  SqlCommand cmd = new SqlCommand(sql, con);
  int i = Convert.ToInt32(cmd.ExecuteNonQuery());
  con.Close();
  if (i > 0)
  {
  Response.Redirect("Manger.aspx");
  }
  else if (i==0)
  {
  Response.Write("<script> alert('用户名和密码错误！')</script>");

  }
  else if(i<0)
  {
  Response.Write("<script> alert('真的错了！')</script>");

  }

  }

---

C# code:

string sql = "select * from Userlogin where username='" + this.TextBox1.Text + "'and Pwd='" + this.TextBox2.Text + "'";
这段