如何得到 TextBox 或 richTextBox 的当前输入光标的屏幕坐标

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Forms;

namespace CSWin
{
public partial class Form1 : Form
{
const int EM_GETSEL = 0xB0;
const int EM_LINEFROMCHAR = 0xC9;
const int EM_LINEINDEX = 0xBB;
[DllImport("user32.dll", EntryPoint = "SendMessage")]
public static extern int SendMessage(
int hwnd,
int wMsg,
int wParam,
ref int lParam
);
private void GetCaretPos(int TextHwnd, ref int LineNo, ref int ColNo)
{
int i = 0, j = 0, lParam = 0, wParam = 0, k = 0;
i = SendMessage(TextHwnd, EM_GETSEL, wParam, ref lParam);
j = i / 65536;
LineNo = SendMessage(TextHwnd, EM_LINEFROMCHAR, j, ref lParam) + 1;
k = SendMessage(TextHwnd, EM_LINEINDEX, -1, ref lParam);
ColNo = j - k + 1;
}
public Form1()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
int lineNo = 0, colNo = 0;
GetCaretPos(this.textBox1.Handle.ToInt32(), ref lineNo, ref colNo);
this.label1.Text = string.Format("Line:{0}, Col:{1}", lineNo, colNo);
}
}
}