如何取得 TextBox 游標位置 (WinForms)

SelectionStart 在反向選取時回傳錯誤值。正確做法呼叫 Win32 API GetCaretPos 取得螢幕座標，再轉換為字元索引。

完整程式碼

using System.Runtime.InteropServices;
using System.Drawing;
using System.Windows.Forms;

public partial class Form1 : Form
{
    [DllImport("user32.dll")]
    private static extern bool GetCaretPos(out Point ppt);

    private void Form1_MouseDown(object sender, MouseEventArgs e)
    {
        Point p;
        GetCaretPos(out p);
        int index = this.textBox1.GetCharIndexFromPosition(p);
        this.Text = index.ToString();
    }
}

關鍵：GetCaretPos 取得插入點螢幕座標，GetCharIndexFromPosition 轉為控件內字元索引。