Get cursor position of winforms textbox

As already stated, the SelectionStart property is not reliable to get the actual CARET position in a TextBox with a selection active. This is caused by the fact that this property points always at the selection start (clue: the name doesn't lie) and depending on how you select the text with the mouse the caret could be positioned on the LEFT or RIGHT side of the selection.

This code (tested with LinqPAD) shows an alternative

public class WinApi
{
    [DllImport("user32.dll")]
    public static extern bool GetCaretPos(out System.Drawing.Point lpPoint);
}

TextBox t = new TextBox();
void Main()
{
    Form f = new Form();
    f.Controls.Add(t);
    Button b = new Button();
    b.Dock = DockStyle.Bottom;
    b.Click += onClick;
    f.Controls.Add(b);
    f.ShowDialog();
}

// Define other methods and classes here
void onClick(object sender, EventArgs e)
{
    Console.WriteLine("Start:" + t.SelectionStart + " len:" +t.SelectionLength);
    Point p = new Point();
    bool result = WinApi.GetCaretPos(out p);
    Console.WriteLine(p);
    int idx = t.GetCharIndexFromPosition(p);
    Console.WriteLine(idx);
}

The API GetCaretPos returns the point in client coordinates where the CARET is. You could return the index of the character after the position using the managed method GetCharIndexFromPosition. Of course you need to add a reference and a using to System.Runtime.InteropServices.

Not sure if there is some drawback to this solution and waiting if someone more expert can tell us if there is something wrong or unaccounted for.