RichTextBox doesn't start selection on mouse down when the form has not focus

Question

I'm using WinForms and on my Form I have a RichTextBox. When my form is out of focus but visible and I try to highlight/select text, it does not allow me to until the form or textbox itself has focus.

I've tried:

txtInput.MouseDown += (s, e) => { txtInput.Focus(); }

but to no avail and I can't seem to find anything online about this issue.

When testing with another program like Notepad, it does possess the desired behavior.

Answer 1

MouseDown is too late.

This is a workaround for sure, but may be all you need:

private void txtInput_MouseMove(object sender, MouseEventArgs e)
{
    txtInput.Focus();
}

or of course:

txtInput.MouseMove += (s, e) => { txtInput.Focus(); }

As it is it may steal focus from other controls on your form when you move over the textbox. If this is a problem you could prevent it by checking if your program is active using one the of answers here..

Answer 2

You can make the selection manually using MouseDown and MouseMove events. The answer is based on Taw's first idea:

int start = 0;
private void richTextBox1_MouseDown(object sender, MouseEventArgs e)
{
    start = richTextBox1.GetTrueIndexPositionFromPoint(e.Location);
    richTextBox1.SelectionStart = start;
}

private void richTextBox1_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button.HasFlag(MouseButtons.Left))
    {
        var current = richTextBox1.GetTrueIndexPositionFromPoint(e.Location);
        richTextBox1.SelectionStart = Math.Min(current, start);
        richTextBox1.SelectionLength = Math.Abs(current - start);
    }
}

And here is the codes for GetTrueIndexPositionFromPoint method which has taken from Justin:

public static class RichTextBoxExtensions
{
    private const int EM_CHARFROMPOS = 0x00D7;
    public static int GetTrueIndexPositionFromPoint(this RichTextBox rtb, Point pt)
    {
        POINT wpt = new POINT(pt.X, pt.Y);
        int index = (int)SendMessage(new HandleRef(rtb, rtb.Handle), EM_CHARFROMPOS, 0, wpt);
        return index;
    }

    [DllImport("User32.dll", EntryPoint = "SendMessage", CharSet = CharSet.Auto)]
    private static extern IntPtr SendMessage(HandleRef hWnd, int msg, int wParam, POINT lParam);
}