Question

I’m planning a C# app where some rather large strings will be displayed in scrollable/resizeable textboxes.

When the user closes the app and returns later, I’d like to redisplay these strings from the point they were displayed on lostFocus.

Is there a way to get the index, within the full text, of the first char that is displayed in a textBox?

Answer 1

These two should help you get the caret position. Get the caret position before you close your application, and then set it back using the SelectionStart property:

textBox1.SelectionStart;
textBox1.SelectionLength;

EDIT:
This works, just tested, as I said, it is important to use SelectionStart property and ScrollToCaret() method. You will have to get the index of the char that is first visible, for this you should use GetCharIndexFromPosition() method from the TextBox:

        int selectionStart;

        private void button1_Click(object sender, EventArgs e)
        {
            Graphics g = textBox1.CreateGraphics();
            SizeF size = g.MeasureString("A", textBox1.Font);
            g.Dispose();
            int index = textBox1.GetCharIndexFromPosition(new Point(0, (int)(2 * size.Height + 0.5)));
            selectionStart = index;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            textBox1.SelectionStart = selectionStart;
            textBox1.ScrollToCaret();
        }