How to implement a "fully viewed" multi-line TextBox?

Question

I am trying to create a form with a multi-line TextBox with the following requirements:

The text in the text box might be short or long.
There is not much space availble.
I want to be sure that the entirety of the text has been seen. I can't be sure that the user has actually read it, but I can at least require that all of it has been seen.

So I'm trying to make a "fully viewed" multi-line TextBox.

This picture should make it clear what I'm trying to do:

If they check the checkbox before they've scrolled through the whole thing, I'll know not to believe them.

I think I need to know:

When the form comes up, was the text that was put into the TextBox short (all visible without scrolling) or long (the vertical scroll bar was shown)?
If the vertical scroll bar is showing, has the user scrolled it all the way to the bottom?

Any ideas about how to achieve this?

See [Get current scroll position from rich text box control?](http://stackoverflow.com/a/10238729/719186) — LarsTech, Sep 11 '15 at 18:57
@LarsTech: Thanks! I'll try that if I can't find a solution using a plain TextBox. — Jeff Roe, Sep 11 '15 at 19:09

Olivier Jacot-Descombes · Accepted Answer · 2015-09-11T19:50:15.907

The TextBox has no scrolling event but the RichTextBox has. Also it has a method that allows you to get the index of the character closest to a point position.

private readonly Point _lowerRightCorner;

public frmDetectTextBoxScroll()
{
    InitializeComponent();
    _lowerRightCorner = new Point(richTextBox1.ClientRectangle.Right,
                                  richTextBox1.ClientRectangle.Bottom);
}

private void richTextBox1_VScroll(object sender, EventArgs e)
{
    int index = richTextBox1.GetCharIndexFromPosition(_lowerRightCorner);
    if (index == richTextBox1.TextLength - 1) {
        // Enable your checkbox here
    }
}

How to implement a "fully viewed" multi-line TextBox?

1 Answers1