Vb.Net: Limit the total width of characters entered into a textbox?

Question

Let me be very clear - I am not looking for a static character limit here (Textbox.MaxLength just doesn't cut it.)

I'm making a simple messaging program and I'd like to implement a character limit. The messages are displayed inside a listbox and can't be horizontally scrolled/wrapped. The solution: impose a limit on every message so that users don't accidentally cut off their own messages.

The problem is that 10 small characters are a lot smaller than 10 full width characters. - E.G. i and W:

iiiiiiiiii
WWWWWWWWWW

I'd like to find a way to limit the characters entered into the text box by the actual amount of pixels the string is wide. so that:

nobody can use all capitals and get cut off, and
nobody can type normally and be stopped by the character limit far earlier than neccesary.

For reference, I'm using Verdana 8.25pt for the listbox.

Any suggestions would be appreciated, Thanks.

So if I change the text scaling in Windows, I can send longer messages? — Joel Coehoorn
– Joel Coehoorn, Commented Sep 14, 2017 at 2:41
Realistically, I'd be looking to create a little snippet of code that dynamically adjusts TextBox.MaxLength based on the pixel width of the specific characters being entered In Verdana 8.25pt. I could potentially use a dictionary to store how many pixels each character will take up and use that to calculate the amount of pixels a string will occupy. I'm not sold on this method, however. But If a better one isn't sugested I'll self-answer later. — PurpleSkyHoliday
– PurpleSkyHoliday, Commented Sep 14, 2017 at 2:50
Look into the "MeasureString()" function. You'll have to call it after every keystroke (on TextChanged). Also... how will you handle pasting larger strings? This whole concept just seems like bad UI. — Joel Coehoorn
– Joel Coehoorn, Commented Sep 14, 2017 at 3:02
For similar purpose, I used System.Windows.Forms.TextRenderer.MeasureText rather than MeasureString. — Craig
– Craig, Commented Sep 14, 2017 at 13:25

David Wilson · Accepted Answer · 2017-09-14 13:52:50Z

This should do the trick for you. I've deliberately chosen the keyUp event because the user will see the letter typed and it disappearing.

Private Sub TextBox1_TextChanged(sender As Object, e As KeyEventArgs) Handles TextBox1.KeyUp Dim text1 As String = TextBox1.Text Dim textboxFont As Font = TextBox1.Font Dim textSize As Size = TextRenderer.MeasureText(text1, textboxFont) If textSize.Width > TextBox1.ClientRectangle.Width Then Dim cursorLocation As Integer = TextBox1.SelectionStart TextBox1.Text = TextBox1.Text.Remove(cursorLocation - 1, 1) If cursorLocation > TextBox1.Text.Length Then TextBox1.SelectionStart = TextBox1.Text.Length Else TextBox1.SelectionStart = cursorLocation End If End If End Sub

Basically what is happening is that the text is rendered (not displayed) using the font of the textbox and measured. If the width of the rendered text is greater than the client area of the textbox, the letter is removed at the point it was typed. This can be anywhere in the text box.

If the cursor is at the end of the text when the letter is removed, .net automatically sets the cursor position to the beginning is a letter is removed. So this sub checks if the initial cursor position was a bigger index than the length of the new textbox contents. If so, the cursor is set to the end again. Otherwise it is moved back 1 because a character was deleted.

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