Как я могу установить textbox maxlenght во время запуска программы.?

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I want to set MaxLength of TextBox when TextChanged in Calculator Program.

İf there is Comma in TextBox I want to set MaxLength 19 if not there is comma in textboxI want to set MaxLength 24

I want this because there are 1000 Separator in TextBox. When user enter digits inside TextBox When put comma then Length must set 24 if not set max 19 digits.

Что я уже пробовал:

Private Sub txtInput_KeyDown(sender As Object, e As KeyEventArgs) Handles txtInput.KeyDown
       If Not InStr(1, txtInput.Text, ",") Then
           txtInput.MaxLength = 19
       Else
           txtInput.MaxLength = 24

       End If

   End Sub

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