Validating String User Entry

Question

My issue is that I am attempting to make the user only able to enter "a, b, c or d". I would rather have it error out if they did not enter one of those four letters than the user only being able to input those letters. I have only been able to find resources that do something similar to this with numeric data (using try catch). Any sites or hints would be great.

                If String.Compare(TextBox2.Text, "a", True) = 0 AndAlso String.Compare(TextBox21.Text, "a", True) = 0 Then
                'MessageBox.Show("A")
                totCorrect = totCorrect + corAns
            ElseIf String.Compare(TextBox2.Text, "b", True) = 0 AndAlso String.Compare(TextBox21.Text, "b", True) = 0 Then
                'MessageBox.Show("B")
                totCorrect = totCorrect + corAns
            ElseIf String.Compare(TextBox2.Text, "c", True) = 0 AndAlso String.Compare(TextBox21.Text, "c", True) = 0 Then
                'MessageBox.Show("C")
                totCorrect = totCorrect + corAns
            ElseIf String.Compare(TextBox2.Text, "d", True) = 0 AndAlso String.Compare(TextBox21.Text, "d", True) = 0 Then
                'MessageBox.Show("D")
                totCorrect = totCorrect + corAns
            Else
                totWrong = totWrong + wrgAns
                Label13.Visible = True
            End If

Stupid question, but if you want the user to only select a, b, c or d, why not use a combobox instead of a textbox? Then you won't have to do all these checks... — yu_ominae, Mar 27 '13 at 01:06

yu_ominae · Accepted Answer · 2013-03-27T01:26:44.503

This should do the trick

Private Sub TextBox1_KeyPress(sender As Object, e As KeyPressEventArgs) Handles TextBox1.KeyPress

    Dim allowableChar As New List(Of Char) From {"a"c, "b"c, "c"c, "d"c}

    Call allowableChar.AddRange(allowableChar.Select(Function(c) Convert.ToChar(c.ToString().ToUpper())).ToList())

    If Not (allowableChar.Contains(e.KeyChar) OrElse e.KeyChar = Convert.ToChar(Keys.Delete) OrElse e.KeyChar = Convert.ToChar(Keys.Back)) Then
        e.Handled = True
    End If

End Sub

The Call allowableChar.AddRange(...) adds the upper case characters to the list as well. As it is now, this generates a new list every time the method executes, which is a bit wasteful... if you use this piece of code I would suggest that you the list of allowable chars a class-level variable and fill it only once in the constructor of your form.

To allow only once character of each type, use this:

Private Sub TextBox1_KeyPress(sender As Object, e As KeyPressEventArgs) Handles TextBox1.KeyPress

    Dim allowableChar As New List(Of Char) From {"a"c, "b"c, "c"c, "d"c}

    Call allowableChar.AddRange(allowableChar.Select(Function(c) Convert.ToChar(c.ToString().ToUpper())).ToList())

    If Not (allowableChar.Contains(e.KeyChar) OrElse e.KeyChar = Convert.ToChar(Keys.Delete) OrElse e.KeyChar = Convert.ToChar(Keys.Back)) Then
        e.Handled = True
    Else
        If Me.TextBox1.Text.Count(Function(c) c = e.KeyChar) >= 1 Then
            e.Handled = True
        End If
    End If

End Sub

That will work...I might be stretching what code in general can do but, do you think it is possible for it to only allow 1 "a" or 1 "b" etc? — Brandon, Mar 27 '13 at 01:21
Sure, I changed the code to include that, although if you actually want to do that you could use a set of checkboxes instead of a textbox. Unless the order in which the letters are entered is important of course. — yu_ominae, Mar 27 '13 at 01:26

Validating String User Entry

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