Restrict TextBox to only certain characters, numeric or symbolic

[kayakerca]

The following are some coding examples on how to customize which characters you want to keep from being entered into a TextBox.

To specify which characters are the only ones that are allowed to be in the TextBox:

(In this example only letters and numbers will be accepted into the textbox)

Public Class MainForm

    Dim charactersAllowed As String = ".1234567890"

    Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
        Dim theText As String = TextBox1.Text
        Dim Letter As String
        Dim SelectionIndex As Integer = TextBox1.SelectionStart
        Dim Change As Integer

        For x As Integer = 0 To TextBox1.Text.Length - 1
            Letter = TextBox1.Text.Substring(x, 1)
            If charactersAllowed.Contains(Letter) = False Then
                theText = theText.Replace(Letter, String.Empty)
                Change = 1
            End If
        Next

        TextBox1.Text = theText
        TextBox1.Select(SelectionIndex - Change, 0)
    End Sub

End Class

Could this be modified to allow only one instance of a character? In this case the "." so you couldn't enter a number with 2 "." if you were entering a double.