ASP.NET

Public Sub RestrictDecimalPlaces(ByVal txtSender As TextBox, ByVal intPlacesToTheRight As Integer)
    '***********************************************************************************
    '   This function will ensure that only one decimal point is allowed in a text box
    '   and that the user may only type in a certain number of digits to the right of
    '   the decimal point (as specified by the intPlacesToTheRight parm).
    '***********************************************************************************
    If txtSender.TextLength <> 0 Then
        Dim strTemp As String = txtSender.Text
        Dim straTemp() As String
        straTemp = strTemp.Split(".")
        Select Case straTemp.Length
            Case 1
                'No decimal point, so no problem
                Return
            Case 2
                'One decimal point, ensure specified chars to the right
                Select Case straTemp(1).Length
                    Case Is > intPlacesToTheRight
                        txtSender.Text = straTemp(0) & "." & straTemp(1).PadRight(intPlacesToTheRight, "0").Substring(0, intPlacesToTheRight)
                        txtSender.SelectionStart = txtSender.Text.Length
                End Select
            Case Else
                'More than one decimal point, remove extra
                txtSender.Text = straTemp(0) & "." & straTemp(1)
                txtSender.SelectionStart = txtSender.Text.Length
        End Select
    End If
End Sub

Private Sub txtInput_KeyUp(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles txtInput.KeyUp
    RestrictDecimalPlaces(sender, 4)
End Sub