Public Sub validation()
        If txtFirstNum.Text = "" Then
            MsgBox("First Number is Empty")
        ElseIf Not IsNumeric(txtFirstNum.Text) Then
            MsgBox("First Number is not numeric")
        Else
            call factorial
        endif
endsub

Public Sub factorial()
        Dim counter As Integer
        Dim result As Long
        Dim x As String = (Val(txtFirstNum.Text))
        counter = 1
        result = 1
        While counter <= x
            result = result * counter
 
            counter = counter + 1
 
            txtAnswer.Text = result
        End While
    End Sub

Private sub cmdcalculate_click(.)
call validation
endsub