Hex - Convert from decimal to hexadecimal

' convert from decimal to hexadecimal
' if you pass the Digits argument, the result is truncated to that number of 
' digits
'
' you should always specify Digits if passing negative values

Function Hex(ByVal value As Long, Optional ByVal digits As Short = -1) As String
    ' convert to base-16
    Dim res As String = Convert.ToString(value, 16).ToUpper()

    ' truncate or extend the number
    If digits > 0 Then
        If digits < res.Length Then
            ' truncate the result
            res = res.Substring(res.Length - digits)
        ElseIf digits > res.Length Then
            ' we must extend the result to the left
            If value >= 0 Then
                ' extend with zeroes if positive
                res = res.PadLeft(digits, "0"c)
            Else
                ' extend with "F"s if negative
                res = res.PadLeft(digits, "F"c)
            End If
        End If
    End If
    ' return to caller
    Return res
End Function