You can make a nullable type to allow a data type that normally cannot accept the value Nothing accept that value. For example, value types such as structures and integers cannot have the value Nothing. When you make such a type nullable, you allow it to take the value Nothing.

To make a data type nullable, add a question mark either after the variable's name or the data type's name as in the following declarations.

    Dim num_points As Integer?
    Dim num_customers? As Integer

To see if a nullable variable contains a value, use its HasValue property. If the variable has a value, use its Value property to get that value.

The following DrawStar method draws a star. The num_points, star_pen, and star_brush parameters can all be Nothing. The star_pen and star_brush parameters can be Nothing because they are object references and those can always be Nothing. The num_points parameter is a nullable integer.

' Draw a star.
Private Sub DrawStar(ByVal gr As Graphics, ByVal num_points _
    As Integer?, ByVal bounds As RectangleF, ByVal star_pen _
    As Pen, ByVal star_brush As Brush)
    ' See if num_points is nothing.
    Dim number_of_points As Integer
    If (num_points.HasValue) Then
        number_of_points = num_points.Value
    Else
        number_of_points = 5
    End If

    ' Make room for the points.
    Dim points(0 To 2 * number_of_points - 1) As PointF

    Dim rx1 As Double = bounds.Width / 2
    Dim ry1 As Double = bounds.Height / 2
    Dim rx2 As Double = rx1 * 0.5
    Dim ry2 As Double = ry1 * 0.5
    Dim cx As Double = bounds.X + rx1
    Dim cy As Double = bounds.Y + ry1

    ' Start at the top.
    Dim theta As Double = -Math.PI / 2
    Dim dtheta As Double = Math.PI / number_of_points
    For i As Integer = 0 To 2 * number_of_points - 1 Step 2
        points(i) = New PointF( _
            CDbl(cx + rx1 * Math.Cos(theta)), _
            CDbl(cy + ry1 * Math.Sin(theta)))
        theta += dtheta

        points(i + 1) = New PointF( _
            CDbl(cx + rx2 * Math.Cos(theta)), _
            CDbl(cy + ry2 * Math.Sin(theta)))
        theta += dtheta
    Next i

    ' Fill the star.
    If (star_brush IsNot Nothing) Then _
        gr.FillPolygon(star_brush, points)

    ' Draw the star.
    If (star_pen Is Nothing) Then star_pen = Pens.Black
    gr.DrawPolygon(star_pen, points)
End Sub

Next the code checks the star_brush parameter and fills the star if the brush isn't null.

It then checks the star_pen parameter. If star_pen is null, the program sets it to the stock black pen. Finally the code draws the star using the pen.