Question

I’m working with layered forms and I found a great example in Visual Basic .NET but came across a problem converting the source to C#. The Visual Basic.NET implementation uses the shadows modifier to effectively replace the Form Class’ Invalidate() method and then does the same for the Paint event.

Public Shadows Event Paint(ByVal G As Drawing.Graphics)

Public Shadows Sub Invalidate()
    Dim B As New Drawing.Bitmap(ClientSize.Width, ClientSize.Height)
    Dim G As Drawing.Graphics = Drawing.Graphics.FromImage(B) : G.SmoothingMode = Drawing.Drawing2D.SmoothingMode.AntiAlias

    RaiseEvent Paint(G)

    G.Dispose()
    SetBits(B)
    B.Dispose()
End Sub

I converted the code to C# so it looks like the following:

public new event PaintEventHandler Paint;

public new void Invalidate()
    {
        Bitmap b = new Bitmap(ClientSize.Width, ClientSize.Height);
        Graphics g = Graphics.FromImage(b);
        g.SmoothingMode = SmoothingMode.AntiAlias;

        Paint(this, new PaintEventArgs(g, Rectangle.Empty));

        g.Dispose();
        SetBits(b);
        b.Dispose();
    }

But when the method is invoked I receive a NullReferenceException – Object reference not set to an instance of an object.

I also tried a different implementation, but received the same exception.

    public delegate void PaintEventHandler(Graphics g);
    public event PaintEventHandler Paint;

Could somebody tell me what I’m doing wrong? Thanks.

Answer 1

You need to check for null before invoking the event:

if (Paint != null)   
    Paint(this, new PaintEventArgs(g, Rectangle.Empty));