Disposing a static brush

Question

I'm writing a biorhythm app. To test it i have a form with a Button and a PictureBox. When I click on the button i do

myPictureBox.Image = GetBiorhythm2();

Which runs ok for the first time, but on the second click it causes the following exception:

System.ArgumentException: Parameter is not valid.
   at System.Drawing.Graphics.CheckErrorStatus
   at System.Drawing.Graphics.FillEllipse
   at Larifari.Biorhythm.Biorhythm.GetBiorhythm2 in c:\delo\Horoskop\Biorhythm.cs:line 157
   at Larifari.test.Button1Click in c:\delo\Horoskop\test.Designer.cs:line 169
   at System.Windows.Forms.Control.OnClick
   at System.Windows.Forms.Button.OnClick
   at System.Windows.Forms.Button.OnMouseUp
   at System.Windows.Forms.Control.WmMouseUp
   at System.Windows.Forms.Control.WndProc
   at System.Windows.Forms.ButtonBase.WndProc
   at System.Windows.Forms.Button.WndProc
   at ControlNativeWindow.OnMessage
   at ControlNativeWindow.WndProc
   at System.Windows.Forms.NativeWindow.DebuggableCallback
   at ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop
   at ThreadContext.RunMessageLoopInner
   at ThreadContext.RunMessageLoop
   at System.Windows.Forms.Application.Run
   at Larifari.test.Main in c:\delo\Horoskop\test.cs:line 20

the cut-down function which causes the error is :

public static Image GetBiorhythm2() {
        Bitmap bmp = new Bitmap(600, 300);
        Image img = bmp;
        Graphics g = Graphics.FromImage(img);

        Brush brush = Brushes.Black;
        g.FillEllipse(brush, 3, 3, 2, 2); //Here the exception is thrown on the second call to the function

        brush.Dispose(); //If i comment this out, it works ok.

        return img;
 }

if I comment-out the brush disposal it works ok, but I am not happy with that and wish to find an alternative solution. Can you help me please ?

Answer 1

It looks like you're trying to dispose of a static, which causes some problems next time it's used:

    Brush brush = Brushes.Black;
    g.FillEllipse(brush, 3, 3, 2, 2); //Here the exception is thrown on the second call to the function

    brush.Dispose(); //If i comment this out, it works ok.

When you set brush = Brushes.Black, you're actually setting brush as a reference (or pointer) to the static Brushes.Black. By disposing it, you're effectively writing:

    Brushes.Black.dispose();

When you come back around to use the black brush again, the runtime says you can't because it's already been disposed of, and isn't a valid argument to g.FillEllipse()

A better way to write this might be just simply:

    g.FillEllipse(Brushes.Black, 3, 3, 2, 2);

Or, if you want to be really complex about it:

    Brush brush = Brushes.Black.Clone();
    g.FillEllipse( brush, 3, 3, 2, 2 );
    brush.Dispose();

Or if you don't care about things looking wrong, just comment out the brush.Dispose(); line in your original code.