Windows application very slow when using transparency

Question

I am writing a windows application with C# in visual studio .net 2005.

In the form , there are some control with transparent Background; the form opens maximised and with full screen background.

The application runs very slow with high CPU usage.

Why is this?

Answer 1

1. Solution using property DoubleBuffered

Sidenote: Only works if you have access to the control as DoubleBuffered is a protected property of Control. Similar as solution 2 (see code behind).

// from within the control class
this.DoubleBuffered = true;

// from the designer, in case you are utilizing images
control.BackgroundImageLayout = System.Windows.Forms.ImageLayout.None;

System.Windows.Forms.Control.DoubleBuffered System.Windows.Forms.Control.BackgroundImageLayout

2. Alternative solution using SetStyle + OptimizedDoubleBuffer:

Sidenote: The control paints itself, window message WM_ERASEBKGND is ignored to reduce flicker, and the control is first drawn to a buffer rather than directly to the screen.

control.SetStyle(UserPaint | AllPaintingInWmPaint | OptimizedDoubleBuffer, true);

System.Windows.Forms.Control.SetStyle(ControlStyles, Boolean) System.Windows.Forms.Control.ControlStyles

3. Alternative solution using SetStyle + DoubleBuffer:

Sidenote: Similar as OptimizedDoubleBuffer, due to legacy reasons it remained in the codebase.

control.SetStyle(UserPaint | AllPaintingInWmPaint | OptimizedDoubleBuffer, true);

System.Windows.Forms.Control.SetStyle(ControlStyles, Boolean) System.Windows.Forms.Control.ControlStyles

Answer 2

Thats because the GDI+ transparency implemented in .NET 2 is not ideally implemented, as Bob Powell explains.