I tried everything, but the WriteableBitmap class just doesn't have the .CopyPixels() method like it is said in the droumentation:https://msdn.microsoft.com/de-de/library/system.windows.media.imaging.writeablebitmap_methods(v=vs.110).aspx
Example of what Microsoft Visual Studio 2015 underlines red:
writeableBitmap.CopyPixels(new Int32Rect(0, 0, 20, 10),pixels, 20 * 4, 0);