Framebuffer -> Image

I am implementing a Phyton tool new in C#. The Phyton tool converts a framebuffer of a monochrome LCD display 240x128 into an image as follows:

image = QImage(buffer, LcdWidth, LcdHeight, QImage.Format_Grayscale8)

In my C# tool I tried it like this:

WriteableBitmap writeableBitmap = new WriteableBitmap(pixelWidth, pixelHeight, 72, 72, PixelFormats.Gray8, null);
writeableBitmap.WritePixels(new Int32Rect(0, 0, (int)writeableBitmap.Width, (int)writeableBitmap.Height), buffer, 1, 1);

Unfortunately, I don't get anywhere with this approach.

Thank you for any hints.