Surface.GetGraphics() in WindowsMobile.DirectX.Direct3D always throw an exception2

My code is2:

Texture texture = new Texture(device, 256, 256, 0, Usage.None, Format.R5G6B5, Pool.SystemMemory);
Surface s = texture.GetSurfaceLevel(0);

Graphics g = s.GetGraphics()); //Exception here

no matther how i change the parameters in creat texture ,It always throw an invalid call exception any help?

[342 byte] By [rapbbit] at [2008-1-1]