Get ImageFormat from System.Drawing.Image.RawFormat

This code fails when trying to call Image.Save(MemoryStream, ImageFormat).

I get the exception:

a Value cannot be null.Parameter name: encoder"

ImageFormat format = generatedImage.RawFormat as ImageFormat;
image.ImageData = generatedImage.Save(format);

It works if I pass in an ImageFormat object directly e.g. ImageFormat.Jpeg.

What is the best way of converting the rawformat to ImageFormat (ideally without a switch statement or lots of if statements)

Thanks Ben