C# .net wrapper for tessnet

Question

I try to write wrapper for tessnet OCR library. I am getting an error saying "Cannot marshal 'return value': Generic types cannot be marshaled." for this code section

List<Word> k = OCRWrapper.DoOCR(new Bitmap(@"C:\Data\pdf2image\auto.png"), new Rectangle(0, 0, 55, 27));

My wrapper class is

class OCRWrapper
{
    [DllImport("TrueMarble.dll")]
    public static extern List<Word> DoOCR(Bitmap b, Rectangle rec);
}

please help me, can any one guide me to write this code

Thanx!