When I try your code, it throws an ArgumentException saying
The input WriteableBitmap needs to have the Pbgra32 pixel format. Use the BitmapFactory.ConvertToPbgra32Format method to automatically convert any input BitmapSource to the right format accepted by this class.\r\nParametername: writeableBitmap
Hence this works:
var bitmap = new WriteableBitmap(400, 400, 96, 96, PixelFormats.Pbgra32, null);
WriteableBitmapExtensions.DrawLine(bitmap, 10, 10, 300, 300, Colors.Black);
image.Source = bitmap;
UPDATE: As noted by Anders, you should perhaps use the portable bitmap factory method provided by WriteableBitmapEx to create your bitmap:
var bitmap = BitmapFactory.New(400, 400);
WriteableBitmapExtensions.DrawLine(bitmap, 10, 10, 300, 300, Colors.Black);
image.Source = bitmap;