// Create the new BitmapSource that will be used to scale the size of the source.
TransformedBitmap myRotatedBitmapSource = new TransformedBitmap();

// BitmapSource objects like TransformedBitmap can only have their properties
// changed within a BeginInit/EndInit block.
myRotatedBitmapSource.BeginInit();

// Use the BitmapSource object defined above as the source for this BitmapSource.
// This creates a "chain" of BitmapSource objects which essentially inherit from each other.
myRotatedBitmapSource.Source = myBitmapImage;

// Flip the source 90 degrees.
myRotatedBitmapSource.Transform = new RotateTransform(90);
myRotatedBitmapSource.EndInit();