For best performance, graphics drawing commands need to be batched into groups so the GPU hardware can process as many operations as possible in parallel. Win2D achieves this through the CanvasDrawingSession API. A drawing session represents an active batch of commands that are in the process of being generated. Use CanvasDrawingSession's various methods to indicate what you want to draw, then when you are done close the CanvasDrawingSession. This will finalize the command batch, optimize it and submit it to the GPU hardware to be carried out. No actual rendering work is performed on the GPU until the CanvasDrawingSession is closed.

CanvasDrawingSession implements the IClosable interface; therefore, you close it by explicitly calling the Close method (which is projected into .NET as Dispose). If you are using C#, you can also use the 'using' statement. If you are using C++/CX, Close is accessed via the 'delete' keyword.

Drawing sessions can be obtained in various different ways. Most often, you should use the one that is passed in to the CanvasControl.Draw or CanvasAnimatedControl.Draw event handler. A drawing session obtained in this way is automatically closed after the event handler returns, so there is no need for you to do anything else. However, if you explicitly created a drawing session you must close it yourself when you are done drawing to it.

When using Direct2D interop, this Win2D class corresponds to the Direct2D interface ID2D1DeviceContext1.

Reference