How do you send a refresh message to a WPF grid or canvas?

Question

How do you send a refresh message to a WPF grid or canvas?
In other words, I have noticed while in debug mode, I can write code that sends a line to the display and then, if that line is not right, I can adjust it -- but the previous line is still there.
Now, the code I am writing sends information to the display based on what the user clicks.  So this must mean that the display is not refreshed each time a new set of lines and boxes and text goes to the grid or canvas in WPF.
Using C# code, how do you send a refresh/repaint message to a WPF grid or canvas?