Programming WPF, 2nd Edition

Code Behind

Separate handling of appearance and behavior is an important design principle for keeping UI code manageable. To help with this separation, XAML supports the concept of code behind, where a XAML file has a corresponding source file containing executable code. The idea is that the XAML file defines the structure of the user interface, and the code-behind file provides its behavior.

Tip

The exact definition of behavior can look a little different in WPF applications compared with what you may be used to. It is possible to use styling and event triggers to make a UI respond automatically to simple stimuli. In older UI technologies, you would typically have used code to achieve this, but in WPF we normally use markup. Although this is behavior in the sense that it is something the application does in response to input, it is essentially superficial behavior—it is part of the look and feel of the application, rather than the functionality. This superficial behavior usually lives in the control template, and you can therefore replace it without altering the underlying behavior of the control, as described in Chapter 9.

In the context of code behind, behavior usually means the application functionality invoked when you click a button, rather than what the button looks like when it is clicked.

Of course, you won't want to put much application logic in your code behind if you care about maintainability and testability. In practice, the code behind is likely to act as the glue between ...