Before diving into the details, let's review why you should use a pattern like MVVM in the first place.It is unnecessary and counterproductive to use design patterns in a simple "Hello, World! Any competent developer can understand a few lines of code at a glance.However, as the number of features in a program increases, the number of lines of code and moving parts increase accordingly.

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It can be a murky blend of data, interaction design, visual design, connectivity, multithreading, security, internationalization, validation, unit testing, and a touch of voodoo.

Considering that a user interface exposes the underlying system and must satisfy the unpredictable stylistic requirements of its users, it can be the most volatile area of many applications.

There are popular design patterns that can help to tame this unwieldy beast, but properly separating and addressing the multitude of concerns can be difficult.

In this article, I'll review some of those best practices for designing and implementing client applications with WPF.

By leveraging some core features of WPF in conjunction with the Model-View-View Model (MVVM) design pattern, I will walk through an example program that demonstrates just how simple it can be to build a WPF application the "right way."By the end of this article, it will be clear how data templates, commands, data binding, the resource system, and the MVVM pattern all fit together to create a simple, testable, robust framework on which any WPF application can thrive.

The demonstration program that accompanies this article can serve as a template for a real WPF application that uses MVVM as its core architecture.

The unit tests in the demo solution show how easy it is to test the functionality of an application's user interface when that functionality exists in a set of View Model classes.

The more complicated the patterns are, the more likely that shortcuts will be used later on which undermine all previous efforts to do things the right way. Sometimes we use complicated design patterns, which require writing a lot of code because the UI platform in use does not lend itself well to a simpler pattern.

What's needed is a platform that makes it easy to build UIs using simple, time-tested, developer-approved design patterns.

Fortunately, Windows Presentation Foundation (WPF) provides exactly that.

As the software world continues to adopt WPF at an increasing rate, the WPF community has been developing its own ecosystem of patterns and practices.