When trying to write reliable, maintainable software, it’s important to try and minimize duplication. When you find yourself writing the same magic number in multiple places, you move it into a named constant to eliminate the duplication. When you find yourself writing the same expression multiple times, you move it into a named function, or perhaps a property. Sometimes, you find yourself dealing with higher level duplication – entire concepts and patterns that you find yourself writing again and again. Many modern languages provide tools to help you eliminate this kind of duplication: class inheritance allows you to share common logic between classes by inheriting that logic from a parent class, and generic types allow you to apply a generic pattern to multiple individual types. Both of these tools work with the aid of the compiler, so they interact well with debugging tools and allow you to catch most errors at compile time. Sometimes, though, you run into duplication that can’t easily be eliminated, even with the aid of the compiler.
In many graphical applications, a common pattern is the need to remember user preferences or inputs. Modern languages provide useful tools that can make this task simpler – for example, both Java and C# provide ways to automatically serialize an entire data structure into XML, which eliminates the need to manually read/write your preference values. Unfortunately, even with the aid of these features, you still find yourself with a lot of duplication – even if the runtime can automatically serialize your data structure, you still have to fill the structure out with your preferences, or read them back in manually. I’m going to demonstrate how you can use a little-known aspect of the new LINQ feature in C# 3.5 to eliminate duplication with the aid of the compiler.
As an example, we’ll be using the simple application shown above. It’s a Windows Forms dialog box, with a few different controls, set up to remember user inputs between sessions. You can download the source code for this application below:
If you run the application, input some values, and click OK, you’ll find that it stores your inputs in a JSON file in the %AppData%\PreferencesExample folder. If you run it again, you’ll see that it pulls your inputs back out of the JSON file to populate the UI.
