Files and Classes: Organize by Use
The default organization method Visual Studio uses when adding classes is to put each class in a separate file. That doesn't mean you should do the same thing. This is especially true in Visual Studio 2012, which combines Class View with the Solution Explorer standard File View (and adds a search capability on top of that).
Even in earlier versions of Visual Studio, you can always get to a class's code just by clicking on the class name in your code and pressing F12. With those tools in place, it's hard to see much advantage in being able to scroll to the class file in the Solution Explorer file view. Keeping each class in a separate file is an option, not a requirement.
Off the top of my head, I can think of at least two occasions where it makes sense to put multiple classes in the same file. First, where you have a class that's used only by one other class, it's probably easier for everybody if you keep those two classes in a single file. Second, EventArgs classes that are generated in one class and returned from that class's events might be best kept in the same file with the class that generates it.
I bet some of you can think of other occasions where it makes more sense to put two classes in the same file than it does to put them in separate files. That's OK.
Posted by Peter Vogel on 10/07/2014