C# – Valid Uses of Static Classes

I noticed that nearly every time I see programmers using static classes in object oriented languages such as C#, they are doing it wrong. The major problems are obviously the global state and the difficulty to swap implementations at runtime or with mocks/stubs during tests.

By curiosity, I've looked at a few of my projects selecting ones which are well tested and when I made an effort to actually think about architecture and design. The two usages of static classes I've found are:

• The utility class—something I would rather avoid if I was writing this code today,

• The application cache: using static class for that is plainly wrong. What if I want to replace it by MemoryCache or Redis?

Looking at .NET Framework, I don't see any example of a valid usage of static classes either. For example:

• File static class makes it really painful to switch to alternatives. For example, what if one needs to switch to Isolated Storage or store files directly in memory or needs a provider which can support NTFS transactions or at least be able to deal with paths longer than 259 characters? Having a common interface and multiple implementations appears as easy as using File directly, with the benefit of not having to rewrite most of the code base when requirements change.

• Console static class makes testing overly complicated. How should I ensure within a unit test that a method outputs a correct data to console? Should I modify the method to send the output to a writeable Stream which is injected at run time? Seems that a non-static console class would be as simple as a static one, just without all the drawbacks of a static class, once again.

• Math static class is not a good candidate either; what if I need to switch to arbitrary precision arithmetic? Or to some newer, faster implementation? Or to a stub which will tell, during a unit test, that a given method of a Math class was indeed called during a test?

On Programmer.SE, I've read:

• Don't Use “Static” in C#? Some answers are quite against static classes. Others assert that “Static methods are fine to use and have a rightful place in programming.”, but don't bake it with arguments.

• When to use a Singleton and when to use a static class. Utility classes are mentioned, given that I mentioned above that utility classes are problematic.

• Why and when should I make a class 'static'? What is the purpose of 'static' keyword on classes? The only valid usage which is mentioned is a container for extension methods. Great.

Aside extension methods, what are the valid uses of static classes, i.e. cases where Dependency Injection or singletons are either impossible or will result in a lower quality design and harder extensibility and maintenance?