A common problem with many c# codebases I see is a lot reliance on static classes and developers who say "we can't unit test our code because we need static classes to make it easy to do stuff". This is the kind of code where you see static constructors and singletons are everywhere.

For example, a logger class which instantiates a Windows event log object in the static constructor but could just as easily created a new file, read from a file or made a network call to initialize code before we have a chance to intervene for testing purposes.

// made up logging example.
public static class Logger
{
	private static EventLog _loggerInstance;
	
	static Logger()
	{	
		// create instances, open files, make network calls....
		_loggerInstance = new EventLog();
	}
	
	public static void Log(string message)
	{	
		_loggerInstance.Log(message, Information);
	}
}

...

// used like this...

Logger.Log("Hello world");

There is a little refactoring that we can use to make this situation a little bit more testable while still preserving the horrific static outer we present to the world. I don't know if there's an official term for it but I like to think of it as splitting the code into a testable core object, containing the subtle code we are worried about, and a static wrapper that we are fairly sure we won't mess up.

Bit by bit we can create a new instance of a core class and gradually move properties and methods across to that class, forwarding those static calls to the instance we create in the static constructor.

public class InstanceLogger
{
	private readonly EventLog _eventLog;
	
	public InstanceLogger()
	{
		this._eventLog = new EventLog();
	}
	
	public void Log(string message)
	{
		this._eventLog.Log(message, Information);
	}
}

public static class Logger
{
	private static InstanceLogger _loggerInstance;
	
	static Logger()
	{	
		_loggerInstance = new InstanceLogger();
	}
	
	public static void Log(string message)
	{	
		_loggerInstance.Log(message);
	}
}

This way, we can write tests against instances of the core class and be sure the functionality is what we expect but also preserve the interface to the outside world with the simpler implementation in the static wrapper class via delegation to the core.