What do you suggest to a developer to improve testability?

To improve testability of code in C#, the following approaches can be suggested:

• Dependency Injection — instead of creating dependencies inside the class, pass them through the constructor or properties. This makes it easy to replace real dependencies with mocks or stubs in tests.

• Interfaces — program against interfaces rather than concrete implementations. This simplifies the creation of test doubles.

• Separation of concerns — follow the Single Responsibility Principle (SRP) so that classes and methods are small and easily testable.

• Avoid static methods and singletons — they hinder isolation and mocking.

• Use mocking frameworks — such as Moq, NSubstitute, which simplify creating stubs.

Example of dependency injection:

public interface ILogger {
    void Log(string message);
}

public class Service {
    private readonly ILogger _logger;
    public Service(ILogger logger) {
        _logger = logger;
    }
    public void DoWork() {
        _logger.Log("Work started");
        // ...
    }
}

In tests, you can pass a mock ILogger to verify calls without actual logging.