When to use static classes in C# and why they are not always bad?

Hi everyone! I'm a beginner C# developer (studying for about 4 months). I think I've grasped the basics of OOP: classes, objects, inheritance, polymorphism. But there's one topic that confuses me — static classes. In tutorial examples, I often see them used for helpers or constants. For example, I wrote this class for mathematical operations: ```csharp public static class MathHelper { public static double CircleArea(double radius) { return Math.PI * radius * radius; } public static double CelsiusToFahrenheit(double celsius) { return celsius * 9 / 5 + 32; } } ``` But then I read on forums that static classes violate OOP principles (specifically Dependency Injection and testability), and they should be avoided. They say