C# - Misusing static variables

Question

I have a class like below (refactored for purpose) in a PR and one of the seniors who is now on holiday stated I was misusing static variables and I should pass variables from method to method.

class Class {
  static int dataPool;
  
  // the below methods are called cyclicly for the duration of the application, 
  // xxxx times, First method is called and then Second is called, 
  // over and over
  public static void First()
  {
    // at the start, reads dataPool count to do computation
    // at the end, set dataPool count based on computation
  }

  public static void Second()
  {
    // at the start, read dataPool count to do some computation
    // at the end, set dataPool count based on computation
  }
}

I want to understand why using variables like the above is 'bad' to learn. Can anyone explain please?

Answer 1

I want to understand why using variables like the above is 'bad' to learn. Can anyone explain please?

Main reasons:

• It means your code is no-longer re-entrant.
• It means your code is no-longer thread-safe (or even less thread-safe) and cannot run concurrently.
• It means your code has state in unexpected places (see: Principle of Least Astonishment ).
• It means your code will perform poorly in a multi-threaded system (even if it's strictly single-threaded and without reentrancy) as having mutable static-state means your compiler cannot optimize your program's use of memory because it cannot guarantee thread ownership of data. Cache coherency is expensive .
• It means you cannot correctly unit-test your code (using the strict definition of "unit test", not Visual Studio's).
• It means the JIT compiler cannot optimize your program as much as it could if it could otherwise because reasoning about the lifetime of static state is very difficult.
• It means your code could be broken if another function or thread decides to mutate your static state without you knowing it.