I had an interesting discussion today with another developer about how to approach a class with a method that accepts a string and outputs string.

Imagine something like the following which is completely made up for the purpose of example

public string GetStringPart(string input) {     //Some input validation which is removed for clarity     if(input.Length > 5)         return input.Substring(0,1);     if(input.Substring(0,1) == "B")         return input.Substring(0,3);     return string.empty; } 

A function which has some logic based on it’s string input is added to a project using DI and has a DI Container in place. Would you add this new class with an interface and inject it where needed, or would you make it a static class? What are the pros and cons of each? Why would you (or not) want to make this something used with constructor injection rather than just accessed when required anywhere.