Command Query Separation Principle

Every [[method]] should either be a [[command]] that performs an action, or a [[query]] that returns data to the caller, but not both.

In other words, asking a question should not change the answer.

There are two major types of functions: - Functions which perform commands - Functions which perform a query and return a result

If we have a function that logs you in and returns the result. We cannot really say if the user was already logged in - etc.

public bool LogOn(string username, string password) { }
if (LogOn("admin", "qwerty")) { }

We should implement this way:

public void LogOn(string username, string password) { }
public bool IsLoggedOn(string username, string password) { }