JetBrains annotations:
Indicates that a method does not make any observable state changes. The same as System.Diagnostics.Contracts.PureAttribute
Microsoft Code Contracts:
Indicates that a type or method is pure, that is, it does not make any visible state changes.
When operating with primitive types it is very easy to know if your method is a pure function or not.
Consider this scenario however:
public class Program
{
public static void Main()
{
var dodgy = new DodgyClass();
string.Format("{0}", dodgy);
}
}
public class DodgyClass
{
public int State = 0;
public override string ToString()
{
State = new Random().Next();
return State.ToString();
}
}
ReSharper tells me that "return value of pure method is not used" which is obviously incorrect since the state does change and in a very visible way. Unless of course they mean that any static method is always pure? Otherwise none of LINQ extension methods are pure since I can do all sort of state manipulation when overriding GetEnumerator().
