c# - Why IsNan is a static method on the Double class instead of an instance property?

Question

The question is in the title, why :

return double.IsNaN(0.6d) && double.IsNaN(x);

Instead of

return (0.6d).IsNaN && x.IsNaN;

I ask because when implementing custom structs that have a special value with the same meaning as NaN I tend to prefer the second.

Additionally the performance of the property is normally better as it avoid copying the struct on the stack to call the IsNaN static method (And as my property isn't virtual there is no risk of auto-boxing). Granted it isn't really an issue for built-in types as the JIT could optimize this easilly.

My best guess for now is that as you can't have both the property and the static method with the same name in the double class they favored the java-inspired syntax. (In fact you could have both as one define a get_IsNaN property getter and the other an IsNaN static method but it will be confusing in any .Net language supporting the property syntax)

Answer 1

Static Methods are thread safe, methods on primitives generally need to be thread safe to support threading in the platform (meaning at least safe from internal race conditions), instance methods take a managed pointer to a structure, meaning that the structure/primitive might be modified concurrently while the method executes, on the other hand static methods take a copy of the structure/primitive and therefore are safe from threading race conditions.

If the structure is intended to be thread safe, then the methods should be made instance methods only if they do atomic operations, else static methods should be chosen.

(As another option, instance methods that use locking could be used but they are more expensive than, copying)

Edit: @VirtualBlackFox I've prepared and example to show that instance methods on structures are not thread safe even on immutable structures:

using System;
using System.Threading;

namespace CA64213434234
{
    class Program 
    {
        static void Main(string[] args)
        {
            ManualResetEvent ev = new ManualResetEvent(false);
            Foo bar = new Foo(0);
            Action a =  () => bar.Display(ev);
            IAsyncResult ar = a.BeginInvoke(null, null);
            ev.WaitOne();
            bar = new Foo(5);
            ar.AsyncWaitHandle.WaitOne();
        }
    }

    public struct Foo
    {
        private readonly int val;
        public Foo(int value)
        {
            val = value;
        }
        public void Display(ManualResetEvent ev)
        {
            Console.WriteLine(val);
            ev.Set();
            Thread.Sleep(2000);
            Console.WriteLine(val);
        }
    }
}

The display Instance method prints: 0 5

even though the structure is immutable. For thread safe methods use static methods.