Comparing Threads for equality

Question

I am looking for a way to explain that it's unreasonable to sprinkle high-level business logic with calls to ReferenceEquals().

Here's a code snippet that I have a problem with (precondition in a method, designed to throw if we're on a wrong thread):

if (!object.ReferenceEquals(Thread.CurrentThread, RequestHandlerThread))

Is it reliable to write this instead:

if (Thread.CurrentThread != RequestHandlerThread)

I suggested to use ManagedThreadIds in the comparison based on what I commonly see in tutorials. Adversary says that comparison for reference equality seems more object-oriented.

Here's (roughly) what I saw in Reflector's view of System.Object with .NET 4.0. Keep in mind that Thread class is sealed, and has no overload for operator==.

public static bool ReferenceEquals(object objA, object objB)
{
    return (objA == objB);
}

public static bool Equals(object objA, object objB)
{
    return (objA == objB || 
        (objA != null && objB != null && objA.Equals(objB)));
}

Here are some basic tests, verifying operation on thread pool... Did I miss any significant tests?

using System.Threading;
using System.Diagnostics;
using System.Threading.Tasks;

namespace ConsoleApplicationX
{
   class Program
   {
      static readonly Thread mainThread;

      static Program()
      {
         mainThread = Thread.CurrentThread;
      }

      static void Main(string[] args)
      {
         Thread thread = Thread.CurrentThread;
         if (thread != Thread.CurrentThread)
            Debug.Fail("");

         if(Thread.CurrentThread != thread)
            Debug.Fail("");

         if (thread != mainThread)
            Debug.Fail("");

         var task = Task.Factory.StartNew(() => RunOnBackground(thread));
         task.Wait();

         var anotherThread = new Thread(new ParameterizedThreadStart(RunInAnotherThread));
         anotherThread.Start(thread);
      }

      static void RunOnBackground(Thread fromInitial)
      {
         if (Thread.CurrentThread == fromInitial)
            Debug.Fail("");

         if (fromInitial != mainThread)
            Debug.Fail("");
      }

      static void RunInAnotherThread(object fromInitialAsObject)
      {
         var fromInitial = (Thread)fromInitialAsObject;

         if (Thread.CurrentThread == fromInitial)
            Debug.Fail("");

         if (fromInitial != mainThread)
            Debug.Fail("");
      }
   }
}

Answer 1

Trying not to preach, but ... In general, if there is an identity property to use, in this case

Thread.ManagedThreadId

it should be used for equality comparisons unless there is a performance constraint for the executing code. The semantics of a pointer comparison are a lot different than equality. Also, the pointer comparison is dependent upon the implementation of Thread in mscorlib.

Answer 2

You are correct. As your reflection inquiry shows, the two ways are equivalent (barring an extra method call in the adversary's way)