SpinLock SynchronizationLockException - The calling thread does not hold the lock

Here is the code from a console app.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace ConsoleApplication48
{
    class Program
    {
        static readonly SpinLock SpinLock = new SpinLock();
        static void Main(string[] args)
        {
            bool lockTaken = false;
            try
            {
                SpinLock.Enter(ref lockTaken);
                if (lockTaken)
                    Console.WriteLine("Lock taken");
            }
            finally
            {
                if (lockTaken)
                    SpinLock.Exit();
            }
            Console.WriteLine("Done");
        }
    }
}

So why would this single-threaded app tell me that the thread trying to call SpinLock.Exit() doesn’t hold the lock?

SpinLock is a value type.  When you mark a field referencing a reference type as readonly you are not only making the field unassignable but also implicitly making the value immutable.  To achieve this .NET will give you a copy of the value each time you read it rather than returning the original, that way if you change anything you will in fact be changing a copy.  So the call to SpinLock.Enter will execute on one copy of the SpinLock, and SpinLock.Exit will execute on a second copy and thus because they are separate instances the call to Exit will throw an exception because the current thread doesn’t hold a lock on it.

The solution is simple; remove readonly.