ThreadStatic v.s. ThreadLocal<T>: is generic better than attribute?

[ThreadStatic] is defined using attribute while ThreadLocal<T> uses generic. Why different design solutions were chosen? What are the advantages and disadvantages of using generic over attributes in this case?

Something the blog post noted in the comments doesn't make explicit, but I find to be very important, is that [ThreadStatic] doesn't automatically initialize things for every thread. For example, say you have this:

[ThreadStatic]
private static int Foo = 42;

The first thread that uses this will see Foo initialized to 42. But subsequent threads will not. The initializer works for the first thread only. So you end up having to write code to check if it's initialized.

ThreadLocal<T> solves that problem by letting you supply an initialization function (as Reed's blog shows) that's run before the first time the item is accessed.

In my opinion, there is no advantage to using [ThreadStatic] instead of ThreadLocal<T>.

ThreadStatic Initialize only on first thread, ThreadLocal Initialize for each thread. Below is the simple demonstration:

    public static ThreadLocal<int> _threadlocal =
        new ThreadLocal<int>(() =>
        {
            return Thread.CurrentThread.ManagedThreadId;
        });

    public static void Main()
    {
        new Thread(() =>
        {
            for (int x = 0; x < _threadlocal.Value; x++)
            {
                Console.WriteLine("First Thread: {0}", x);
            }
        }).Start();

        new Thread(() =>
        {
            for (int x = 0; x < _threadlocal.Value; x++)
            {
                Console.WriteLine("Second Thread: {0}", x);
            }
        }).Start();

        Console.ReadKey();
    }

The main idea behind ThreadStatic is to maintain a separate copy of the variable for each thread.

class Program
    {
        [ThreadStatic]
        static int value = 10;

        static void Main(string[] args)
        {
            value = 25;

            Task t1 = Task.Run(() =>
            {
                value++;
                Console.WriteLine("T1: " + value);
            });
            Task t2 = Task.Run(() =>
            {
                value++;
                Console.WriteLine("T2: " + value);
            });
            Task t3 = Task.Run(() =>
            {
                value++;
                Console.WriteLine("T3: " + value);
            });

            Console.WriteLine("Main Thread : " + value);

            Task.WaitAll(t1, t2, t3);
            Console.ReadKey();
        }
    }

In the above snippet, we have a separate copy of value for each thread, including the main thread.

So, a ThreadStatic variable will be initialized to its default value on other threads except the thread on which it is created.

If we want to initialize the variable on each thread in our own way, use ThreadLocal.