Maximum number of threads in a .NET app?

Question

There is no inherent limit. The maximum number of threads is determined by the amount of physical resources available. See this article by Raymond Chen for specifics.

If you need to ask what the maximum number of threads is, you are probably doing something wrong.

[Update: Just out of interest: .NET Thread Pool default numbers of threads:

• 1023 in Framework 4.0 (32-bit environment)
• 32767 in Framework 4.0 (64-bit environment)
• 250 per core in Framework 3.5
• 25 per core in Framework 2.0

(These numbers may vary depending upon the hardware and OS)]

Mitch is right. It depends on resources (memory).

Although Raymond's article is dedicated to Windows threads, not to C# threads, the logic applies the same (C# threads are mapped to Windows threads).

However, as we are in C#, if we want to be completely precise, we need to distinguish between "started" and "non started" threads. Only started threads actually reserve stack space (as we could expect). Non started threads only allocate the information required by a thread object (you can use reflector if interested in the actual members).

You can actually test it for yourself, compare:

    static void DummyCall()
    {
        Thread.Sleep(1000000000);
    }

    static void Main(string[] args)
    {
        int count = 0;
        var threadList = new List<Thread>();
        try
        {
            while (true)
            {
                Thread newThread = new Thread(new ThreadStart(DummyCall), 1024);
                newThread.Start();
                threadList.Add(newThread);
                count++;
            }
        }
        catch (Exception ex)
        {
        }
    }

with:

   static void DummyCall()
    {
        Thread.Sleep(1000000000);
    }

    static void Main(string[] args)
    {
        int count = 0;
        var threadList = new List<Thread>();
        try
        {
            while (true)
            {
                Thread newThread = new Thread(new ThreadStart(DummyCall), 1024);
                threadList.Add(newThread);
                count++;
            }
        }
        catch (Exception ex)
        {
        }
    }

Put a breakpoint in the exception (out of memory, of course) in VS to see the value of counter. There is a very significant difference, of course.

i did a test on a 64bit system with c# console, the exception is type of out of memory, using 2949 threads.

I realize we should be using threading pool, which I do, but this answer is in response to the main question ;)

You should be using the thread pool (or async delgates, which in turn use the thread pool) so that the system can decide how many threads should run.

I would recommend running ThreadPool.GetMaxThreads method in debug

ThreadPool.GetMaxThreads(out int workerThreadsCount, out int ioThreadsCount);

Docs and examples: https://docs.microsoft.com/en-us/dotnet/api/system.threading.threadpool.getmaxthreads?view=netframework-4.8

Jeff Richter in CLR via C#:

"With version 2.0 of the CLR, the maximum number of worker threads default to 25 per CPU in the machine and the maximum number of I/O threads defaults to 1000. A limit of 1000 is effectively no limit at all."

Note this is based on .NET 2.0. This may have changed in .NET 3.5.

[Edit] As @Mitch pointed out, this is specific to the CLR ThreadPool. If you're creating threads directly see the @Mitch and others comments.