await task completion in a thread ends (breaks) this thread in C# .net6

have a problem when using await in a thread function.
It seems that await breaks the thread. Below the test class to demonstrate this.

    public class Question {
    public static async Task<double> testTask() {
        // [2]
        await Task.Run(() => {
            Console.WriteLine("start testTask {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
            for (int i = 0; i < 100; i++) {
                Task.Delay(10).Wait();
            }
            Console.WriteLine("end testTask {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
        });

        return 42.00;

    }

    static async Task ThreadFunction() {
        Console.WriteLine("ThreadFunction begin tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
        // [1]
        //double x = await test.testTask().ConfigureAwait(true);
        double x = await test.testTask();


        int inserted = 0;
        for (long i = 0; i < 100; i++)
            System.Threading.Thread.Sleep(100);

        // [3]
        Console.WriteLine("ThreadFunction end tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
    }


    public static void startTestThread() {

        Console.WriteLine("start startTestThread. tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
        Thread TestThread = new Thread(() => ThreadFunction());
        TestThread.Start();
        TestThread.Join();
        // [4]
        Console.WriteLine("end startTestThread. tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
    }
}

startTestThread() is called from outside, a console application
in the new manner without a main function.

When program reaches
double x = await test.testTask();
Thread "ThreadFunction" returns.
Rhis is what i do not understand.
The thread should wait for function
testTask()
to get completed. The function is called and Task started
but the thread gets interrupted.
So the program execution sequence is not:
1 2 [3] [4]
instead it is:
1 2 [4]

, so code at [3] not executed. Why is this so? Many thanks to the community

You are missing async and await in various places.

Furthermore, using Thread.Start to run a Task is wrong on so many levels: the thread will finish as soon as you hit the first await. Instead, if you want to run it synchronously, use Task.Run and Wait, but note that you may get a deadlock if you have a SynchronizationContext set up (not usually the case in a console app).

Ideally you shouldn't do sync-over-async ever. Take a read of this and this article for may reasons why this is a bad idea.

public class Question
{
    public static async Task<double> testTask()
    {
        // [2]
        await Task.Run(async () => {   // async HERE!!
            Console.WriteLine("start testTask {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
            for (int i = 0; i < 100; i++) {
                await Task.Delay(10);     // await HERE!!
            }
            Console.WriteLine("end testTask {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
        });

        return 42.00;
    }

    static async Task ThreadFunction()
    {
        Console.WriteLine("ThreadFunction begin tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
        // [1]
        //double x = await test.testTask().ConfigureAwait(true);
        double x = await test.testTask();

        int inserted = 0;
        for (long i = 0; i < 100; i++)
            await Task.Delay(100);  // await HERE!!

        // [3]
        Console.WriteLine("ThreadFunction end tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
    }

    public static void startTestThread() {

        Console.WriteLine("start startTestThread. tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
        Task.Run(ThreadFunction).Wait();
        // [4]
        Console.WriteLine("end startTestThread. tid={0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
    }
}

I have no idea what you are trying to test with ManagedThreadId but it's unlikely to be useful. If using tasks with await then you shouldn't care which thread you are on, the synchronization context will sort that out. And if you were using threads then of course you remain on the same thread, you are still in the same function call.