#nullable enable

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using System;

using System.Threading;

using System.Threading.Tasks;

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namespace Test

{

    class Program

    {

        class CustomSynchronizationContext : SynchronizationContext { };

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        static readonly SynchronizationContext context = new CustomSynchronizationContext();

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        static void SetContextAndTrace(string id)

        {

            SynchronizationContext.SetSynchronizationContext(context);

            Console.WriteLine($"{id}: {Thread.CurrentThread.ManagedThreadId}");

        }

        static async Task GoAsync(bool continueOnCapturedContext)

        {

            SetContextAndTrace("G1");

            var tcs = new TaskCompletionSource<bool>();

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            async Task funcAsync()

            {

                SetContextAndTrace("F1");

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                await tcs.Task

                    .ContinueWith(_ => SetContextAndTrace("F2"), TaskContinuationOptions.ExecuteSynchronously)

                    .ConfigureAwait(continueOnCapturedContext);

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                SetContextAndTrace("F3");

            }

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            var task = funcAsync();

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            new Thread(_ =>

            {

                SetContextAndTrace("T1");

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                tcs.TrySetResult(true);

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                SetContextAndTrace("T2");

                Thread.Sleep(1000);

            }).Start();

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            await task

                .ContinueWith(_ => SetContextAndTrace("G2"), TaskContinuationOptions.ExecuteSynchronously)

                .ConfigureAwait(continueOnCapturedContext);

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            SetContextAndTrace("G3");

        }

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        static void RunTest(bool continueOnCapturedContext)

        {

            Console.WriteLine(new String('-', 80));

            Console.WriteLine($"continueOnCapturedContext: {continueOnCapturedContext}");

            SetContextAndTrace("R1");

            GoAsync(continueOnCapturedContext).Wait();

            SetContextAndTrace("R2");

        }

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        static void Main()

        {

            RunTest(continueOnCapturedContext: false);

            RunTest(continueOnCapturedContext: true);

        }

    }

}

​