SynchronizationContextExtensions - Task<T> ExecuteAsync<T>

SynchronizationContextExtensions.cs

public static class SynchronizationContextExtensions {

    public static Task<T> ExecuteAsync<T> (this SynchronizationContext synchronizationContext, Func<T> factory, CancellationToken cancellationToken = default (CancellationToken)) {
        var state = new State<T> (factory,
                                    cancellationToken);
        synchronizationContext.Post (ExecuteAsync<T>,
                                        state);
        return state.Task;
    }

    private static void ExecuteAsync<T> (object state) {
        var asyncState = (State<T>)state;
        var taskCompletionSource = asyncState.TaskCompletionSource;

        var cancellationToken = asyncState.CancellationToken;
        if (cancellationToken.IsCancellationRequested) {
            taskCompletionSource.TrySetCanceled (cancellationToken);
            return;
        }

        try {
            var value = asyncState.Factory ();
            taskCompletionSource.TrySetResult (value);
        }
        catch (Exception exception) {
            taskCompletionSource.TrySetException (exception);
        }
    }

    private sealed class State<T> {

        public State (Func<T> factory, CancellationToken cancellationToken) {
            CancellationToken = cancellationToken;
            TaskCompletionSource = new TaskCompletionSource<T> (factory);
        }

        public CancellationToken CancellationToken { get; }

        public TaskCompletionSource<T> TaskCompletionSource { get; }

        public Task<T> Task => TaskCompletionSource.Task;
        public Func<T> Factory => (Func<T>)Task.AsyncState;

    }

}