What is the correct way to call an async method from synchronous code, and block until the task is complete? [duplicate]

What is the correct way to do this?

The correct way would be to do what you don't want to do, expose two different methods, one which is completely synchronous and another which is completely asynchronous.

Exposing async over sync and sync over async are poth anti-patterns when dealing with async code.

If the methods aren't aren't naturally async, i would expose a synchronous version only and let the invoker decide if he wants to wrap it with a Task.Run himself.

If the method is naturally async, i would expose the async method only. If you have both sync and async versions, expose them independently.

Async methods go "all the way", and they slowly bubble up and down the call stack as you use them. Embrace that fact, and you'll avoid many pitfalls along the way.

There is a Github project for that : AsyncBridge

Example included in the project homepage :

public async Task<string> AsyncString()
{
    await Task.Delay(1000);
    return "TestAsync";
}

public void Test()
{
    string string1 = "";
    string string2 = "";
    using (var A = AsyncHelper.Wait)
    {
        A.Run(AsyncString(), res => string1 = res);
        A.Run(AsyncString(), res => string2 = res);
    }
}

I used it on several project without issue.

How do I do a blocking wait.

// Looks like it might work, but what if I want to return a value?
DoStuffAsync().Wait();

And it does. Regarding the

what if I want to return a value?

Suppose you also had this async method that returned a Task<string>:

public async Task<string> MyStringGetterAsync() {
    return await GetStringAsync();
}  

You could to the following to perform a blocking wait and retrieve its result:

var myString = MyStringGetterAsync().Result;

With regards to your intention:

I want to provide two methods with the same functionality: an async method which runs asynchronously, and a non-async method that blocks, and either of these two methods can be called depending on whether the code I'm calling from is also async.

Creating a MyWhatEver() overload that just does MyWhatEverAsync.Wait() is not a recommended pattern, because it may have unintended side effects for clients calling that method, as explained e.g. here. The client calling your synchronous version of the method, might not expect its thread to be blocked as the result of calling your method, and cannot decide for itself if that is acceptable (e.g. it cannot specify ConfigureAwait). As mentioned by @Default in the comments, this could lead to deadlocks.

The same is generally true for the reverse case: i.e. having a MyWhatEverAsync() that only does return Task.Run(() => MyWhatEver()); If a client needs to, they could do this themselves and include e.g. a cancellation token, so they can control the task.

It is better to be explicit in your interface and only provide methods that do what their signature says they do.

For your real world problem (after question edit):

I have a UnitOfWork class that wraps the the SaveChanges() and SaveChangesAsync() methods of an EF6 DbContext. I also have a List<MailMessage> of emails that need to be sent only when the SaveChanges() succeeds.

And (from the comments)

The success of sending an email isn't critical.

Your current UnitOfWork design for the Save and SaveAsync methods is not correct, because it violates the atomicity promises it makes.

In the scenario where "SaveChanges" succeeds, but an exception occurs on sending one of the emails. The client code calling one of the Save variants, will observe that exception, will (and should) assume that saving the changes failed, and could retry applying them.

If a client was wrapping this operation in a transaction context with a two phase commit, the "SaveChanges" would be rolled back, and when a client retries the operation, it will attempt to send the emails again, potentially causing a number of recipients to receive the email multiple times, although the changes they received the email about were not committed successfully (and may never be if sending emails fails repeatedly).

So you will need to change something.

If you really don't care about the success of sending the emails, I suggest you add the following method.

private Task SendEmailsAndIgnoreErrors(IEmailService emailService, List<MailMessage> emailQueue) {
    return Task.WhenAll(emailQueue.Select(async (m) => {
        try {
            await emailService.SendAsync(m).ConfigureAwait(false);
        } catch {
            // You may want to log the failure though,
            // or do something if they all/continuously fail.
        }
    }));
}

And use it like this:

public async Task SaveAsync() {
    await _ctx.SaveChangesAsync();
    await SendEmailsAndIgnoreErrors(_email, _emailQueue);
}

I would personally omit the synchronous variant, and let clients decide how to wait on the task, but if you believe you really need it, you could do this:

public void Save() {
    _ctx.SaveChanges();
    SendEmailsAndIgnoreErrors(_email, _emailQueue).Wait().ConfigureAwait(false);
}