Confused with the flow of async / await functions

Question

I have the following classes:

DataAccessFactory:

public class DataAccessFactory
{
    public static IUserAccessLayer User() => new UserAccessLayer(new DataContext());
    public static IAuthenticationAccessLayer Authentication() => new AuthenticationAccessLayer(new DataAccess.DataContext());
}

AuthenticationAccessLayer:

public class AuthenticationAccessLayer : IAuthenticationAccessLayer
{
    private readonly DataContext context;

    public AuthenticationAccessLayer(DataContext context)
    {
        this.context = context;
    }

    public async void RegisterAsync(UserRegisterModel model)
    {
        context.User.Add(new UserModel()
        {
            EmailAddress = model.Email,
            PasswordHash = model.PasswordHash,
            PasswordSalt = model.PasswordSalt
        });
    }

    public async Task<bool> EmailExist(string email)
    {
        var user = await context.User.Where(x => x.EmailAddress.Equals(email)).FirstOrDefaultAsync();
        if (user == null)
            return false;
        else
            return true;
    }
}

UserStore:

public class UserStore : ViewModelBase
{
    public UserStore()
    {
    }

    public UserStore(int userID)
    {
        this.UserID = userID;
    }

    #region Authentication

    public async Task<bool> AuthenticateAsync(LoginModel model)
    {
        return await DataAccessFactory.Authentication().LoginAsync(model);
    }

    public async void RegisterUserAsync(UserRegisterModel model)
    {
        var store = DataAccessFactory.Authentication();

        //check if unique email
        if(await store.EmailExist(model.Email))
            throw new ValidationException($"Email {model.Email} is already registered.");

        store.RegisterAsync(model);
    }

    #endregion

}

My question is, in the UserStore, in the RegisterUserAsync function, will the UserRegisterModel get added to the DB before the EmailExist function returns or throw an exception?

Answer 1

No, your RegisterUserAsync method will be executed after the EmailExist method returns.

msdn

The await operator is applied to a task in an asynchronous method to suspend the execution of the method until the awaited task completes. The task represents ongoing work.
...
An await expression does not block the thread on which it is executing. Instead, it causes the compiler to sign up the rest of the async method as a continuation on the awaited task. Control then returns to the caller of the async method. When the task completes, it invokes its continuation, and execution of the async method resumes where it left off.

Answer 2

It will throw the ValidateException if the email exists, because you are awaiting the EmailExist function.

Answer 3

It will depend on the thread scheduler probably. However, if you define RegisterUserAsync to be something like

    public async Task RegisterUserAsync(UserRegisterModel model)
    {
        var store = DataAccessFactory.Authentication();

        //check if unique email
        if(await store.EmailExist(model.Email))
            throw new ValidationException($"Email {model.Email} is already registered.");

        await store.RegisterAsync(model);
    }

Then RegisterAsync will execute after EmailExist. (notice the Task return type and the await keyword.)