Reuse a IAsyncEnumerable instance without having to iterate again

Question

I am currently writing a method like this:

public async Task<UserModel> GetUserByUserIdAsync(string userId)
{
    IQueryable<UserEntity> usersQuery = BuildQueryable(userId);

    bool any = await usersQuery.ExecuteQuery().AnyAsync();
    
    if (!any) return null; // wanna do other logic in the future

    return (await usersQuery.ExecuteQuery().SingleAsync()).ToUserModel();
}

As you can see, I am calling the await usersQuery.ExecuteQuery() twice, and ExecuteQuery() is a method which iterates my database and could be considered an expensive operation. Is there any way I could save my IAsyncEnumerable<T> like I normally would with IEnumerable<T> and re-use it throughout my code?

I thought about using ToListAsync() on it, but I am unsure whether that is considered good practice or not. I've also read that I could return a Task<IAsyncEnumerable<T>> and do something with that maybe. What is the best way to handle this? I'd like to implement the most efficient solution.

Answer 1

Why not simply use SingleOrDefaultAsync? Assuming your entity is a reference type you can get your single item, check if it is null to handle the empty-case. Another alternative is always to convert the enumerable to a list. Then you can iterate over it however many times you want to.

Answer 2

In case there is any possibility for the single returned UserEntity to be null, and you want to differentiate between no-entity and one-null-entity, you could install the System.Linq.Async package and do this:

public async Task<UserModel> GetUserByUserIdAsync(string userId)
{
    IQueryable<UserEntity> usersQuery = BuildQueryable(userId);

    var (userEntity, exists) = await usersQuery
        .AsAsyncEnumerable()
        .Select(x => (x, true))
        .FirstOrDefaultAsync();

    if (!exists) return null; // wanna do other logic in the future
    return userEntity.ToUserModel();
}

This query exploits the fact that the default value of a ValueTuple<UserEntity, bool> is (null, false).

Using the AsAsyncEnumerable may not be as efficient as using the SingleOrDefaultAsync method though, because the data provider may create a less optimized execution plan.