EF Core DbContext Where Async

Question

I'm trying to execute a query that contains a WHERE as an Async function. Just like using the FirstAsync action, but there's no WhereAsync so I was wondering if there is some workaround.

I have an ApplicationRepository object that has an GetEntitiesAsync function and I tried this:

public async Task<IEnumerable<TEntity>> GetEntitiesAsync<TEntity>(Func<TEntity, bool> selector) where TEntity : class => 
            await _context.Set<TEntity>().Where(selector).AsQueryable().ToArrayAsync();

However, this line of code throws an exception:

System.InvalidOperationException: The source IQueryable doesn't implement IAsyncEnumerable<OneStopApp.Models.CustomForm>. Only sources that implement IAsyncEnumerable can be used for Entity Framework asynchronous operations.

Answer 1

There is an ToListAsync method that can be called asynchronously to get the data.

var list = await db.Accounts.Where(x => true).ToListAsync();

Fetching the data takes the bulk of time, therefore async is not on the Where method, but it is on methods that get the data like ToArrayAsync or FirstAsync .

Answer 2

The Where clause doesn't actually do anything, it's deferred execution. You can just use FirstAsync , ToListAsync , or ToArrayAsync with Where .

In your code, you should remove the AsQueryable() part. Without it, you should be OK:

await _context.Set<TEntity>().Where(selector).ToArrayAsync();

And yes, you should use an Expresion instead of Func . It's possible the DbSet or DbContext doesn't offer an overload for Where that accepts Func . That' pretty common, actually.