Asynchronous Programming

Asynchronous operations avoid blocking a thread while the query is executed in the database. Async operations are important for keeping a responsive UI in rich client applications, and can also increase throughput in web applications where they free up the thread to service other requests in web applications.

Following the .NET standard, EF Core provides asynchronous counterparts to all synchronous methods which perform I/O. These have the same effects as the sync methods, and can be used with the C# async and await keywords. For example, instead of using DbContext.SaveChanges, which will block a thread while database I/O is performed, DbContext.SaveChangesAsync can be used:

var blog = new Blog { Url = "http://sample.com" };
context.Blogs.Add(blog);
await context.SaveChangesAsync();

For more information, see the general C# asynchronous programming docs.

Warning

EF Core doesn't support multiple parallel operations being run on the same context instance. You should always wait for an operation to complete before beginning the next operation. This is typically done by using the await keyword on each async operation.

Note

EF Core passes cancellation tokens down to the underlying database provider in use (e.g. Microsoft.Data.SqlClient). These tokens may or may not be honored - consult your database provider's documentation.

Async LINQ operators

In order to support executing LINQ queries asynchronously, EF Core provides a set of async extension methods which execute the query and return results. These counterparts to the standard, synchronous LINQ operators include ToListAsync, SingleAsync, AsAsyncEnumerable, etc.:

var blogs = await context.Blogs.Where(b => b.Rating > 3).ToListAsync();

Note that there are no async versions of some LINQ operators such as Where or OrderBy, because these only build up the LINQ expression tree and don't cause the query to be executed in the database. Only operators which cause query execution have async counterparts.

Important

The EF Core async extension methods are defined in the Microsoft.EntityFrameworkCore namespace. This namespace must be imported for the methods to be available.

Client-side async LINQ operators

The async LINQ operators discussed above can only be used on EF queries - you cannot use them with client-side LINQ to Objects query. To perform client-side async LINQ operations outside of EF, use the System.Interactive.Async package; this package can be especially useful for performing operations on the client that cannot be translated for evaluation at the server.

Unfortunately, referencing System.Interactive.Async causes ambiguous invocation compilation errors on LINQ operators applied to EF's DbSets; this makes it hard to use both EF and System.Interactive.Async in the same project. To work around this issue, add AsQueryable to your DbSet:

var groupedHighlyRatedBlogs = await context.Blogs
    .AsQueryable()
    .Where(b => b.Rating > 3) // server-evaluated
    .AsAsyncEnumerable()
    .GroupBy(b => b.Rating) // client-evaluated
    .ToListAsync();