Entity Framework has two methods for converting a query to a list or an array: ToListAsync and ToArrayAsync. Of course there is also the sync version of these methods: ToList and ToArray. And sometimes, you should use the latter one!
The bug
In one of my projects, I have the following code:
public static async Task<IPagedList<T>> ToPagedListAsync<T>(this IQueryable<T> source, int page, int pageSize, CancellationToken token = default)
{
var count = await source.CountAsync(token);
if (count > 0)
{
var items = await source
.Skip((page - 1) * pageSize)
.Take(pageSize)
.ToListAsync(token);
return new PagedList<T>(items, count, page, pageSize);
}
return PagedList<T>.Empty;
}
That looks fairly trivial - and of course we want ToListAsync here, or do we? Well: Not necessarily! To give a bigger picture, this call is used in that very blog post you are reading right now. And the datamodel of a blog post has the following column: Content NVARCHAR(MAX) NOT NULL. NVARCHAR(MAX) (as well as VARBINARY(MAX) and friends) cause a known issue with Entity Framework since almost 10 years: Performance degradation. The async version spawns a whole lot of tasks, see the stackoverflow article.
If you want to follow this in more depth: Reading large data (binary, text) asynchronously is extremely slow .
The fix
The fix is fairly simple: Just use the synchronous version:
public static async Task<IPagedList<T>> ToPagedListAsync<T>(this IQueryable<T> source, int page, int pageSize, CancellationToken token = default)
{
var count = await source.CountAsync(token);
if (count > 0)
{
// I tried ToListAsync and it performed just poorly!
// Mainly because we have a VARCHAR(max) column
// See here: https://stackoverflow.com/questions/28543293/entity-framework-async-operation-takes-ten-times-as-long-to-complete/28619983
var items = source
.Skip((page - 1) * pageSize)
.Take(pageSize)
.ToList();
return new PagedList<T>(items, count, page, pageSize);
}
return PagedList<T>.Empty;
}
And yes, adding a comment here is absolutely necessary and okay! Especially because CountAsync is still async and the whole method is marked as async.
What is the takeaway?
Well, check if you are handling large binary data via ToListAsync in your selector? If so, maybe you are suffering from the issue!
