When to use @await Html.PartialAsync in a View in MVC 6

Question

I noticed on one of Scott Hanselman's blogs he uses the following code in his Views when using .Net 5 (MVC 6):

@await Html.PartialAsync("_LoginPartial") 

vs.

@Html.Partial("_LoginPartial") 

Is there any documentation yet on when which one should be used?

Answer 1

This is actually a pretty interesting question and scenario. To a certain extent, async is the new hotness (though it's really not all that new). Entity Framework 6 hit with async methods and every... single... piece... of... documentation... suddenly starting using async for everything. I think we're seeing a little of the same here. MVC 6 supports async for things like rendering partials, so OMG we've all just have to use async now.

Async serves one very specific purpose. It allows the active thread to be returned to the pool to field other tasks while the current task is in a wait state. The key part of that is "wait state". Certain tasks are just flat out incompatible with async. CPU-bound work like complex financial analysis never allows the thread to enter a wait state so everything is effectively run as sync even if you set it up as async. On the other hand, things involving network latency (requesting a resource from a web API, querying a database, etc.) or that are I/O bound (reading/writing files, etc.) can at times have periods where the thread is waiting around for some other process to complete before it continues processing.

Looking specifically at rendering a partial, the only piece that's not entirely CPU-bound is reading the view file itself from the filesystem. While that's technically enough to make it eligible for async, how long is it really going to take to read what's essentially a text file that's probably less than 50KB max. By the time the thread is handed back to the pool, it's probably time to request it back, so you're actually using resources more inefficiently at that point.

Long and short, don't fall into the trap of "it can be done async, so I must do it async". Each use should be evaluated in terms of whether there's actually value in it. Async has a lot of overhead, and if you're only talking about a few milliseconds of wait time, it's probably not worth all that extra overhead.

Answer 2

As per the ASP.NET MVC documentation on partial views. https://docs.asp.net/en/latest/mvc/views/partial.html

The PartialAsync method is available for partial views containing asynchronous code (although code in views is generally discouraged):

Also the note on the page.

If your views need to execute code, the recommended pattern is to use a view component instead of a partial view.

So you should use Partial and avoid PartialAsync, and if you find yourself with a PartialAsync you should question yourself whether you're doing something wrong, maybe you should be using a ViewComponent instead or move the logic from the view to the controller.