Avoid allocations from LINQ.ToArray()

[paulomorgado]

Enumerable.Array() enumerates and allocates items for no real purposes in this use case.

[JamesNK]

Why change this to use foreach + Reverse?

[paulomorgado]

After removing the invocation of ToArray(), it made more sense to me to move the invocation of Reverse() to its usage.

[paulomorgado]

I'm sorry, but I don't get your point.

[JamesNK]

The goal is to avoid allocations from the Reverse, right? Why not remove it and loop through the collection backwards?

[paulomorgado]

Sorry! It was a typo due to copy+paste.

In this case the goal is to avoid Enumerable.ToArray().

[Tratcher]

Why are you worried about ToArray in one scenario and Reverse in the other? Shouldn't these use the same approach?

[paulomorgado]

It's not about one or the other. It's about both.

In one case, the size of the collection is checked, so I kept ToArray to check its size. Now I don't need to reverse it because I can iterate it in reverse order.

In the other case, the collection is only iterated, so Reverse serves the purpose.

[paulomorgado]

Now that you mention it, this is a pattern I often use:

var filters = HostingServiceProvider.GetRequiredService<IEnumerable<IStartupConfigureServicesFilter>>()
    .Reverse();
var enumerator = filters.GetEnumerator();
try
{
    // If there are no filters just run startup (makes IServiceProvider ConfigureServices(IServiceCollection services) work.
    if (!enumerator.Next())
    {
        return startup(services);
    }
    Action<IServiceCollection> pipeline = InvokeStartup;
    do
    {
        pipeline = enumerator.Current.ConfigureServices(pipeline);
    } while (enumerator.Next());
    pipeline(services);
    // We return null so that the host here builds the container (same result as void ConfigureServices(IServiceCollection services);
    return null;
}
finally
{
    enumerator.Dispose();
}

No ToArray!

(ducks and hides)

[Tratcher]

filters.Length == 0 could be an .Any() check instead.

[paulomorgado]

NO! Don't do that!

If the type provides a Length/Count, use it. Otherwise, there will be interface implementation checks and an enumerator allocation.

Now, if you go with Reverse and use Any you'll pay that cost when you can avoid it with this code.

[Tratcher]

Thanks