ArrayPool<T>/MemoryPool<T> Best Practice

[GF-Huang]

When should they be used? Are there any guidelines?

And the docs said:

Using the ArrayPool class to rent and return buffers (using the Rent and Return methods) can improve performance in situations where arrays are created and destroyed frequently, resulting in significant memory pressure on the garbage collector.

It's not very clear, how frequent is frequent?

Is it premature optimization to use them instead of temporary arrays which created in the all method bodies?
Even refactor all short life object like the follow, is this bad?

class ShortLife : IDisposable {
    private byte[] _buffer;
    
    public ShortLife(...) {
        _buffer = ArrayPool<byte>.Shared.Rent(xxx);
    }
    public void Dispose() {
        ArrayPool<byte>.Shared.Return(_buffer);
    }
}

// then use
using var shortLife = new ShortLife(...);

[davidfowl]

The shared array pool is optimized for synchronous rent and return. I think that's where the short lived advice comes from.

[Clockwork-Muse]

The other thing that should be mentioned here is that if your array is also small (in bytes used - size of type * number of elements), in addition to being short-lived, it may behoove you to look at using stackalloced arrays.

[GF-Huang]

In the async method body, stackalloc seems cumbersome to use and requires a static local function.

[Clockwork-Muse]

And that's just another part of the determination. Although note that if you're in an async method, they may not "live" long enough - since in many cases you use the array as a buffer to an async call, you'd run into problems at that point.