C#•2y ago

❔ Does this method cause boxing of value types?

I was looking through the list<T> class and noticed that the Contains<T>(T item) method attemps to cast the 'item' to an object - '(object) item', does this not cause boxing for value types?

15 Replies

jokiOP•2y ago

or does that not matter because they are already allocated on the heap? since theyre in an array

Sossenbinder•2y ago

It will box a value type if you cast it to an object Not sure about the array part, an array of value types will most likely end up on the heap, but things can become twisted Technically there's stuff like stackalloc etc So I wouldn't worry too much If you really want to have non-boxing equality comparison, then I think EqualityComparer works Since it's generic-based

jokiOP•2y ago

well not most likely, it will end up on the heap, [] is a reference type those create pointers to unmanaged memory

Sossenbinder•2y ago

That's true, I mentioned likely since it is technically not written in stone, IIRC all of these details are only an implementation detail, so they could always swap out details

Anton•2y ago

it should be optimized out for structs as that check is always true for structs

jokiOP•2y ago

false you mean?

cap5lut•2y ago

shouldnt if (item == default) avoid boxing? u cant avoid boxing if ya check for null

Aaron•2y ago

you don't want == default this doesn't box the entire branch is elided for value types because a value type can never be null it does not, the JIT is smart enough to know that a value type can never be null, and will get rid of the entire if

Anton•2y ago

yeah of course

Aaron•2y ago

to clarify on why this is, this is a fast path specifically for checking for null reference types, so it can skip using EqualityComparer a struct being the default doesn't really mean you can skip EqualityComparer

Sossenbinder•2y ago

Oh, TIL, that is very good to know It makes sense, but I had no idea that this will outright be skipped

MODiX•2y ago

Windows10CE#8553

sharplab.io (click here)

public class C {
    [JitGeneric(typeof(int))]
    public static void M<T>(T t) {
        if ((object)t == null) Console.WriteLine("a");
    }
}

public class C {
    [JitGeneric(typeof(int))]
    public static void M<T>(T t) {
        if ((object)t == null) Console.WriteLine("a");
    }
}

React with ❌ to remove this embed.

Aaron•2y ago

you can see the assembly doesn't have the WriteLine

Sossenbinder•2y ago

Indeed

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Does this method cause boxing of value types?