Does List<T> create garbage in C# in foreach

Question

Correct me if im wrong but while doing a foreach an IEnumerable<T> creates garbage no matter what T is. But I'm wondering if you have a List<T> where T is Entity. Then say there is a derived class in the list like Entity2D. Will it have to create a new enumerator for each derived class? Therefore creating garbage?

Also does having an interface let's say IEntity as T create garbage?

Answer 1

List<T>'s GetEnumerator method actually is quite efficient.

When you loop through the elements of a List<T>, it calls GetEnumerator. This, in turn, generates an internal struct which holds a reference to the original list, an index, and a version ID to track for changes in the list.

However, since a struct is being used, it's really not creating "garbage" that the GC will ever deal with.

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As for "create a new enumerator for each derived class" - .NET generics works differently than C++ templates. In .NET, the List<T> class (and it's internal Enumerator<T> struct) is defined one time, and usable for any T. When used, a generic type for that specific type of T is required, but this is only the type information for that newly created type, and quite small in general. This differs from C++ templates, for example, where each type used is created at compile time, and "built in" to the executable.

In .NET, the executable specifies the definition for List<T>, not List<int>, List<Entity2D>, etc...

Answer 2

I think you may be interested in this article which explains why List(T) will not create "garbage", as opposed to Collection(T):

Now, here comes the tricky part. Rumor has it that many of the types in System.Collections.Generic will not allocate an enumerator when using foreach. List's GetEnumerator, for example, returns a struct, which will just sit on the stack. Look for yourself with .NET Reflector, if you don't believe me. To prove to myself that a foreach over a List doesn't cause any heap allocations, I changed entities to be a List, did the exact same foreach loop, and ran the profiler. No enumerator!

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However, there is definitely a caveat to the above. Foreach loops over Lists can still generate garbage. [Casting List to IEnumerable] Even though we're still doing a foreach over a List, when the list is cast to an interface, the value type enumerator must be boxed, and placed on the heap.