CODEWITHSUNDEEP
c#.netlinq.net-core
user2037593
user2037593

Reputation:

What does IPartition do in .NET Core?

I had a look at the .NET Core source in order to find out whether calls to IEnumerable.Last() (LINQ) are optimized when the collection implements IList which I suspected to be the case after some quick benchmarking. It turns out that yes, the input is specifically checked for IList, however, even before that, it is checked whether the input implements IPartition. IPartition is defined here, but I do not understand at all what it is supposed to do.

What is the purpose of IPartition and how could it possibly be faster than the usually constant-time indexing of IList (or if it isn't, why is the input checked for IPartition earlier than IList)?

Upvotes: 8

Views: 1148

Answers (1)

usr
usr

Reputation: 171246

It's an optimization for enumerable methods that act on indexes (Take/Skip and others) when the collection has support for indexes. IIListProvider is related.

There are a few implementations. Search the file for IPartition.

There are issues about this.

IPartition allows to delegate higher level operations to the underlying collection. For example, new int[10].Skip(1) is slow because all data runs through two IEnumerable<int>. With the IPartition implementation this turns into effectively for (int i = 1 ... 9) emit(list[i]);. So it's not using intermediate enumerators and it's calling the list indexer which is slightly faster than enumerating.

This is a very rough description. I encourage you to check out the Github discussions and the code.

Upvotes: 8

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