In .NET, which loop runs faster, 'for' or 'foreach'?

Question

In C#/VB.NET/.NET, which loop runs faster, for or foreach?

Ever since I read that a for loop works faster than a foreach

Answer 1

This has the same two answers as most "which is faster" questions:

1) If you don't measure, you don't know.

2) (Because...) It depends.

It depends on how expensive the "MoveNext()" method is, relative to how expensive the "this[int index]" method is, for the type (or types) of IEnumerable that you will be iterating over.

The "foreach" keyword is shorthand for a series of operations - it calls GetEnumerator() once on the IEnumerable, it calls MoveNext() once per iteration, it does some type checking, and so on. The thing most likely to impact performance measurements is the cost of MoveNext() since that gets invoked O(N) times. Maybe it's cheap, but maybe it's not.

The "for" keyword looks more predictable, but inside most "for" loops you'll find something like "collection[index]". This looks like a simple array indexing operation, but it's actually a method call, whose cost depends entirely on the nature of the collection that you're iterating over. Probably it's cheap, but maybe it's not.

If the collection's underlying structure is essentially a linked list, MoveNext is dirt-cheap, but the indexer might have O(N) cost, making the true cost of a "for" loop O(N*N).