How is foreach implemented in C#? [duplicate]

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It doesn’t use an anonymous function, no. Basically the compiler converts the code into something broadly equivalent to the while loop you’ve shown here.

foreach isn’t a function call – it’s built-into the language itself, just like for loops and while loops. There’s no need for it to return anything or “take” a function of any kind.

Note that foreach has a few interesting wrinkles:

  • When iterating over an array (known at compile-time) the compiler can use a loop counter and compare with the length of the array instead of using an IEnumerator
  • foreach will dispose of the iterator at the end; that’s simple for IEnumerator<T> which extends IDisposable, but as IEnumerator doesn’t, the compiler inserts a check to test at execution time whether the iterator implements IDisposable
  • You can iterate over types which don’t implement IEnumerable or IEnumerable<T>, so long as you have an applicable GetEnumerator() method which returns a type with suitable Current and MoveNext() members. As noted in comments, a type can also implement IEnumerable or IEnumerable<T> explicitly, but have a public GetEnumerator() method which returns a type other than IEnumerator/IEnumerator<T>. See List<T>.GetEnumerator() for an example – this avoids creating a reference type object unnecessarily in many cases.

See section 8.8.4 of the C# 4 spec for more information.

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