foreach是否更喜欢IEnumerable <T> 或IEnumerator <T>

Question

如果集合或列表同时实现IEnumerable <T>和IEnumerator <T>，foreach会使用什么？

Answer 1

IEnumerable<T>有一个方法GetEnumerator - 这将是一个IEnumerator<T> ，它是用于迭代的（使用MoveNext方法）。

Answer 2

You can't use a foreach on an IEnumerator<T> at all.

You can only foreach over an object that has a GetEnumerator method (whether or not it implements IEnumerable .

So to answer your question, it uses the GetEnumerator method from IEnumerable<T> .

It's simple to prove. Just try to compile this:

IEnumerator<char> str = "asdf".GetEnumerator();
foreach (char c in str) { }

Answer 3

Also notice, that this is not a requirement for collection to implement IEnumerable/IEnumerator in order be used in foreach loop. Foreach uses pattern-based approach as described here Eric Lippert blog

Answer 4

Another (more relevant) question would be: What if a type implements IEnumerable<> explicitly and at the same time has a regular public method GetEnumerator() that is suitable? Answer: Then the public method is used.

And what if the type implements both IEnumerable<Foo> and IEnumerable<Bar> ? Then, if there's not a regular instance method GetEnumerator() (that can happen if both interfaces are implemented explicitly), a compile-time error arises.