How does foreach work when looping through function results?

Question

Suppose I have the following code:

foreach(string str in someObj.GetMyStrings()) {     // do some stuff } 

Will someObj.GetMyStrings() be called on every iteration of the loop? Would it be better to do the following instead:

List<string> myStrings = someObj.GetMyStrings(); foreach(string str in myStrings) {     // do some stuff } 

?

Answer 1

The function's only called once, to return an IEnumerator<T>; after that, the MoveNext() method and the Current property are used to iterate through the results:

foreach (Foo f in GetFoos()) {     // Do stuff } 

is somewhat equivalent to:

using (IEnumerator<Foo> iterator = GetFoos().GetEnumerator()) {     while (iterator.MoveNext())     {         Foo f = iterator.Current;         // Do stuff     } } 

Note that the iterator is disposed at the end - this is particularly important for disposing resources from iterator blocks, e.g.:

public IEnumerable<string> GetLines(string file) {     using (TextReader reader = File.OpenText(file))     {         string line;         while ((line = reader.ReadLine()) != null)         {             yield return line;         }     } } 

In the above code, you really want the file to be closed when you finish iterating, and the compiler implements IDisposable cunningly to make that work.