How does foreach work when passing through a function?

Question

Suppose I have the following code:

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

Will someObj.GetMyStrings() be called on each iteration of the loop? It would be better to do the following:

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

?

Answer 1

A function called only once returns IEnumerator<T> ; after that, the MoveNext() method and the Current property are used to repeat the results:

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

somewhat equivalent:

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

Note that the iterator is located at the end - this is especially important for allocating resources from iterator blocks, for example:

 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 the iteration, and the compiler implements IDisposable cunningly to make this work.

Answer 2

No. the function will be called once to get IEnumerable .., and then call MoveNext and Current again.

Answer 3

GetMyStrings () migrates an object of type IEnumerable. The runtime knows how to deal with this. It calls IEnumerable.GetEnumerator (), and then this enumerator object calls MoveNext () and Current.

Answer 4

Just to round out the question if you find yourself in "all for the same"; for (int x = 0; x <ProvideCeiling (); x ++) calls the function every time.