How does foreach work when looping through function results?

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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.

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