public struct EnumeratorStruct : IDisposable
{
    public int Current { get; }

    public bool MoveNext() => foo;

    public EnumeratorStruct GetEnumerator() => this;

    public void Dispose() => Console.WriteLine("Disposed");
}

foreach (int element in new EnumeratorStruct())
{
    ...
}

public static EnumeratorStruct Enumerate(this Foo source) => new EnumeratorStruct(source);


Foo source = new Foo();
foreach (int element in source.Enumerate())
{
    ...
}

EnumeratorStruct enumerator = new EnumeratorStruct();
foreach (int element in enumerator)
{
    ...
}