Cleanup That Still Happens

try/finally is allowed around a yield return, and it does what you would hope — even when the consumer walks away early.

static IEnumerable<string> Lines()
{
    Console.WriteLine("opening");

    try
    {
        yield return "first";
        yield return "second";
        yield return "third";
    }
    finally
    {
        Console.WriteLine("closing");
    }
}

static void Main()
{
    foreach (var line in Lines())
    {
        Console.WriteLine(line);
        if (line == "second") break;
    }
}
   

opening
first
second
closing

The loop broke after the second value, and closing still printed.

That works because the generated state machine implements IDisposable, and foreach compiles to a try/finally that calls Dispose() on the enumerator. Disposing an iterator that is parked inside a try runs its finally blocks.

This is why File.ReadLines can hand you a lazy sequence and still close the file when you stop reading. The using sits inside the iterator, and your break disposes it.