C# Ensuring an iterator method finishes gracefully

Question

I tested this block of code and find that the GetInts method does not exit the method and print "GetInts disconnected" as i would expect, traditionally. I want to write a scroll control that incrementally downloads datarows from the database with yield return, but I am unsure of the correct method.

On the other hand, wrapping the yield return block with using block will guarantee the call on dispose(), but should I go that way?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace IteratorPattern
{
    class Program
    {
        static IEnumerator<int> _mIter;

        static void Main(string[] args)
        {
            // get an enumerator to enumerate values in the database, at a later time
            _mIter = GetInts(100).GetEnumerator();

            // simulate some scrolling, which will add values to my scroll box
            Scroll(10);

            // suppose this control is now redundant,
            // but this does not disconnect the data source 
            _mIter.Dispose();

            // program ends will connection still open?
            Console.WriteLine("Program End");
        }
        // iterate and cache (not implemented) values
        private static void Scroll(int units)
        {
            Console.WriteLine("Scroll()");

            while(units-- != 0 && _mIter.MoveNext())
            {
                Console.WriteLine(_mIter.Current);
            }

            Console.WriteLine("Scroll() completed");
        }
        // connect to database, yield-return each datarow, and disconnect (hopefully)
        static IEnumerable<int> GetInts(int i)
        {
            Console.WriteLine("GetInts connected");

            using (var ds = new DataSourceWrapper())
            {
                while (i-- != 0)
                {
                    Console.WriteLine("yield {0}", i);
                    yield return i;
                }
            }

            // not called! 
            Console.WriteLine("GetInts disconnected");
        }
    }
    // try using a datasource wrapper to ensure Dispose() is called to disconnect the connection.
    public class DataSourceWrapper : IDisposable
    {
        public void Dispose()
        {
            Console.WriteLine("DataSource Disconnected");
        }
    }
}

Answer 1

When I run it, it does disconnect:

Scroll()
GetInts connected
yield 99
99
...snip...
yield 90
90
Scroll() completed
DataSource Disconnected
Program End

Note that it would disconnect itself if you were reading to the end of the data; however, you are asking for 100 ints, and scrolling through 10 of them; as fas as the iterator is concerned you've left it hanging 10% in. If you exhaust an iterator block, it cleans up any using etc; if you don't exhaust it, it needs to be explicitly disposed. You can illustrate this by changing it to GetInts(5) (leaving all other code the same):

Scroll()
GetInts connected
yield 4
4
yield 3
3
yield 2
2
yield 1
1
yield 0
0
DataSource Disconnected
GetInts disconnected
Scroll() completed
Program End

The reason it doesn't show "GetInts disconnected" otherwise is that ... it never gets there unless you exhaust it! That is what your code says: "print connected; yield i items; print disconnected" - it won't print disconnected unless it has first done all the yields. If you use a finally though (using the GetInts(100) again), then this changes:

    static IEnumerable<int> GetInts(int i)
    {
        Console.WriteLine("GetInts connected");
        try
        {
            using (var ds = new DataSourceWrapper())
            {
                while (i-- != 0)
                {
                    Console.WriteLine("yield {0}", i);
                    yield return i;
                }
            }
        }
        finally
        {
            // not called! 
            Console.WriteLine("GetInts disconnected");
        }
    }

Then it works:

...
yield 90
90
Scroll() completed
DataSource Disconnected
GetInts disconnected
Program End

Basically, finally is mapped into the Dispose() of the iterator.

Additionally, _mIter is IEnumerator<T>; it explicitly implements IDisposable, and you are responsible for calling Dispose() at some point. Do that, and it will all work. Even with IEnumerator (non-generic, not explicitly IDisposable) you should follow the same approach as the compiler, and check if the enumerator is IDisposable, and call Dispose() accordingly.

Since you aren't consuming the data in one chunk, you can't use using, but you must still dispose it. This might mean making your type implement IDisposable, and passing along the call. Also, you might want to explicitly dispose and release the enumerator when you reach the end. I can't really change your code to illustrate this, as it doesn't make sense on static data, but I would hope that an enumerator would not be static on real code anyway.

Answer 2

You claim that it doesn't "disconnect the database connection" - but the code you've shown does print out "DataSource disconnected"... so it's working fine as far as I can see.

Yes, you need to make sure that something calls Dispose on the iterator - which should normally be done either with a using statement or (more usually) by iterating over an IEnumerable with a foreach loop.

You can't guarantee that the iterator block will run to completion itself, but any appropriate finally blocks will be executed when the iterator is disposed.

EDIT: So if you want to make sure you'll see "GetInts disconnected" you can just put that in a finally block:

static IEnumerable<int> GetInts(int i)
{
    try
    {
        Console.WriteLine("GetInts connected");

        using (var ds = new DataSourceWrapper())
        {
            while (i-- != 0)
            {
                Console.WriteLine("yield {0}", i);
                yield return i;
            }
        }
    }
    finally
    {
        Console.WriteLine("GetInts disconnected");
    }
}