Iterators OR Excuse me waiter theres a goto in my C sharp by ThinqLinq

Iterators OR Excuse me waiter theres a goto in my C sharp

At Codestock '09, I gave my LINQ Internals talk and had a number of people express shock when I showed the underlying implementation of their beloved iterators when looking at the code through Reflector. Let's look first at the C# that we wrote. This is similar to the implementation of LINQ to Object's Where method as shown in the sequence.cs file that's part of the C# Samples.


public static IEnumerable Where(this QueryableString source, Func predicate)
{
   foreach (char curChar in source)
        if (predicate(curChar))
            yield return curChar;
}

C# Iterators aren't really first class citizens, but syntactic sugar around the actual implementation. The meat of the implementation occurs in a generated class that implements the actual MoveNext method as we foreach over the results. The results are much less pretty:


private bool MoveNext()
{
    bool CS$1$0000;
    try
    {
        switch (this.1__state)
        {
            case 0:
                break;

            case 2:
                goto Label_0087;

            default:
                goto Label_00A5;
        }
        this.1__state = -1;
        this.7__wrap2 = this.4__this.GetEnumerator();
        this.1__state = 1;
        while (this.7__wrap2.MoveNext())
        {
            this.<curString>5__1 = this.7__wrap2.Current;
            if (!this.predicate(this.<curString>5__1))
            {
                continue;
            }
            this.2__current = this.<curString>5__1;
            this.1__state = 2;
            return true;
        Label_0087:
            this.1__state = 1;
        }
        this.m__Finally4();
    Label_00A5:
        CS$1$0000 = false;
    }
    fault
    {
        this.System.IDisposable.Dispose();
    }
    return CS$1$0000;
}

As you can see, the iterator sets up a switch (Select Case) statement that checks to see where we are in the loop (using a state variable). Essentially this is a state machine. The first time through we set up the environment. As we iterate over the results, we call the predicate that was passed in. If the predicate evaluates as true, we exit out of the method returning true.

The next time we return to the MoveNext, we use goto Label_0087 to re-enter the loop and continue the iteration. It's at this point that the jaws dropped in my presentation. Yes, Virginia, there are "Goto's" in C#. Spaghetti code isn't limited to VB. It's this point in my presentation where I quipped that the reason why iterators aren't in VB yet is because we want to do them "Right". While this is partly a joke, there is a level of seriousness in the comment. If you want to dig deeper on iterators, I recommend the following for your reading pleasure (note, these are NOT for the faint of heart):

After reading these, I'm sure you will have a better understanding of why it is taking so long to get iterators in VB. In the mean time, you might also find Bill McCarthy's recent article on using Iterators in VB Now to be interesting.

Posted on - Comment
Categories: VB - C# - LINQ -
comments powered by Disqus