Question

I have to implement a call graph for expressions like Id = Id(Param); and that wasn’t a problem.

Now I have to implement an enumerator which lists one at a time all topological orderings among the calls that satisfy the order of dependencies.

And here’s the trouble.

This is a simple node for the call graph:

class CallGraphNode
{
    private string name;
    public List<CallGraphNode> dependents = new List<CallGraphNode>();
    public int dependencies;
    private bool executed = false;
    public bool Executable { get { return dependencies == 0; } }
    public bool Executed { get { return executed; } set { executed = value; } }

    public CallGraphNode(string name)
    {
        this.name = name;
        dependencies = 0;    
    }

    public override string ToString()
    {
        return name;
    }

    public void AddDependent(CallGraphNode n)
    {
        dependents.Add(n);
    }          
}

And this is the call graph class itself:

class CallGraph : IEnumerable<List<CallGraphNode>>
{
    public List<CallGraphNode> nodes = new List<CallGraphNode>();

    public void AddNode(CallGraphNode n)
    {
        nodes.Add(n);
    }

    public static void Show(IEnumerable<CallGraphNode> n)
    {
        foreach (CallGraphNode node in n)
        {
            Console.Write("{0} ", node);
        }
        Console.WriteLine();
    }

    static IEnumerable<List<CallGraphNode>> EnumerateFunctions(List<CallGraphNode> executable, List<CallGraphNode> res)
    {
        if (executable.Count == 0)
            yield return res;
        else foreach (CallGraphNode n in executable)
            {
                if (!n.Executed)
                    res.Add(n);
                List<CallGraphNode> next_executable = new List<CallGraphNode>(executable);
                executable.Remove(n);
                foreach (CallGraphNode m in n.dependents)
                    if (--m.dependencies == 0)
                        next_executable.Add(m);
                foreach (List<CallGraphNode> others in EnumerateFunctions(next_executable, res))
                    yield return others;
                foreach (CallGraphNode m in n.dependents)
                    m.dependencies++;
                if (!n.Executed)
                    res.Remove(n);
            }
    }

    IEnumerator<List<CallGraphNode>> IEnumerable<List<CallGraphNode>>.GetEnumerator()
    {
        List<CallGraphNode> executable = new List<CallGraphNode>();
        foreach (CallGraphNode n in nodes)
            if (n.Executable || n.Executed)
                executable.Add(n);
        List<CallGraphNode> output = new List<CallGraphNode>();
        foreach (List<CallGraphNode> list in EnumerateFunctions(executable, output))
            yield return list;
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    { throw new NotImplementedException(); }
}

Now, the problem is it just won’t work. When I try to create an IEnumerator and assign it the GetEnumerator() return value, I get a casting error and that’s honestly what I expected when trying to do so:

IEnumerator<List<CallGraphNode>> lt = cg.GetEnumerator();

Then I’ve tried:

System.Collections.Generic.List<CallGraphNode>.Enumerator en = cg.nodes.GetEnumerator();

This works, but the method EnumerateFunctions is never called and the enumerator just contains the original list of graph nodes.

Any ideas?

Answer 1

The problem is that you’re implementing both IEnumerable<T> and IEnumerable using explicit interface implementation.

You probably want to change this declaration:

IEnumerator<List<CallGraphNode>> IEnumerable<List<CallGraphNode>>.GetEnumerator()

to be a “normal” interface implementation:

public IEnumerator<List<CallGraphNode>> GetEnumerator()

Alternatively, you could stick with explicit interface implementation, but use:

IEnumerable<List<CallGraphNode>> sequence = cg;
IEnumerator<List<CallGraphNode>> lt = sequence.GetEnumerator();