I found this algorithm here.

I have a problem, I cant seem to understand how to set up and pass my heuristic function.

    static public Path<TNode> AStar<TNode>(TNode start, TNode destination,
        Func<TNode, TNode, double> distance,
        Func<TNode, double> estimate) where TNode : IHasNeighbours<TNode>
    {
        var closed = new HashSet<TNode>();
        var queue = new PriorityQueue<double, Path<TNode>>();
        queue.Enqueue(0, new Path<TNode>(start));
        while (!queue.IsEmpty)
        {
            var path = queue.Dequeue();
            if (closed.Contains(path.LastStep))
                continue;
            if (path.LastStep.Equals(destination))
                return path;
            closed.Add(path.LastStep);
            foreach (TNode n in path.LastStep.Neighbours)
            {
                double d = distance(path.LastStep, n);
                var newPath = path.AddStep(n, d);
                queue.Enqueue(newPath.TotalCost + estimate(n), newPath);
            }
        }
        return null;
    }

As you can see, it accepts 2 functions, a distance and a estimate function.

Using the Manhattan Heuristic Distance function, I need to take 2 parameters. Do I need to modify his source and change it to accepting 2 parameters of TNode so I can pass a Manhattan estimate to it? This means the 4th param will look like this:

Func<TNode, TNode, double> estimate) where TNode : IHasNeighbours<TNode>

and change the estimate function to:

queue.Enqueue(newPath.TotalCost + estimate(n, path.LastStep), newPath);

My Manhattan function is:

    private float manhattanHeuristic(Vector3 newNode, Vector3 end)
    {
        return (Math.Abs(newNode.X - end.X) + Math.Abs(newNode.Y - end.Y));
    }