Question

I’m building a graph and verify uniqueness of a vertex to add using HashSet<T>. Its method Add() returns true if an item was not added already.

What technique can I use for edges? I need to verify that either pair A-B or pair B-A pair were added once.

Here’s a lightweight version of the data I’m using:

interface INode
{
    INode[] Previous { get; }

    Node Next { get; }
}

Then I have:

INode current;
INode[] allPrevious = ExternalMethod1(current);
INode[] allNext = ExternalMethod2(current);

So I can’t build a graph while go round recursively, I have all the data stored up to iterate over.

Answer 1

Create your own equality comparer implementation for edges and set HashSet<T>.Comparer to an instance of it. For example:

class Edge
{
    public string From { get; set; }
    public string To { get; set; }
}

class EdgeEqualityComparer : IEqualityComparer<Edge>
{
    public bool Equals(Edge lhs, Edge rhs)
    {
        return (lhs.From.Equals(rhs.From) && lhs.To.Equals(rhs.To)) ||
               (lhs.From.Equals(rhs.To) && lhs.To.Equals(rhs.From));
    }

    public int GetHashCode(Edge e)
    {
        return e.From.GetHashCode() ^ e.To.GetHashCode();
    }
}

Of course you would also have to check for nullity. All the standard caveats apply to the GetHashCode implementation as well.