class Node
{
    FooType Data; // I can save Data to file with extension .foo
    void Save()
    {
       // save Data to .foo file
    }
}

Now ,

class Graph
{
    List<Node> Nodes; 
    void Save()
    {
        foreach(Node node in Nodes)
        {
            node.Save();
        }
    }
}

Now when I invoke someGraph.Save(); then it creates Nodes.Count number of files.

I would like to appear those files as one file with extension somename.graph and be able to read it again as Nodes.

How can I do this? Is there a way to bundle files into a single different file?