Recursive generic types

Question

Is it possible to define a generic type in C# that references itself?

E.g. I want to define a Dictionary<> that holds its type as TValue (for a hierarchy).

Dictionary<string, Dictionary<string, Dictionary<string, [...]>>>

Answer 1

Try:

class StringToDictionary : Dictionary<string, StringToDictionary> { }

Then you can write:

var stuff = new StringToDictionary
        {
            { "Fruit", new StringToDictionary
                {
                    { "Apple", null },
                    { "Banana", null },
                    { "Lemon", new StringToDictionary { { "Sharp", null } } }
                }
            },
        };

General principle for recursion: find some way to give a name to the recursive pattern, so it can refer to itself by name.

Answer 2

Another example would be generic tree

public class Tree<TDerived> where TDerived : Tree<TDerived>
{
    public TDerived Parent { get; private set; }
    public List<TDerived> Children { get; private set; }
    public Tree(TDerived parent)
    {
        this.Parent = parent;
        this.Children = new List<TDerived>();
        if(parent!=null) { parent.Children.Add(this); }
    }
    public bool IsRoot { get { return Parent == null; } }
    public bool IsLeaf { get { return Children.Count==0; } }
}

Now to use it

public class CoordSys : Tree<CoordSys>
{
    CoordSys() : base(null) { }
    CoordSys(CoordSys parent) : base(parent) { }
    public double LocalPosition { get; set; }
    public double GlobalPosition { get { return IsRoot?LocalPosition:Parent.GlobalPosition+LocalPosition; } }
    public static CoordSys NewRootCoordinate() { return new CoordSys(); }
    public CoordSys NewChildCoordinate(double localPos)
    {
        return new CoordSys(this) { LocalPosition = localPos };
    }
}

static void Main() 
{
    // Make a coordinate tree:
    //
    //                  +--[C:50] 
    // [A:0]---[B:100]--+         
    //                  +--[D:80] 
    //

    var A=CoordSys.NewRootCoordinate();
    var B=A.NewChildCoordinate(100);
    var C=B.NewChildCoordinate(50);
    var D=B.NewChildCoordinate(80);

    Debug.WriteLine(C.GlobalPosition); // 100+50 = 150
    Debug.WriteLine(D.GlobalPosition); // 100+80 = 180
}

Note that you cannot directly instantiate Tree<TDerived>. It has to be a base class to the node class in the tree. Think class Node : Tree<Node> { }.