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> { }.