可空和泛型

Question

I'm trying to create a generic node class which can hold any number of children nodes, a string for the node key, and a data node which may or may not be null. However, I'm having problems getting the syntax right to have it accept the generic into the generic parameter for Nullable.

internal class TrieNode<E>
{
    Nullable<E> Data;
    string Key;
    List<TrieNode<E>> Children;

    public TrieNode(E? data, string key)
    {
        Data = data;
        Key = key;
        Children = new List<TrieNode<E>>();
    }
}

On compile time I get the following error messsage:

The type 'E' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

Is there a way to guarantee that E is a non-nullable type, or some other way around this?

Answer 1

You just need:

internal class TrieNode<E> where E : struct

The : struct clause limits E to value types excluding Nullable<> , which allows Nullable<E> or E? to work fine.

Answer 2

Maybe try internal class TrieNode where E : struct

I think that Nullable types need to be value types rather than reference types (Reference types are already nullable)

Answer 3

Why not just declare the node to use object ? An object can refer to any data type, and can also be null.