使用此泛型代码，为什么我会收到“参数 1：无法从“TopologyLibrary.RelationshipBase”转换？至“T关系”

Question

有人知道为什么我在 CreateRelationship() 的下面的代码中收到“参数 1：无法从 'ToplogyLibrary.RelationshipBase' 转换为 'TRelationship'” 吗？

public class TopologyBase<TKey, TNode, TRelationship>
    where TNode : NodeBase<TKey>, new()
    where TRelationship : RelationshipBase<TKey>, new()
{
    // Properties
    public Dictionary<TKey, TNode> Nodes { get; private set; }
    public List<TRelationship> Relationships { get; private set; }

    // Constructors
    protected TopologyBase()
    {
        Nodes = new Dictionary<TKey, TNode>();
        Relationships = new List<TRelationship>();
    }

    // Methods
    public TNode CreateNode(TKey key)
    {
        var node = new TNode {Key = key};
        Nodes.Add(node.Key, node);
        return node;
    }

    public void CreateRelationship(TNode parent, TNode child)
    {
        // Validation
        if (!Nodes.ContainsKey(parent.Key) || !Nodes.ContainsKey(child.Key))
        {
            throw new ApplicationException("Can not create relationship as either parent or child was not in the graph: Parent:" + parent.Key + ", Child:" + child.Key);
        }

        // Add Relationship
        var r = new RelationshipBase<TNode>();
        r.Parent = parent;
        r.Child = child;
        Relationships.Add(r);  // *** HERE *** "Argument 1: cannot convert from 'ToplogyLibrary.RelationshipBase<TNode>' to 'TRelationship'" 

    }


}

public class RelationshipBase<TNode>
{
    public TNode Parent { get; set; }
    public TNode Child { get; set; }

}

public class NodeBase<T>
{
    public T Key { get; set; }

    public NodeBase()
    {
    }

    public NodeBase(T key)
    {
        Key = key;
    }      


}

Answer 1

您对 TRelationship 的约束为 RelationshipBase。您可能是想说 RelationshipBase 吗？

Answer 2

使用这些行：

where TRelationship : RelationshipBase<TNode>, new()

您并不是说 TRelationship = RelationshipBase，而是 TRelationship 继承自 RelationshipBase。

但是您不能将基类隐式转换为其后代。

那么，您确实需要这个：

List<TRelationship>

或者

List<RelationshipBase<TNode>>

这对您来说就足够了？

或者也许查看你的代码：
你为什么不改变这一行：

var r = new RelationshipBase<TNode>();

与：

var r = new TRelationship();

??

编辑：
正如 AakashM 所说，我认为你指的是 TNode 而不是 TKey