具有遞迴型別規範的基類

具有遞迴型別說明符的通用基類的一次定義。每個節點都有一個父節點和多個子節點。

/// <summary>
/// Generic base class for a tree structure
/// </summary>
/// <typeparam name="T">The node type of the tree</typeparam>
public abstract class Tree<T> where T : Tree<T>
{
    /// <summary>
    /// Constructor sets the parent node and adds this node to the parent's child nodes
    /// </summary>
    /// <param name="parent">The parent node or null if a root</param>
    protected Tree(T parent)
    {
        this.Parent=parent;
        this.Children=new List<T>();
        if(parent!=null)
        {
            parent.Children.Add(this as T);
        }
    }
    public T Parent { get; private set; }
    public List<T> Children { get; private set; }
    public bool IsRoot { get { return Parent==null; } }
    public bool IsLeaf { get { return Children.Count==0; } }
    /// <summary>
    /// Returns the number of hops to the root object
    /// </summary>
    public int Level { get { return IsRoot ? 0 : Parent.Level+1; } }
}

每次需要定義物件的樹層次結構時，可以重複使用上述內容。樹中的節點物件必須從基類繼承

public class MyNode : Tree<MyNode>
{
    // stuff
}

每個節點類都知道它在層次結構中的位置，父物件是什麼以及子物件是什麼。幾種內建型別使用樹結構，如 Control 或 XmlElement，上面的 Tree<T> 可以用作程式碼中任何型別的基類。

例如，要建立一個部件層次結構，其中總重量是根據所有子項的權重計算的，請執行以下操作：

public class Part : Tree<Part>
{
    public static readonly Part Empty = new Part(null) { Weight=0 };
    public Part(Part parent) : base(parent) { }
    public Part Add(float weight)
    {
        return new Part(this) { Weight=weight };
    }
    public float Weight { get; set; }

    public float TotalWeight { get { return Weight+Children.Sum((part) => part.TotalWeight); } }
}

用作

// [Q:2.5] -- [P:4.2] -- [R:0.4]
//    \
//      - [Z:0.8]
var Q = Part.Empty.Add(2.5f);
var P = Q.Add(4.2f);
var R = P.Add(0.4f);
var Z = Q.Add(0.9f);

// 2.5+(4.2+0.4)+0.9 = 8.0
float weight = Q.TotalWeight;

另一個例子是相對座標系的定義。在這種情況下，座標系的真實位置取決於所有父座標系的位置。

public class RelativeCoordinate : Tree<RelativeCoordinate>
{
    public static readonly RelativeCoordinate Start = new RelativeCoordinate(null, PointF.Empty) { };
    public RelativeCoordinate(RelativeCoordinate parent, PointF local_position)
        : base(parent)
    {
        this.LocalPosition=local_position;
    }
    public PointF LocalPosition { get; set; }
    public PointF GlobalPosition
    {
        get
        {
            if(IsRoot) return LocalPosition;
            var parent_pos = Parent.GlobalPosition;
            return new PointF(parent_pos.X+LocalPosition.X, parent_pos.Y+LocalPosition.Y);
        }
    }
    public float TotalDistance
    {
        get
        {
            float dist = (float)Math.Sqrt(LocalPosition.X*LocalPosition.X+LocalPosition.Y*LocalPosition.Y);
            return IsRoot ? dist : Parent.TotalDistance+dist;
        }
    }
    public RelativeCoordinate Add(PointF local_position)
    {
        return new RelativeCoordinate(this, local_position);
    }
    public RelativeCoordinate Add(float x, float y)
    {
        return Add(new PointF(x, y));
    }
}

用作

// Define the following coordinate system hierarchy
//
// o--> [A1] --+--> [B1] -----> [C1]
//             |     
//             +--> [B2] --+--> [C2]
//                         |
//                         +--> [C3]

var A1 = RelativeCoordinate.Start;
var B1 = A1.Add(100, 20);
var B2 = A1.Add(160, 10);

var C1 = B1.Add(120, -40);
var C2 = B2.Add(80, -20);
var C3 = B2.Add(60, -30);

double dist1 = C1.TotalDistance;