Prevent self-reference in Node hierarchy with ArgumentException

Promete.Test/NodeTests.cs

@@ -0,0 +1,80 @@
+using FluentAssertions;
+using Promete.Nodes;
+
+namespace Promete.Test;
+
+public class NodeTests
+{
+    [Fact]
+    public void AddSelfAsChild_ShouldThrowArgumentException()
+    {
+        // Arrange
+        var container = new Container();
+
+        // Act & Assert
+        var act = () => container.Add(container);
+        act.Should().Throw<ArgumentException>()
+            .WithMessage("*自分自身*");
+    }
+
+    [Fact]
+    public void InsertSelfAsChild_ShouldThrowArgumentException()
+    {
+        // Arrange
+        var container = new Container();
+
+        // Act & Assert
+        var act = () => container.Insert(0, container);
+        act.Should().Throw<ArgumentException>()
+            .WithMessage("*自分自身*");
+    }
+
+    [Fact]
+    public void AddDifferentNode_ShouldSucceed()
+    {
+        // Arrange
+        var container = new Container();
+        var child = new Container();
+
+        // Act
+        container.Add(child);
+
+        // Assert
+        container.Count.Should().Be(1);
+        child.Parent.Should().Be(container);
+    }
+
+    [Fact]
+    public void InsertDifferentNode_ShouldSucceed()
+    {
+        // Arrange
+        var container = new Container();
+        var child = new Container();
+
+        // Act
+        container.Insert(0, child);
+
+        // Assert
+        container.Count.Should().Be(1);
+        child.Parent.Should().Be(container);
+    }
+
+    [Fact]
+    public void InsertNodeWithExistingParent_ShouldMoveNode()
+    {
+        // Arrange
+        var oldParent = new Container();
+        var newParent = new Container();
+        var child = new Container();
+
+        oldParent.Add(child);
+
+        // Act
+        newParent.Insert(0, child);
+
+        // Assert
+        oldParent.Count.Should().Be(0);
+        newParent.Count.Should().Be(1);
+        child.Parent.Should().Be(newParent);
+    }
+}

Promete/Nodes/ContainableNode.cs

@@ -70,6 +70,11 @@ protected void SortChildrenIfNeeded()
 
     protected void Add(Node node)
     {
+        if (node == this)
+        {
+            throw new ArgumentException("ノードの子要素に自分自身を追加することはできません。", nameof(node));
+        }
+
         node.Parent?.Remove(node);
 
         node.Parent = this;
@@ -92,7 +97,12 @@ protected void Clear()
 
     protected void Insert(int index, Node node)
     {
-        node.Parent?.Remove(this);
+        if (node == this)
+        {
+            throw new ArgumentException("ノードの子要素に自分自身を追加することはできません。", nameof(node));
+        }
+
+        node.Parent?.Remove(node);
 
         children.Insert(index, node);
         node.Parent = this;