未将对象引用设置到对象的实例。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;namespace Exp1
{
    class treeNode
    {
        string attribute;        public string Attribute
        {
            get { return attribute; }
            set { attribute = value; }
        }
        List<string> valueofAttr;        public List<string> ValueofAttr
        {
            get { return valueofAttr; }
            set { valueofAttr = value; }
        }
        treeNode parent;        internal treeNode Parent
        {
            get { return parent; }
            set { parent = value; }
        }
        public List<treeNode> child;        //internal List<treeNode> Child
        //{
        //    get { return child; }
        //    set { child = value; }
        //}
        string parent_attr_value;        public string Parent_attr_value
        {
            get { return parent_attr_value; }
            set { parent_attr_value = value; }
        }        string yesorno;        public string Yesorno
        {
            get { return yesorno; }
            set { yesorno = value; }
        }        public void addChild(treeNode n) {
            this.child.Add(n); 这里提示“未将对象引用设置到对象的实例。”
        }        public treeNode(string attribute) {
            this.attribute = attribute;
            this.child = null;
            this.parent = null;
            this.parent_attr_value = null;
            this.valueofAttr = null;
            this.yesorno = null;
        }
    }
} treeNode n = new treeNode(root.Attribute);
                    n.Parent = root;
                    n.Parent_attr_value = s;
                    n.Yesorno = temp.First().PlayBall.ToString();
                    //List<treeNode> hg = new List<treeNode>();
                    //hg = root.child;
                    //hg.Add(n);
                    root.addChild(n);