递归实现treeView下的省市联动

namespace 省市联动_递归_
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            CreateTreeNode(0,treeView1.Nodes);
        }
    
        private List<Model> GetAreaNameById(int id) {
            List<Model> ls = new List<Model>();
            string sql = "select * from TblArea where AreaPId=@id";
            SqlParameter sp1 = new SqlParameter("@id", SqlDbType.Int) { Value = id };
            using (SqlDataReader reader = SqlHelp.ExecuteReader(sql, sp1))
            {
                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        int AreaId = reader.GetInt32(0);
                        string AreaName = reader.GetString(1);
                        int AreaPId = reader.GetInt32(2);
                        ls.Add(new Model(AreaId,AreaName,AreaPId));
                    }
                }
            }
            return ls;
        
        }
        private  void CreateTreeNode(int id, TreeNodeCollection tnc) {
            List<Model> ls= GetAreaNameById(id);
            foreach (Model item in ls )
            {
              TreeNode tn= tnc.Add(item.AreaName);
              tn.Tag = item.AreaId;
              CreateTreeNode(item.AreaId, tn.Nodes);
            }
        }
    }
}

namespace 省市联动_递归_
{
    class Model
    {
        public int AreaId { set; get; }
        public string AreaName { set; get; }
        public int AreaPId { set; get; }
        public Model(int areaId,string areaName,int areaPId) { 
            this.AreaId=areaId;
            this.AreaName = areaName;
            this.AreaPId = areaPId;     
        }
    }

 sqlhelp

namespace 省市联动_递归_
{
    public  static class SqlHelp
    {
        static readonly string constr = ConfigurationManager.ConnectionStrings["MyconnectionString1"].ConnectionString;
        public static int ExecuteNoneQuery(string sql,params SqlParameter[] p) {
            using (SqlConnection conn=new SqlConnection(constr) )
            {
                using (SqlCommand cmd=new SqlCommand(sql,conn))
                {
                    if (p!=null)
                    {
                        cmd.Parameters.AddRange(p); 
                    }
                    if (conn.State==ConnectionState.Closed)
                    {
                        conn.Open();
                    }
                    return cmd.ExecuteNonQuery();
                }
            }
        }
        public static int ExecuteScalar(string sql, params SqlParameter[] p) {
            using (SqlConnection conn=new SqlConnection(constr))
            {
                using (SqlCommand cmd=new SqlCommand(sql,conn))
                {
                    if (p!=null)
                    {
                        cmd.Parameters.AddRange(p); 
                    }
                    if (conn.State==ConnectionState.Closed)
                    {
                        conn.Open();
                    }
                    return (int)cmd.ExecuteScalar();
                }
            }
        }
        public static SqlDataReader ExecuteReader(string sql,params SqlParameter[] p)
        {
            SqlConnection conn=new SqlConnection(constr);
            using(SqlCommand cmd=new SqlCommand(sql,conn))
            {
                if (p!=null)
                {
                    cmd.Parameters.AddRange(p); 
                }
                try 
                {            
                    conn.Open();
                    return cmd.ExecuteReader(System.Data.CommandBehavior.CloseConnection);
                }
                catch (Exception)
                {
                    conn.Close();
                    conn.Dispose();
                    throw;
                }
            }
      
        }
    }
}