声明一个Model类：

public class TreeNode
{
    /// <summary>
    /// 子id
    /// </summary>
    public string Id { get; set; }

    /// <summary>
    /// 父id
    /// </summary>
    public string ParentId { get; set; }

    /// <summary>
    /// 名称
    /// </summary>
    public string Name { get; set; }

    /// <summary>
    /// 子节点
    /// </summary>
    public List<TreeNode> Children { get; set; }

    /// <summary>
    /// 无参构造函数
    /// </summary>
    public TreeNode()
    {
        Children = new List<TreeNode>();
    }

    /// <summary>
    /// 有参构造函数
    /// </summary>
    /// <param name="id">子id</param>
    /// <param name="name">名称</param>
    /// <param name="parentId">父id</param>
    public TreeNode(string id, string name, string parentId)
    {
        this.Id = id;
        this.Name = name;
        this.ParentId = parentId;
        Children = new List<TreeNode>();
    }

    /// <summary>
    /// 有参构造函数
    /// </summary>
    /// <param name="id">子id</param>
    /// <param name="name">名称</param>
    /// <param name="parent">父节点</param>
    public TreeNode(string id, string name, TreeNode parent)
    {
         this.Id = id;
         this.Name = name;
         this.ParentId = parent.Id;
         Children = new List<TreeNode>();
     }
}

生成一个转换前的list集合：

TreeNode treeNode1 = new TreeNode("1", "山东", "0");
TreeNode treeNode2 = new TreeNode("2", "北京", "0");

TreeNode treeNode3 = new TreeNode("3", "历下区", treeNode1);
TreeNode treeNode4 = new TreeNode("4", "高新区", treeNode1);
TreeNode treeNode5 = new TreeNode("5", "历城区", treeNode1);
TreeNode treeNode6 = new TreeNode("6", "甸柳庄", treeNode3);
TreeNode treeNode7 = new TreeNode("7", "济南长途汽车站东站", treeNode6);


TreeNode treeNode8 = new TreeNode("8", "朝阳区", treeNode2);
TreeNode treeNode9 = new TreeNode("9", "海淀区", treeNode2);
TreeNode treeNode10 = new TreeNode("10", "金盏乡", treeNode8);


List<TreeNode> list = new List<TreeNode>();

list.Add(treeNode1);
list.Add(treeNode2);
list.Add(treeNode3);
list.Add(treeNode4);
list.Add(treeNode5);
list.Add(treeNode6);
list.Add(treeNode7);
list.Add(treeNode8);
list.Add(treeNode9);
list.Add(treeNode10);

递归方式

调用方式：

List<TreeNode> trees1 = TreeHelper.BulidTreeByRecursive(list, new List<TreeNode>(), "0");

实现代码：

/// <summary>
/// 使用递归方法建树
/// </summary>
public static List<TreeNode> BulidTreeByRecursive(List<TreeNode> treeNodes, List<TreeNode> resps, string pID)
{
    resps = new List<TreeNode>();
    List<TreeNode> tempList = treeNodes.Where(c => c.ParentId == pID).ToList();
            
    for (int i = 0; i < tempList.Count; i++)
    {
        TreeNode node = new TreeNode();
        node.Id = tempList[i].Id;
        node.ParentId = tempList[i].ParentId;
        node.Name = tempList[i].Name;
        node.Children = BulidTreeByRecursive(treeNodes, resps, node.Id);
        resps.Add(node);
    }

   return resps;
}

双层循环

调用方式：

List<TreeNode> trees = TreeHelper.BulidTree(list);

实现代码：

/// <summary>
/// 双层循环
/// </summary>
/// <param name="treeNodes"></param>
/// <returns></returns>
public static List<TreeNode> BulidTree(List<TreeNode> treeNodes)
{
    try
    {
         List<TreeNode> trees = new List<TreeNode>();

         foreach (var treeNode in treeNodes)
         {
              if ("0" == (treeNode.ParentId))
              {
                  trees.Add(treeNode);
              }

              foreach (var it in treeNodes)
              {
                   if (it.ParentId == treeNode.Id)
                   {
                       treeNode.Children.Add(it);
                   }
               }
          }
          return trees;
     }
     catch (Exception ex)
     {
          throw ex;
     }
}