//其中StudentInfo是實體類，可以換成自己的實體類，
public static string ListToJson(List<StudentInfo> list, string rootProperty)
        {
            StringBuilder Json = new StringBuilder();
            if (list.Count > 0)
            {
                Json.Append("[");
                for (int i = 0; i < list.Count; i++)
                {
                    if(list[i].children.Count>0){
                        Json.Append("{\"" + rootProperty+ "\":[");
                        for (var j = 0; j < list[i].children.Count; j++)
                        {
                            Json.Append("{\"" + rootProperty+ "\":[],");
                            Json.Append("\"" + "id" + "\":" + "\"" + list[i].children[j].id.ToString() + "\",");
                            Json.Append("\"" + "text" + "\":" + "\"" + list[i].children[j].text.ToString() + "\",");
                            Json.Append("\"" + "leaf" + "\":" + "false,");
                            Json.Append("\"" + "checked" + "\":" + "false");
                            Json.Append("}");
                            if (j < list[i].children.Count - 1)
                            {
                                Json.Append(",");
                            }
                        }
                        Json.Append("],");
                    }
                    else {
                      Json.Append("{\"" + rootProperty+ "\":[],");
                    }
                    Json.Append("\"" +"id" + "\":" + "\"" + list[i].id.ToString() + "\",");
                    Json.Append("\"" +"text" + "\":" + "\"" + list[i].text.ToString() + "\",");
                    if (list[i].children.Count == 0)
                    {
                        Json.Append("\"" + "leaf" + "\":" + "false,");
                        Json.Append("\"" + "checked" + "\":" + "false");
                    }
                    else {
                        Json.Append("\"" + "leaf" + "\":" + "false");
                    }
                      
                    Json.Append("}");
                    if (i < list.Count - 1)
                    {
                        Json.Append(",");
                    } 
                 }
                }
             Json.Append("]");
            return Json.ToString();
        }
 

效果如下:

   {
        [
          { "children": [], "id": "1", "text": "Item1", "leaf": false, "checked": false },
          { "children": [], "id": "2", "text": "Item2", "leaf": false, "checked": false },
          { "children": [], "id": "3", "text": "Item3", "leaf": false, "checked": false },
          { "children": [], "id": "4", "text": "Item4", "leaf": false, "checked": false },
          { "children": [], "id": "5", "text": "Item5", "leaf": false, "checked": false },
          { "children": [], "id": "6", "text": "Item6", "leaf": false, "checked": false },
          { "children": [], "id": "7", "text": "Item7", "leaf": false, "checked": false },
          { "children": [], "id": "8", "text": "Item8", "leaf": false, "checked": false },
          { "children": [], "id": "9", "text": "Item9", "leaf": false, "checked": false },
          { "children": [
                         { "children": [], "id": "11", "text": "Item11", "leaf": false, "checked": false },
                         { "children": [], "id": "12", "text": "Item12", "leaf": false, "checked": false }
                        ], "id": "10", "text": "Item10", "leaf": false
          },
          { "children": [], "id": "13", "text": "Item13", "leaf": false, "checked": false },
          { "children": [], "id": "14", "text": "Item14", "leaf": false, "checked": false },
          { "children": [
                          { "children": [], "id": "15", "text": "Item15", "leaf": false, "checked": false },
                          { "children": [], "id": "16", "text": "Item16", "leaf": false, "checked": false }
                        ]
          }
       ]
    }