Implemented service for returning tree view categories

Diligent.WebAPI.Business/Services/CategoryService.cs

             return dto;
         }
 
-        public async Task<List<TreeCategoryItem>> GetAllCategories()
+        public async Task<List<TreeViewCategoryResponse>> GetAllCategories()
         {
             var res = await _context.Categories.Include(k => k.ParentCategory).ToListAsync();
-            return _mapper.Map<List<TreeCategoryItem>>(res);
+
+            List<TreeViewCategoryResponse> response = new();
+
+            foreach (var item in res)
+            {
+                TreeViewCategoryResponse treeViewCategory = new();
+                if(item.ParentCategory == null)
+                {
+                    treeViewCategory.Id = item.Id;
+                    treeViewCategory.Name = item.Name;
+                    treeViewCategory.TreeViewCategories = GetTreeCategoryItem(res, item.Id);
+                    response.Add(treeViewCategory);
+                }
+
+            }
+
+            return response;
+        }
+
+        private List<TreeViewCategoryResponse> GetTreeCategoryItem(List<Category> items, int id)
+        {
+            List<TreeViewCategoryResponse> response = new();
+
+            foreach (var item in items)
+            {
+                if(item.ParentCategory != null && item.ParentCategory.Id == id) 
+                {
+                    TreeViewCategoryResponse treeViewCategory = new();
+                    treeViewCategory.Id = item.Id;
+                    treeViewCategory.Name = item.Name;
+                    treeViewCategory.TreeViewCategories = GetTreeCategoryItem(items, item.Id);
+                    response.Add(treeViewCategory);
+                }
+            }
+
+            return response;
         }
     }
 }

Diligent.WebAPI.Business/Services/Interfaces/ICategoryService.cs

         Task<CategoriesParentChild> GetRootCategories(int userId,int categoryId);
         Task<Category> GetCategoryEntityById(int? id);
         Task<List<IsGrantedCategory>> GetCategories(int userId);
-        Task<List<TreeCategoryItem>> GetAllCategories();
+        Task<List<TreeViewCategoryResponse>> GetAllCategories();
     }
 }

Diligent.WebAPI.Contracts/DTOs/Categories/TreeViewCategoryResponse.cs

+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Diligent.WebAPI.Contracts.DTOs.Categories
+{
+    public class TreeViewCategoryResponse
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+        public List<TreeViewCategoryResponse> TreeViewCategories { get; set; } = new();
+    }
+}