Added additional handlers

src/CleanArchitecture.Web/Handlers/CreateItemHandler.cs

+using CleanArchitecture.Core.Entities;
+using CleanArchitecture.Core.Interfaces;
+using CleanArchitecture.Web.ApiModels;
+using CleanArchitecture.Web.Commands;
+using MediatR;
+
+namespace CleanArchitecture.Web.Handlers
+{
+
+    public class CreateItemHandler : IRequestHandler<CreateItemCommand, ToDoItemDTO>
+    {
+        private readonly IRepository<ToDoItem> _todoRepository;
+
+        public CreateItemHandler(IRepository<ToDoItem> todoRepository)
+        {
+            _todoRepository = todoRepository;
+        }
+        public ToDoItemDTO Handle(CreateItemCommand message)
+        {
+            var itemToCreate = message.ItemToCreate;
+            var todoItem = new ToDoItem()
+            {
+                Title = itemToCreate.Title,
+                Description = itemToCreate.Description
+            };
+            _todoRepository.Add(todoItem);
+            return ToDoItemDTO.FromToDoItem(todoItem);
+
+        }
+    }
+}

src/CleanArchitecture.Web/Handlers/GetItemHandler.cs

+using CleanArchitecture.Core.Entities;
+using CleanArchitecture.Core.Interfaces;
+using CleanArchitecture.Web.ApiModels;
+using CleanArchitecture.Web.Commands;
+using MediatR;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace CleanArchitecture.Web.Handlers
+{
+
+    public class GetItemHandler : IRequestHandler<GetItemCommand, ToDoItemDTO>
+    {
+        private readonly IRepository<ToDoItem> _todoRepository;
+
+        public GetItemHandler(IRepository<ToDoItem> todoRepository)
+        {
+            _todoRepository = todoRepository;
+        }
+        public ToDoItemDTO Handle(GetItemCommand message)
+        {
+            return ToDoItemDTO.FromToDoItem(_todoRepository.GetById(message.Id));
+        }
+    }
+}

src/CleanArchitecture.Web/Handlers/MarkCompleteHandler.cs

+using CleanArchitecture.Core.Entities;
+using CleanArchitecture.Core.Interfaces;
+using CleanArchitecture.Web.Commands;
+using MediatR;
+
+namespace CleanArchitecture.Web.Handlers
+{
+    public class MarkCompleteHandler : IRequestHandler<MarkItemCompleteCommand>
+    {
+        private readonly IRepository<ToDoItem> _todoRepository;
+
+        public MarkCompleteHandler(IRepository<ToDoItem> todoRepository)
+        {
+            _todoRepository = todoRepository;
+        }
+        public void Handle(MarkItemCompleteCommand message)
+        {
+            var item = _todoRepository.GetById(message.Id);
+            item.MarkComplete();
+            _todoRepository.Update(item);
+        }
+    }
+}