实体框架-实体查找不抛出无参数构造函数

我让"Find"方法在执行时抛出这个异常:异常已被调用的目标抛出。——>系统。InvalidOperationException:类的NWatch.Entities。NWatchConvertMethod'没有无参数构造函数。

将ConvertMethod添加到dbContext并保存更改没有问题。

[TestMethod]
        public void Delete()
        {
            // Add the entry
            var convertMethod = RenderConvertMethod();
            dbContext.ConvertMethods.Add(convertMethod);
            dbContext.SaveChanges();
            int id = convertMethod.ConvertMethodId;
            // Remove entry
            dbContext.ConvertMethods.Remove(convertMethod);
            dbContext.SaveChanges();
            // Ensure the entry no longer exists in the DB
            // BELOW LINES THROWS THE EXCEPTION
            var convertMethodFromDb = dbContext.ConvertMethods.Find(id);
            Assert.IsNull(convertMethodFromDb);
        }
        private NWatchConvertMethod RenderConvertMethod()
        {
            var convertMethod = new NWatchConvertMethod("TestConvertMethod")
            {
                ConvertMethodDesc = "my description"
            };
            return convertMethod;
        }