当我尝试更新视图模型时，在此上下文中仅支持原始类型(例如Int32，String和Guid) [英] Only primitive types ('such as Int32, String, and Guid') are supported in this context when I try updating my viewmodel

问题描述

我要编写的linq查询遇到一些麻烦.

I am having some trouble with a linq query I am trying to write.

我正在尝试使用存储库模式，但运气不佳.基本上，我有一个交易列表和一个第二个列表，其中包含描述字段，该描述字段与我的案例StoreItemID中的字段相对应

I am trying to use the repository pattern without to much luck. Basically I have a list of transactions and a 2nd list which contains the description field that maps against a field in my case StoreItemID

    public static IList<TransactionViewModel> All()
    {
        var result = (IList<TransactionViewModel>)HttpContext.Current.Session["Transactions"];
        if (result == null)
        {
            var rewardTypes = BusinessItemRepository.GetItemTypes(StoreID);
            HttpContext.Current.Session["Transactions"] = 
                result =
              (from item in new MyEntities().TransactionEntries
                 select new TransactionViewModel()
                                   {
                            ItemDescription = itemTypes.FirstOrDefault(r=>r.StoreItemID==item.StoreItemID).ItemDescription,
                     TransactionDate = item.PurchaseDate.Value,
                     TransactionAmount = item.TransactionAmount.Value,
                                   }).ToList();
        }
        return result;
    }


     public static List<BusinessItemViewModel>GetItemTypes(int storeID)
    {
        var result = (List<BusinessItemViewModel>)HttpContext.Current.Session["ItemTypes"];
        if (result == null)
        {
            HttpContext.Current.Session["ItemTypes"] = result =
                (from items in new MyEntities().StoreItems
                 where items.IsDeleted == false && items.StoreID == storeID
                 select new BusinessItemViewModel()
                 {
                     ItemDescription = items.Description,
                     StoreID = items.StoreID,
                     StoreItemID = items.StoreItemID
                 }).ToList();
        }
        return result;

但是我收到此错误

无法创建类型为'MyMVC.ViewModels.BusinessItemViewModel'的常量值.在这种情况下，仅支持基本类型(例如Int32，String和Guid).

Unable to create a constant value of type 'MyMVC.ViewModels.BusinessItemViewModel'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

我知道它的这一行代码，好像我将它注释掉了一样

I know its this line of code as if I comment it out it works ok

ItemDescription = itemTypes.FirstOrDefault(r=>r.StoreItemID==item.StoreItemID).ItemDescription,

如何将ItemDescription映射到我的itemTypes列表上?

How can I map ItemDescription against my list of itemTypes?

任何帮助都会很棒:)

推荐答案

此行有问题:

 ItemDescription = itemTypes.FirstOrDefault(r=>r.StoreItemID==item.StoreItemID)
                            .ItemDescription,

由于您使用的是FirstOrDefault，因此如果没有满足条件的项目，则会将null作为参考类型的默认值，然后在尝试访问ItemDescription时会遇到异常-要么使用First()如果始终存在至少一个匹配项，或者检查并定义默认属性值，以供ItemDescription在没有属性时使用:

Since you are using FirstOrDefault you will get null as default value for a reference type if there is no item that satifies the condition, then you'd get an exception when trying to access ItemDescription - either use First() if there always will be at least one match or check and define a default property value for ItemDescription to use if there is none:

 ItemDescription = itemTypes.Any(r=>r.StoreItemID==item.StoreItemID) 
                           ? itemTypes.First(r=>r.StoreItemID==item.StoreItemID)
                                      .ItemDescription
                           : "My Default",

这篇关于当我尝试更新视图模型时，在此上下文中仅支持原始类型(例如Int32，String和Guid)的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！