C# 多对多.add和.saveChanges后未刷新实体对象（System.NullReferenceException）

我有一个名为VendorReader的多对多实体，它看起来是这样的：

public partial class VendorReader
{
    public int ID { get; set; }
    public int VendorID { get; set; }
    public string AssetValue { get; set; }

    public virtual Vendor Vendor { get; set; }
    public virtual AssetReader AssetReader { get; set; }
}

public class VendorReaderRepository : Repository<VendorReader>
{
    public VendorReaderRepository() { }
    public VendorReaderRepository(EventBadgesEntities ctx) : base(ctx) { }
    public List<VendorReader> GetVendorReadersByEventID(int eventID)
    {
        return ctx.VendorReaders
                   .Where(e => e.Vendor.EventID == eventID)
                   .ToList();
    }

    public int GetVendorIDByReaderSN(string readerSN) {
        VendorReader thisList = ctx.VendorReaders
            .Where(e => e.AssetReader.ReaderSN == readerSN)
            .SingleOrDefault();
        return thisList.VendorID;
    }
}

我有一个VendorReader的存储库，看起来是这样的：

public partial class VendorReader
{
    public int ID { get; set; }
    public int VendorID { get; set; }
    public string AssetValue { get; set; }

    public virtual Vendor Vendor { get; set; }
    public virtual AssetReader AssetReader { get; set; }
}

public class VendorReaderRepository : Repository<VendorReader>
{
    public VendorReaderRepository() { }
    public VendorReaderRepository(EventBadgesEntities ctx) : base(ctx) { }
    public List<VendorReader> GetVendorReadersByEventID(int eventID)
    {
        return ctx.VendorReaders
                   .Where(e => e.Vendor.EventID == eventID)
                   .ToList();
    }

    public int GetVendorIDByReaderSN(string readerSN) {
        VendorReader thisList = ctx.VendorReaders
            .Where(e => e.AssetReader.ReaderSN == readerSN)
            .SingleOrDefault();
        return thisList.VendorID;
    }
}

在基本存储库中执行此操作的是：

public T Add(T entity)
{
    ctx.Set<T>().Add(entity);
    ctx.SaveChanges();
    return entity;
}

这是什么意思

public MapVendorModel(int eventID) 
{
    // steps to create a selectlist
    VendorRepository vr = new VendorRepository();
    listVendor = vr.GetVendorsByEventID(eventID)
        .ToList()
        .OrderBy(x => x.VendorName)
        .ToSelectList(x => x.VendorName, x => x.ID.ToString(), null)
        .ToList();

    EventID = eventID;

    // now let's get the vendor reader list
    VendorReaderRepository mvmVrr = new VendorReaderRepository();
    VendorReaderList = mvmVrr.GetVendorReadersByEventID(eventID)
        .ToList()
        .OrderBy(x => x.Vendor.VendorName)
        .ToList();
}

---

但当我这样做时，我在x.Vendor.VendorName b/c上得到一个空引用异常错误，似乎在对象中找不到添加的新供应商。我已经研究过了，我想到的是SaveChanges应该可以解决这个问题。这在我添加AssetReader之前没有发生过，它存储我出于各种原因需要的读卡器的实际序列号。

尝试将AssetReaderID属性添加到VendorReader类中以与导航属性一起使用

public partial class VendorReader
{
    public int ID { get; set; }

    public string AssetValue { get; set; }

    public int VendorID { get; set; }

    public virtual Vendor Vendor { get; set; } 

    public int AssetReaderID { get; set; }

    public virtual AssetReader AssetReader { get; set; }
}

---

如果您不想依赖默认的EF代码第一约定，那么也可以考虑使用或替代地显式配置关系。< /P> < P>这样，在使用实体模型和定义了我不需要的ID的图表之后，创建了对AsStEdValk的唯一约束，而不是映射和表。改变和各种各样的墙，我最终得到了一个简单的解决方案

                thisVendorReader.vendorID = model.AddEditVendorReader.vendorID;
                thisVendorReader.AssetValue = model.AddEditVendorReader.AssetValue;
                // need to add vendor too (even though we add the vendorID 
                // b/c entity doesn't automatically give you the vendor in a single call)
                vendor newVendor = new vendor();
                vendorRepository vRepo = new vendorRepository();
                newVendor = vRepo.GetById(thisVendorReader.vendorID);
                thisVendorReader.vendor = newVendor;
                vrr.Add(thisVendorReader);