dotnet核心3-获得System.NullReferenceException可能是因为我尚未加载正确的相关实体 [英] dotnet core 3 - Getting System.NullReferenceException maybe because I have not loaded the correct related entities
问题描述
我遇到以下错误:
System.NullReferenceException:'对象引用未设置为对象的实例.'
System.NullReferenceException: 'Object reference not set to an instance of an object.'
我通过以下方法得到它:
I am getting it on the following method:
private ClientIndexViewModel MapClientToVm(Client client) {
ClientDATARepository.RelatedSuburbEntities(client.Suburb); // Bring all the related entities for "Suburb" down.
var model = new ClientIndexViewModel {
Id = client.Id,
ClientNo = client.ClientNo,
Company = client.Company,
IsWarrantyCompany = client.IsWarrantyCompany,
ClientFirstName = client.ClientFirstName,
ClientLastName = client.ClientLastName,
CompanyName = client.CompanyName,
MobilePhone = client.MobilePhone,
Active = client.Active,
DeActivated = client.DateDeActivated != null
? client.DateDeActivated.Value.ToString("dd/MM/yyyy", CultureInfo.CurrentCulture)
: "n/a",
Activity = client.Activity,
Address = new AddressViewModel {
Address1 = client.Address1,
Address2 = client.Address2,
Suburb = new SuburbViewModel {
SuburbName = client.Suburb.SuburbName,
PostCode = client.Suburb.PostCode,
State = new StateViewModel {
Id = client.Suburb.State.Id,
StateName = client.Suburb.State.StateName,
StateShortName = client.Suburb.State.StateShortName
}
}
},
NumberOfJobs = client.Jobs.Count.ToString(CultureInfo.CurrentCulture)
};
// Obtain any jobs the client has including any visits for those jobs.
if (client.Jobs.Count > 0) {
JobDATARepository
.RelatedJobEntities(client.Jobs); // Bring all the related entities for "Jobs" collection down.
foreach (var job in client.Jobs) {
var jobViewModel = new ClientJobListingViewModel {
Id = job.Id,
AgentJobNo = job.AgentJobNo,
JobNo = job.JobNo,
Type = job.Type.Name,
Status = job.Status.StatusName,
WarrantyCompany = job.WarrantyCompany.CompanyName,
NumberOfVisits = job.Visits.Count.ToString(CultureInfo.CurrentCulture)
};
if (job.Visits.Count > 0)
foreach (var visit in job.Visits) {
var visitModel = new VisitViewModel {
Id = visit.Id,
VisitDate = visit.VisitDate.ToString("dd/MM/yyyy", CultureInfo.CurrentCulture),
StartTime = visit.StartTime.ToString("hh:mm", CultureInfo.CurrentCulture),
EndTime = visit.EndTime.ToString("hh:mm", CultureInfo.CurrentCulture)
};
jobViewModel.Visits.Add(visitModel);
}
model.Jobs.Add(jobViewModel);
}
}
return model;
}
，特别是本节内容:
foreach (var job in client.Jobs)
{
var jobViewModel = new ClientJobListingViewModel {
Id = job.Id,
AgentJobNo = job.AgentJobNo,
JobNo = job.JobNo,
Type = job.Type.Name,
Status = job.Status.StatusName,
WarrantyCompany = job.WarrantyCompany.CompanyName,
NumberOfVisits = job.Visits.Count.ToString(CultureInfo.CurrentCulture)
};
问题出在:
WarrantyCompany = job.WarrantyCompany.CompanyName,
问题是作业中的WarrantyCompany设置为允许空值.这是工作实体，您会注意到它已将guaranteeCompany设置为允许空值:
The thing is that WarrantyCompany in jobs is set to allow nulls. Here is the job entity and you will note that it has warrantyCompany set to allow nulls:
public class Job : IEntityBase, IAuditedEntityBase
{
public Job()
{
Visits = new List<Visit>();
}
public string? JobNo { get; set; }
public string? AgentJobNo { get; set; }
public int ClientId { get; set; } = default!;
public Client Client { get; set; } = default!;
public int? BrandId { get; set; }
public Brand? Brand { get; set; }
public int? TypeId { get; set; }
public JobType? Type { get; set; }
public int? StatusId { get; set; }
public Status? Status { get; set; }
public int? WarrantyCompanyId { get; set; }
public Client? WarrantyCompany { get; set; }
public string? Model { get; set; }
public string? Serial { get; set; }
public string? ProblemDetails { get; set; }
public string? SolutionDetails { get; set; }
public string? Notes { get; set; }
public virtual ICollection<Visit> Visits { get; }
public int Id { get; set; }
}
#nullable disable
具体地说，
public int? WarrantyCompanyId { get; set; }
public Client? WarrantyCompany { get; set; }
因此Job的WarrantyCompany可以取空...并且在很多情况下都可以.我也要放置作业的ViewModel如下:
So Job has WarrantyCompany able to take nulls... and it does in a lot of cases. The ViewModel that I am putting the jobs too is as follows:
public class ClientJobListingViewModel {
public ClientJobListingViewModel() {
Visits = new List<VisitViewModel>();
}
public int Id { get; set; }
public string JobNo { get; set; }
public string? AgentJobNo { get; set; }
public string Type { get; set; }
public string Status { get; set; }
public string? WarrantyCompany { get; set; }
public string NumberOfVisits { get; set; }
public ICollection<VisitViewModel> Visits { get; }
}
您将注意到，在此视图模型中的WarrantyCompany也可以为空...
and you will note that the WarrantyCompany in this view model can also take nulls...
public string? WarrantyCompany { get; set; }
当该工作不是保修工作时，该客户的保固编号和保固公司-客户-为空:
When the job is not a warranty job the warrantyId and warrantyCompany - a client - is null:
错误显示如下:
那么如何在不引发上述错误的情况下将其设置为null?
So how can I get it to set a null to this without throwing the above error?
推荐答案
请启用STARTUP.CS中的延迟加载以进行导航
PLEASE ENABLE LAZY LOAD IN STARTUP.CS for navigation
services.AddDbContext<AppDbObjectContext>(options =>
{
options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")).UseLazyLoadingProxies();
});
请参阅Microsoft网址"https://docs.microsoft.com/en-us/ef/core/querying/related-data"
refer to microsoft url "https://docs.microsoft.com/en-us/ef/core/querying/related-data"
还在导航属性中添加虚拟
Also add virtual in navigation property
public virtual Client? WarrantyCompany { get; set; }
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