Asp.net 计算字段上的AspNetCore NullReferenceException异常

我有一些带有计算字段的模型，可以正确地显示值，但当我试图编辑条目或创建新条目时，会出现NullReferenceException

型号：

public class MMSPostage
{
    public int ID { get; set; }

    [Display(Name = "Pieces")]
    public int PieceCount { get; set; }

    [DisplayFormat(DataFormatString = "{0:C3}")]
    public decimal Rate { get; set; }

    [DisplayFormat(DataFormatString = "{0:C3}")]
    [Display(Name = "Total")]
    public decimal MMSSubTotal
    {
        get
        {
            return (Rate + JobType.Cost) * PieceCount;
        }
    }
    public JobType JobType { get; set; }
}

保存时引发NullReferenceException的编辑页面包括：

MMSPostage = await _context.MMSPostage
            .Include(m => m.JobType)
            .SingleOrDefaultAsync(m => m.ID == id);

上下文模型：

modelBuilder.Entity("SPM_Postage_Billing.Models.MMSPostage", b =>
                {
                    b.Property<int>("ID")
                        .ValueGeneratedOnAdd();

                    b.Property<int>("PieceCount");

                    b.Property<decimal>("Rate");

                    b.HasKey("ID");

                    b.HasIndex("JobTypeID");

                    b.ToTable("MMSPostage");
                });

错误：

SPM_Postage_Billing.Models.mmspastage.get_MMSSubTotal（）输入 mmspastage.cs + 退货（费率+作业类型成本）*计件；Microsoft.Extensions.Internal.PropertyHelper.CallNullSafePropertyGetter（Func getter，对象目标）

---

希望这是我忽略的简单的事情。我是AspNetCore的新手，因此非常感谢您的帮助

如果

JobType

和

mmspastage

之间的关系只是一对多，即1

mmspastage

有1

JobType

和1

JobType

有多个

mmspastage

s，则可以设置如下：

注意：

• 我喜欢将持久性模型与域模型分开。因此，我的应用程序中有两组模型（除了视图模型）
• 这是使用Microsoft.EntityFrameworkCore

  v2.0.3。对于v2.1.x，延迟加载功能已经恢复，实体可能看起来不同，因为您可能需要那里的
  virtual
  关键字

JobTypeEntity.cs 然后，您需要使用数据注释或Fluent API将实体属性配置为SQL表列。您可以像以前一样直接在

OnModelCreating（）

上执行此操作，或者为每个实体创建配置类并在其中应用配置

JobTypeConfiguration.cs

---

免责声明：我手工编写了所有示例，因此它们未经测试，甚至可能未经编译。

我发现

JobType

和

mmspastage

之间存在关系。您需要向我们展示您是如何通过

EF-Core

配置关系的？异常看起来显然是

JobType

为空（您在这里执行了

JobType.Cost

）。您好，David，感谢您的回复。在SQL中，我有一个使用EF Core建立的外键关系。约束[FK_MMSPostage_JobType_JobTypeID]外键（[JobTypeID]）在删除集DEFAULTmodelBuilder.Entity（“SPM_Postage_Billing.Models.MMSPostage”，b=>{b.Property（“ID”）.ValueGeneratedAnd（）；b.Property（“计件计数”）上引用[dbo].[JobType]（[ID]）b.财产（“费率”）；b.HasKey（“ID”）；b.HasIndex（“JobTypeID”）；b.ToTable（“MMSPastage”）；}；你能把它贴在评论里吗？很难读。是的，当然，我错了

CONSTRAINT [FK_MMSPostage_JobType_JobTypeID] FOREIGN KEY ([JobTypeID]) REFERENCES [dbo].[JobType] ([ID]) ON DELETE SET DEFAULT

GO
CREATE NONCLUSTERED INDEX [IX_MMSPostage_JobTypeID]
    ON [dbo].[MMSPostage]([JobTypeID] ASC);

namespace DL.SO.Persistence.EFCore.Entities
{
    public class JobTypeEntity
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public double Cost { get; set; }

        public List<MMSPostageEntity> MMSPostages { get; set; }
    }
}

namespace DL.SO.Persistence.EFCore.Entities
{
    public MMSPostageEntity
    {
        public int Id { get; set; }
        public int PieceCount { get; set; }
        public double Rate { get; set; }

        public int JobTypeId { get; set; }
        public JobTypeEntity JobType { get; set; }

        public double SubTotal
        {
            get
            {
               return (Rate + JobType.Cost) * PieceCount;
            }
        }
    }
}

namespace DL.SO.Persistence.EFCore.Configurations
{
    public class JobTypeConfiguration : IEntityTypeConfiguration<JobTypeEntity>
    {
        public void Configure<EntityTypeBuilder<JobTypeEntity> builder)
        {
            builder.HasKey(x => x.Id);
            builder.Property(x => x.Name).IsRequired();
            builder.HasIndex(x => x.Name).IsUnique();
            builder.Property(x => x.Cost).IsRequired();

            builder.ToTable("JobType");
        }
    }
}

namespace DL.SO.Persistence.EFCore.Configurations
{
    public class MMSPostageConfiguration : IEntityTypeConfiguration<MMSPostageEntity>
    {
        public void Configure<EntityTypeBuilder<MMSPostageEntity> builder)
        {
            builder.HasKey(x => x.Id);

            // By default, public properties with getter and setter
            // will be included!
            // Since the computed property SubTotal only has getter,
            // it should not be included by default.
            // Just in case you worry about, you can do Ignore() too!
            builder.Ignore(x => x.SubTotal);

            // This is how you configure the 1-to-many relationship!
            builder.HasOne(x => x.JobType)
                .WithMany(jt => jt.MMSPostages)
                .HasForeignKey(x => x.JobTypeId);

            builder.ToTable("MMSPostage");
        }
    }
}

namespace DL.SO.Persistence.EFCore
{
    public class AppDbContext : DbContext
    {
        public class AppDbContext(DbContextOptions<AppDbContext> options)
            : base(options) { }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            base.OnModelCreating(builder);

            // You apply those configurations
            builder.ApplyConfiguration(new JobTypeConfiguration());
            builder.ApplyConfiguration(new MMSPostageConfiguration());
        }

        public DbSet<JobTypeEntity> JobTypes { get; set; }
        public DbSet<MMSPostageEntity> MMSPostages { get; set; }
    }
}

namespace DL.SO.Web.UI.Controllers
{
    public class PostageController : Controller
    {
        private readonly AppDbContext _dbContext;

        public PostageController(AppDbContext dbContext)
        {
           _dbContext = dbContext;
        }

        public IActionResult Edit(int id)
        {
            var postageEntity = _dbContext.MMSPostages
                    .AsNoTracking()
                    .Include(x => x.JobType)
                    .SingleOrDefault(x => x.Id == id);
            if (postageEntity == null)
            {
                return NotFound();
            }

            // Then you can construct your view model here!
            // Don't send your db entities directly to views man!
            var vm = new EditPostageViewModel
            {
                PostageId = postageEntity.Id,
                PieceCount = postageEntity.PieceCount,
                Rate = postageEntity.Rate,
                SubTotal = postageEntity.SubTotal,
                JobType = postageEntity.JobType.Name
            };

            return View(vm);
        }
    }
}