Add-Migration异常：System.ArgumentNullException：值不能为null。 参数名称：属性

Question

我将以下代码放入.NET Core 2.1控制台应用程序中：

public class ProductsContext : DbContext
    {
        public DbSet<Person> People { get; set; }
        public DbSet<Product> Products { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<PersonProduct>().HasKey(sc => new { sc.PersonId, sc.ProductId });
            modelBuilder.Entity<Product>()
           .ToTable("Product")
           .HasDiscriminator<string>("Description")
           .HasValue<Product1>("Product1")
           .HasValue<Product2>("Product2");

            modelBuilder.Entity<PersonProduct>()
                .HasOne<Person>(sc => sc.Person)
                .WithMany(s => s.PersonProduct)
                .HasForeignKey(sc => sc.PersonId)
                .OnDelete(DeleteBehavior.Cascade);

            modelBuilder.Entity<PersonProduct>()
                .HasOne<Product>(sc => sc.Product)
                .WithMany(s => s.PersonProduct)
                .HasForeignKey(sc => sc.ProductId)
            .OnDelete(DeleteBehavior.Cascade);

            var navigation = modelBuilder.Entity<Person>().Metadata.FindNavigation(nameof(Person));
            navigation.SetPropertyAccessMode(PropertyAccessMode.Field);
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=PQ;Trusted_Connection=True;MultipleActiveResultSets=true");
        }
    }

    public abstract class Product
    {
        public Guid Id { get; private set; }
        public string Description;
        public List<PersonProduct> PersonProduct { get; set; }

        public Product(Guid id)
        {
            Id = id;
        }

    }

    public class Product1 : Product
    {
        public Product1(Guid id)
            : base(id)
        {

        }
    }

    public class Product2 : Product
    {
        public Product2(Guid id)
            : base(id)
        {

        }
    }

    public sealed class Person
    {
        public Guid Id { get; private set; }
        public string Name { get; private set; }
        //public List<PersonSport> PersonSport { get; private set; }
        private readonly List<PersonProduct> _PersonProduct = new List<PersonProduct>();
        public IEnumerable<PersonProduct> PersonProduct => _PersonProduct.AsReadOnly();

        public Person(Guid id, string name)
        {
            Id = id;
            Name = name;
        }

        public void AddEntry(PersonProduct PersonProduct)
        {
            _PersonProduct.Add(PersonProduct);
        }

    }

    public sealed class PersonProduct
    {
        public Person Person { get; set; }
        public Guid PersonId { get; set; }
        public Product Product { get; set; }
        public Guid ProductId { get; set; }
    }

我已经在Nuget软件包管理器控制台中运行以下命令： ADD-MIGRATION InitialCreate

我得到的错误是：

System.ArgumentNullException：值不能为null。 参数名称：Microsoft.EntityFrameworkCore.Utilities.Check.NotNull [T]（T值，字符串parameterName）处的属性， 1 propertyAccessMode) at ConsoleApp1.ProductsContext.OnModelCreating(ModelBuilder modelBuilder) in C:\\GenieDevelopment\\REST\\ConsoleApp1\\Program.cs:line 42 at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder, IModelValidator validator) at System.Lazy处的Microsoft.EntityFrameworkCore.MutablePropertyBaseExtensions.SetPropertyAccessMode（IMutablePropertyBase属性，可空1 propertyAccessMode) at ConsoleApp1.ProductsContext.OnModelCreating(ModelBuilder modelBuilder) in C:\\GenieDevelopment\\REST\\ConsoleApp1\\Program.cs:line 42 at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder, IModelValidator validator) at System.Lazy 1.ViaFactory（LazyThreadSafetyMode模式）
在System.Lazy 1.ExecutionAndPublication(LazyHelper executionAndPublication, Boolean useDefaultConstructor) at System.Lazy （）处在Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model（）
在Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConjector.Scopes范围在（Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService（IServiceProvider provider，Type serviceType）在Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService [T]（IServiceProvider provider）在Microsoft.EntityFramework_Core（DbContext。 ）
在Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider（）处在Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext 1 accessor) at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func 1工厂）在Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext（IInfrastructure 1 accessor) at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func ）在Microsoft.EntityFrameworkCore。 Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl（字符串名称，字符串outputDir，Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration（字符串名称，字符串outputDir，字符串contextType）的Design.Internal.DbContextOperations.CreateContext（String contextType） Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase。<> c__DisplayClass3_0`1.b__0（）处的字符串contextType）。Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute（Action action）值不能为null。 参数名称：属性

问题是什么？ 我花了两个小时在Google上搜索，但没有找到答案。 例如，我看过这里： Add-Migration Value不能为null。 参数名称：语言

Answer 1

似乎navigation变量为null- FindNavigation在类Person找不到名称为“ Person”的属性。

我想你想做这样的事情：

        var navigation = modelBuilder.Entity<Person>().Metadata.FindNavigation(nameof(PersonProduct));
        navigation.SetPropertyAccessMode(PropertyAccessMode.Field);