EF Core 7.x+MySQL使用JSON列执行add migration时遇空引用异常

我有一个Student模型类，希望其中一个属性以JSON格式存储在数据库中，使用的技术栈是.NET Core 7.x、EF Core 7.x和MySQL。

Student模型代码：

public class Student : BaseEntity
{
    public string Name { get; set; }
    public List<StudentData> Data {get; set;} = new ();
}

相关的其他对象模型：

public class StudentData
{
    public virtual Day Day { get; set; }
    public virtual StudentValue Value { get; set; }
}

public class StudentValue
{
    public string Key { get; set; }
    public string Value { get; set; }
}

public class Day
{
    public int DateOfMonth { get; set; }
    public string DayHE { get; set; }
    public int DayOfWeek { get; set; }
}

OnModelCreating配置：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Student>().OwnsMany(
        entity => entity.Data, builder =>
        {
            builder.ToJson();
            builder.OwnsOne(x => x.Day);
            builder.OwnsOne(x => x.Value);
        });
}

执行迁移命令：

dotnet-ef migrations add AddStudentModel

报错信息：

Object reference not set to an instance of an object

完整错误栈：

Build started...
Build succeeded.
2023-07-10 16:40:54.2833|0|INFO|Program|Server is running in "Development" mode, on port 5053 |Program.<Main>$
System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.EntityFrameworkCore.Metadata.Internal.RelationalModel.CreateTableMapping(IRelationalTypeMappingSource relationalTypeMappingSource, IEntityType entityType, IEntityType mappedType, StoreObjectIdentifier mappedTable, RelationalModel databaseModel, List`1 tableMappings, Boolean includesDerivedTypes, Nullable`1 isSplitEntityTypePrincipal)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.RelationalModel.AddTables(RelationalModel databaseModel, IEntityType entityType, IRelationalTypeMappingSource relationalTypeMappingSource)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.RelationalModel.Create(IModel model, IRelationalAnnotationProvider relationalAnnotationProvider, IRelationalTypeMappingSource relationalTypeMappingSource, Boolean designTime)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.RelationalModel.Add(IModel model, IRelationalAnnotationProvider relationalAnnotationProvider, IRelationalTypeMappingSource relationalTypeMappingSource, Boolean designTime)
   at Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelRuntimeInitializer.InitializeModel(IModel model, Boolean designTime, Boolean prevalidation)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelRuntimeInitializer.Initialize(IModel model, Boolean designTime, IDiagnosticsLogger`1 validationLogger)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.GetModel(DbContext context, ModelCreationDependencies modelCreationDependencies, Boolean designTime)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel(Boolean designTime)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.EntityFrameworkCore.Infrastructure.EntityFrameworkServicesBuilder.<>c.<TryAddCoreServices>b__8_4(IServiceProvider p)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass2_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider)
   at Microsoft.EntityFrameworkCore.DbContext.get_DbContextDependencies()
   at Microsoft.EntityFrameworkCore.DbContext.get_ContextServices()
   at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
   at Microsoft.EntityFrameworkCore.DbContext.Microsoft.EntityFrameworkCore.Infrastructure.IInfrastructure<System.IServiceProvider>.get_Instance()
   at Microsoft.EntityFrameworkCore.Infrastructure.Internal.InfrastructureExtensions.GetService[TService](IInfrastructure`1 accessor)
   at Microsoft.EntityFrameworkCore.Infrastructure.AccessorExtensions.GetService[TService](IInfrastructure`1 accessor)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Object reference not set to an instance of an object.

使用-v参数执行也没有更多有效信息，请问如何解决？

这个空引用异常是因为EF Core 7中OwnsMany配合ToJson时嵌套OwnsOne的写法存在兼容性问题，MySQL的EF Core提供程序对这种嵌套JSON映射支持不完善，或是EF Core本身的bug导致。可以通过以下几种方式解决：

方法一：调整映射配置并移除virtual关键字 #

EF Core的JSON映射不需要使用virtual（该关键字用于延迟加载），先移除StudentData中的virtual关键字，再将OwnsOne配置移到ToJson之前：

1. 修改StudentData模型：

public class StudentData
{
    public Day Day { get; set; }
    public StudentValue Value { get; set; }
}

2. 调整OnModelCreating配置：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Student>().OwnsMany(
        entity => entity.Data, builder =>
        {
            builder.OwnsOne(x => x.Day);
            builder.OwnsOne(x => x.Value);
            builder.ToJson();
        });
}

方法二：使用自定义JSON值转换器 #

如果上述方法无效，可以放弃OwnsMany+ToJson的组合，改用EF Core的自定义值转换器直接将List<StudentData>序列化为JSON字符串：

1. 在OnModelCreating中配置值转换器：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    var jsonOptions = new JsonSerializerOptions
    {
        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
        WriteIndented = false
    };

    modelBuilder.Entity<Student>()
        .Property(s => s.Data)
        .HasConversion(
            v => JsonSerializer.Serialize(v, jsonOptions),
            v => JsonSerializer.Deserialize<List<StudentData>>(v, jsonOptions)
        )
        .HasColumnType("json"); // MySQL中使用json类型
}

2. 确保StudentData、Day、StudentValue都是可序列化的简单类（已移除virtual关键字）。

方法三：升级EF Core版本 #

该问题在EF Core 8中已被修复，如果项目允许，可以升级到EF Core 8.x及对应的MySQL EF Core提供程序，原有的配置写法即可正常工作。

内容的提问来源于stack exchange，提问作者Yaniv Amrami