NullReferenceException when attempting to explicitly map a proxy type

[ibasin]

Attempting to create an application that connects to two databases seems to fail. For example, if I try to connect to SQLite and InMemory database at the same time (other combinations also fail), attempt to access ChangeTracker on the second DbContext results in a System.NullReferenceException: 'Object reference not set to an instance of an object.' This exception appears to happen in the ChangeTracker getter.

See the exception details below:

Exception message: System.NullReferenceException: 'Object reference not set to an instance of an object.'
System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Microsoft.EntityFrameworkCore
  StackTrace:
   at Microsoft.EntityFrameworkCore.Metadata.Internal.PropertyParameterBindingFactory.GetCandidatePropertyNames(String parameterName)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.PropertyParameterBindingFactory.TryBindParameter(IMutableEntityType entityType, Type parameterType, String parameterName)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.ConstructorBindingFactory.<>c__DisplayClass3_0.<TryBindConstructor>b__0(ParameterInfo p)
   at System.Linq.Enumerable.SelectArrayIterator`2.ToList()
   at Microsoft.EntityFrameworkCore.Metadata.Internal.ConstructorBindingFactory.TryBindConstructor(IMutableEntityType entityType, ConstructorInfo constructor, ConstructorBinding& binding, IEnumerable`1& failedBindings)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConstructorBindingConvention.Apply(InternalModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConventionDispatcher.ImmediateConventionScope.OnModelBuilt(InternalModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.Model.FinalizeModel()
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder, IModelValidator validator, DiagnosticsLoggers loggers)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.<>c__DisplayClass5_0.<GetModel>b__1()
   at System.Lazy`1.ViaFactory(LazyThreadSafetyMode mode)
   at System.Lazy`1.ExecutionAndPublication(LazyHelper executionAndPublication, Boolean useDefaultConstructor)
   at System.Lazy`1.CreateValue()
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.GetModel(DbContext context, IConventionSetBuilder conventionSetBuilder, IModelValidator validator, DiagnosticsLoggers loggers)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, 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 singletonCallSite, 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 singletonCallSite, 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.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_InternalServiceProvider()
   at Microsoft.EntityFrameworkCore.DbContext.get_ChangeTracker()
   at EFCoreTester.Program.<Main>d__0.MoveNext() in C:\Users\ilyabasin\Documents\Projects\TDM.Core\Frameworks\Testers\EFCoreTester\Program.cs:line 30

Steps to reproduce

The error is easy to reproduce.

Use the following code snippet to reproduce:

            using (var context = new InMemoryDbContext())
            {
                var ct = context.ChangeTracker;
                context.Set<Author>().Add(new Author { Name = "Author1" });
                await context.SaveChangesAsync();
            }

            using (var context = new SqliteDbContext())
            {
                var ct = context.ChangeTracker;
                context.Set<Author>().Add(new Author { Name = "Author2" });
                await context.SaveChangesAsync();
            }

Both SqliteDbContext and InMemoryDbContext are plain vanilla. Just note that I have lazy loading turned on for both contexts.

Further technical details

EF Core version: 3.0.0-preview5.19227.1
Database Provider: Microsoft.EntityFrameworkCore.Sqlite & Microsoft.EntityFrameworkCore.InMemory
Operating system: Windows 10
IDE: Visual Studio 2019 Preview

[ajcvickers]

@ibasin I have not been able to reproduce this--see my code below. Can you post a small, runnable project/solution or complete code listing (like below) that reproduces what you are seeing?

public class Author
{
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Book> Books { get; set; }
}

public class Book
{
    public int Id { get; set; }
    public string Name { get; set; }
    
    public int AuthorId { get; set; }
    public virtual Author Author { get; set; }
}

public class InMemoryDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseLazyLoadingProxies()
            .UseInMemoryDatabase("In-mem");

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Author>();
    }
}

public class SqliteDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseLazyLoadingProxies()
            .UseSqlite("Data Source = sqllitetest.db");

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Author>();
    }
}

public class Program
{
    public static async Task Main()
    {
        using (var context = new InMemoryDbContext())
        {
            context.Database.EnsureCreated();

            var ct = context.ChangeTracker;
            context.Set<Author>().Add(new Author { Name = "Author1" });
            await context.SaveChangesAsync();
        }

        using (var context = new SqliteDbContext())
        {
            context.Database.EnsureCreated();

            var ct = context.ChangeTracker;
            context.Set<Author>().Add(new Author { Name = "Author2" });
            await context.SaveChangesAsync();
        }
    }
}

[ibasin]

Please see attached a simple solution that reproduces the problem.

MultiRepoTest.zip

[ajcvickers]

@ibasin This is happening because the code is attempting to register a dynamically generated proxy as an entity type:

We should throw a better exception for this.

[ibasin]

@ajcvickers

Thank you! This makes perfect sense. The second one picks up the dynamic proxies generated by the first one. Fixed !

[ajcvickers]

Triage: better exception.

[lajones]

Poaching.