Efcore: NullReferenceException when executing simple query

[joshmouch]

I'm using RC1, and I have a DbContext that I'm executing a simple query on, and a null exception is being thrown deep into the EF stack. The query is:

context.Clients.FirstOrDefault(c => c.Code == "test");

The exception is:

System.NullReferenceException was unhandled by user code
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=Anonymously Hosted DynamicMethods Assembly

And the stack track is:

       at lambda_method(Closure , ValueBuffer )
       at Microsoft.Data.Entity.Query.EntityLoadInfo.Materialize()
       at Microsoft.Data.Entity.Query.Internal.QueryBuffer.GetEntity(IKeyValue keyValue, EntityLoadInfo entityLoadInfo, Boolean queryStateManager)
       at Microsoft.Data.Entity.Query.ExpressionVisitors.RelationalEntityQueryableExpressionVisitor.CreateEntity[TEntity](IQuerySource querySource, ValueBuffer valueBuffer, Int32 valueBufferOffset, QueryContext queryContext, IEntityType entityType, Boolean trackingQuery, KeyValueFactory keyValueFactory, Func`2 materializer, Boolean allowNullResult, Boolean useQueryBuffer)
       at lambda_method(Closure , ValueBuffer )
       at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
       at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
       at lambda_method(Closure , QueryContext )
       at Microsoft.Data.Entity.Query.Internal.QueryCompiler.<>c__DisplayClass18_1`1.<CompileQuery>b__1(QueryContext qc)
       at Microsoft.Data.Entity.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
       at Microsoft.Data.Entity.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
       at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source, Expression`1 predicate)

....

       at Microsoft.Extensions.DependencyInjection.ServiceLookup.FactoryService.Invoke(ServiceProvider provider)
       at Microsoft.Extensions.DependencyInjection.ServiceProvider.ScopedCallSite.Invoke(ServiceProvider provider)
       at Microsoft.Extensions.DependencyInjection.ServiceLookup.ConstructorCallSite.Invoke(ServiceProvider provider)
       at Microsoft.Extensions.DependencyInjection.ServiceProvider.ScopedCallSite.Invoke(ServiceProvider provider)
       at Microsoft.Extensions.DependencyInjection.ServiceLookup.ConstructorCallSite.Invoke(ServiceProvider provider)
       at Microsoft.Extensions.DependencyInjection.ServiceProvider.ScopedCallSite.Invoke(ServiceProvider provider)
       at Microsoft.Extensions.DependencyInjection.ServiceProvider.<>c__DisplayClass12_0.<RealizeService>b__0(ServiceProvider provider)
       at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType)

....

       at Microsoft.AspNet.Mvc.Filters.ActionFilterAttribute.<OnActionExecutionAsync>d__6.MoveNext()

[joshmouch]

If it helps at all, if I access just a single property of the entity, that works as expected.

context.Clients.Where(c => c.Code == "test").Select(c => c.Code).FirstOrDefault();

[rowanmiller]

@joshmouch can you share your model (entity classes and context) to help us reproduce this issue.

[rowanmiller]

I'm guessing the exception happens when it is trying to materialize one particular property of your entity

[joshmouch]

That was my guess, as well. Is there a way I can share it with you privately?

[joshmouch]

So there are a few things with this entity:

1. It implements an interface.
2. It has a DataContract attribute.
3. One property is a nullable enum of type byte (not int).
4. It has one read-only property (setter is private).
5. It has a DateTime property configured with .ValueGeneratedOnAdd().
6. The table name and SQL property names are all manually configured.

[joshmouch]

Actually, I just figured it out. It was #5 in the above list.
The DateTime property is non-nullable, but is represented as a nullable smalldate with a default value in the database. The row I was trying to read had a null value. I guess it couldn't fit the null database value into the non-nullable property.

This was working in EF5, so not sure if you'd consider this a bug or not.

[divega]

@joshmouch I would expect the same situation to cause an exception in EF5, although likely with a more informative message. Is that what you mean when you said this was working in EF5 or am I missing some other detail?

[joshmouch]

You could be right. I'm looking through the EF5 code and database, and I can't say with confidence that this didn't throw an exception.

[joshmouch]

So maybe the bug is just give a better error message?

[rowanmiller]

@joshmouch are you mapping to an existing database? If I use EF to create the database then the column is created as non-nullable.

Model

``` c#
class TestContext : DbContext
{
public DbSet Blogs { get; set; }

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

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>()
            .Property(b => b.Created)
            .ValueGeneratedOnAdd()
            .HasDefaultValueSql("GETDATE()");
    }
}

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }
    public DateTime Created { get; set; }
}

Generated schema

``` SQL
CREATE TABLE [Blog] (
    [BlogId] int NOT NULL IDENTITY,
    [Created] datetime2 NOT NULL DEFAULT (GETDATE()),
    [Url] nvarchar(max),
    CONSTRAINT [PK_Blog] PRIMARY KEY ([BlogId])
);

[joshmouch]

Yes, this is an existing database. So the code first model and the database schema aren't in sync. I think the problem is just with the error message making it difficult to determine the reason for the error.

[rowanmiller]

@joshmouch thanks for confirming. We will discuss on our end... this is a very perf-sensitive area of the stack, so we need to be very careful about adding logic to handle negative cases.

[rowanmiller]

@joshmouch opened #4289 to look at the overall story of better errors in query (in a way that does not adversely affect performance)

[joshmouch]

@rowanmiller Thanks. My case is an outside one, so probably not high priority.