任何ToList（）的EF6 NullReferenceException

Question

Using C#, .NET 4.5.2, Entity Framework 6.1.3 and System.Linq I have encountered a confusing exception. The exception itself does not seem to contain useful information to determined why it is being raised.

The following line of code when executed results in a NullReferenceException: dbCtx.Customers.ToList();

However, the following line runs without exception and returns the correct result: (dbCtx.Customers).ToList();

Running the parenthesis surrounded expression first will allow both forms to execute without exception:

var result1 = (dbCtx.Customers).ToList();
var result2 = dbCtx.Customers.ToList();

I would also like to note that adding entities works as expected:

dbCtx.Customers.Add(new Customer() { Enabled = true, Name = "Test" });

Customer entity class:

public sealed class Customer : BaseEntity
{
    public bool Enabled { get; set; }

    [Required]
    public string Name { get; set; }
}

BaseEntity class:

public abstract class BaseEntity
{
    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }
}

DbContext class:

public class MyDbContext : DbContext
{
    public MyDbContext() : base(@"Server=.\SQLExpress;Database=MyDatabase;Trusted_Connection=Yes;")
    {
        Configuration.LazyLoadingEnabled = true;
    }

    public virtual DbSet<Customer> Customers { get; set; }
}

What could possibly be causing this behavior?

EDIT: This problem occurs with any entity when anything like .ToList(), .Count(), etc is invoked.

Exception Details:

System.NullReferenceException occurred
    HResult=0x80004003
    Message=Object reference not set to an instance of an object.
    Source=EntityFramework
    StackTrace:
      at System.Data.Entity.Internal.Linq.InternalSet`1.get_Expression()
      at System.Data.Entity.Infrastructure.DbQuery`1.System.Linq.IQueryable.get_Expression()
      at MyProjTests.Test1.Test(MyDbContext dbCtx) in E:\ProgrammingProjects\WorkInProgress\MyProjRoot\MyProjTests\Test1.cs:line 51
      at MyProjTests.Test1.TestMethod1() in E:\ProgrammingProjects\WorkInProgress\MyProjRoot\MyProjTests\Test1.cs:line 43

EDIT 2:

After experimentation I've narrowed it down to a call to dbCtx.Database.CompatibleWithModel(bool) . Whether the argument supplied is true or false makes no difference. When I commented it out no NullReferenceException is raised later in the code. I have no idea why . Calling dbCtx.Database.Exists() works fine. In addition, dbCtx.Database.Initialize(false); Also reliably produces the error (not at callsite, but on xyz.ToList()).

Answer 1

For all I know the two expressions (with and without brackets) are absolutely equivalent and cannot possibly cause a different behavior. You can check that by looking at the IL code generated (here's how: A tool for easy IL code inspection ). In obscure cases like this, I would generally suspect some form of multithreading to be the culprit. Try isolating the offending code to the bare minimum and see if you can still reproduce this.

Answer 2

I have found a resolution. As best as I can tell, it is caused when certain methods like Database.Initialize() and Database.CompatibleWithModel() are called from outside the scope of a InitializeDatabase() method of a child of IDatabaseInitializer . Perhaps there is some unknown side effect involved with those methods..?

I'll include an example specialization of IDatabaseInitializer just for clarity for anyone else who may stumble across this problem:

public class CreateOrMigrateDatabaseInitializer<TContext, TConfiguration>
    : CreateDatabaseIfNotExists<TContext>, IDatabaseInitializer<TContext>
    where TContext : DbContext
    where TConfiguration : DbMigrationsConfiguration<TContext>, new()
{

    void IDatabaseInitializer<TContext>.InitializeDatabase(TContext context)
    {
        if (context.Database.Exists())
        {
            if (!context.Database.CompatibleWithModel(throwIfNoMetadata: false))
            {
                var migrationInitializer = new MigrateDatabaseToLatestVersion<TContext, TConfiguration>(true);
                migrationInitializer.InitializeDatabase(context);
            }
        }

        base.InitializeDatabase(context);
    }
}