Question

To introduce myself to the entity framework I created a console application that works with it.

There are two entities: ClassA and ClassB. There is a one-to-many relationship between them. One instance of ClassA can have multiple instances of ClassB. One instance of ClassB has 0 or 1 instance of ClassA.

The code for the classes is:

public class ClassA {
    public virtual int Id {get; set;}
    public virtual string Name {get; set;}
    public virtual ICollection<ClassB> ClassBs {get; set;}
}

public class ClassB {
    public virtual int Id {get; set;}
    public virtual string Name {get; set;}
    public virtual ClassA ClassA {get; set;}
}

The code for my database context and initializer is:

public class Context : DbContext {
    public DbSet<ClassA> ClassAs {get; set;}
    public DbSet<ClassB> ClassBs {get; set;}

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {
        modelBuilder.Entity<ClassA>()
            .HasMany(classA => classA.ClassBs)
            .WithRequired(classB => classB.ClassA);
        base.OnModelCreating(modelBuilder);
    }
}

public class Initializer : DropCreateDatabaseAlways<Context> {
    protected override void Seed(Context context) {
        base.Seed(context);
        for (int i = 1; i <= 3; i++) {
            var classA = new ClassA{
                Name = "A-" + i,
                ClassBs = new LinkedList<ClassB>()
            };
            for (int j = 1; j <= 3; j++) {
                var classB = new ClassB{
                    Name = "B-" + i + "-" + j,
                    ClassA = classA
                };
                classA.ClassBs.Add(classB);
                context.ClassBs.Add(classB);
            }
            context.ClassAs.Add(classA);
        }
        context.SaveChanges();
    }
}

My Main method is:

    static void Main(string[] args) {
        Database.SetInitializer(new Initializer());

        Context db = new Context();

        foreach (var classA in db.ClassAs) {
            Console.WriteLine(classA.Name);
            foreach (var classB in classA.ClassBs)
                Console.WriteLine("\t" + classB.Name);
        }

        Console.Write("\nFIN");
        Console.ReadKey();
    }

When the second foreach loop in the Main method starts the following exception is thrown:

System.Data.EntityCommandExecutionException was unhandled
  Message=An error occurred while executing the command definition. See the inner exception for details.
  Source=System.Data.Entity
  InnerException: System.InvalidOperationException
       Message=There is already an open DataReader associated with this Command which must be closed first.
       Source=System.Data
       StackTrace:
            at System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand command)
            at System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String method, SqlCommand command)
            at System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async)
            at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
            at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
            at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
            at System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
            at System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior)
            at System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior)

It seems that the ClassBs property in ClassA is not loading properly. However I did specify in the context that ClassA has many ClassB.

The database connection seems fine, since the first instance of ClassA is displayed on the console. I can also acces the database in Visual Studio and look at the data generated in the initializer.

I used the training videos of ASP.NET MVC to get started with this console application.

Do you know what’s going wrong?

Answer 1

You need to set multipleactiveresultsets in your connection string to true.

MultipleActiveResultSets=true;