Entity Framework error: There is already an open DataReader associated with this Command which must be closed first

On a recent project I started to use Entity Framework. So far the experience has been good, but sometimes I get some unexpected issues(and I have to force myself to not go back to NHibernate ).

Let’s have a look at the code first:

	public class DemoContext:DbContext
	{
	public DbSet<Category> Categories
	{
	get;
	set;
	}
	}

view raw DemoContext.cs hosted with ❤ by GitHub

	var context=new DemoContext();
	var categories=context.Categories;
	foreach (var category in categories)
	{
	Debug.WriteLine(category.Products.Count.ToString());
	}

view raw Sample.cs hosted with ❤ by GitHub

I think there’s nothing special about this code. I first load all the categories and then walk through all the products(which will cause a n+1 issue, but that’s another discussion). When I first ran this code it failed with the following error message:

"An error occurred while executing the command definition. See the inner exception for details."

When I drilled down to the innermost exception, the following message was shown:

"There is already an open DataReader associated with this Command which must be closed first."

  To solve this, you have to add an extra part to your connection string:

Data Source=srv01;Initial Catalog=Northwind;Persist Security Info=True;Integrated Security=True;MultipleActiveResultSets=true