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

Question

I have this query and I get the error in this function:

var accounts = from account in context.Accounts
               from guranteer in account.Gurantors
           


        

Answer 1

Most likely this issue happens because of "lazy loading" feature of Entity Framework. Usually, unless explicitly required during initial fetch, all joined data (anything that stored in other database tables) is fetched only when required. In many cases that is a good thing, since it prevents from fetching unnecessary data and thus improve query performance (no joins) and saves bandwidth.

In the situation described in the question, initial fetch is performed, and during "select" phase missing lazy loading data is requested, additional queries are issued and then EF is complaining about "open DataReader".

Workaround proposed in the accepted answer will allow execution of these queries, and indeed the whole request will succeed.

However, if you will examine requests sent to the database, you will notice multiple requests - additional request for each missing (lazy loaded) data. This might be a performance killer.

A better approach is to tell to EF to preload all needed lazy loaded data during the initial query. This can be done using "Include" statement:

using System.Data.Entity;

query = query.Include(a => a.LazyLoadedProperty);

This way, all needed joins will be performed and all needed data will be returned as a single query. The issue described in the question will be solved.