Entity Framework LINQ SelectMany exception

Question

Why does Entity Framework (4.1), with the data model you might expect behind the following query, return an InvalidOperationException ("There is already an open DataReader associated with this Command which must be closed first") on this?

((IEnumerable<Company>)db.Companies).SelectMany(x => x.Department).ToList();

score 2 · Accepted Answer · edited May 23 '17 at 12:27

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Your query is likely not a problem. You may need to turn on the MultipleActiveResultSets attribute in your connection string. Check out the answer for this one.

edited May 23 '17 at 12:27

Community

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answered Feb 10 '12 at 19:03

Bear In Hat

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score 0 · Answer 2 · answered Feb 10 '12 at 18:07

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Because using Linq to EF as an IEnumerable mantains a DataReader open, thus you can't make other queries before closing it. Try materializing the collection as an IList, like this:

db.Companies.ToList().SelectMany(x => x.Department).ToList();

answered Feb 10 '12 at 18:07

Felipe Castro

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You can make this one: ((IEnumerable)db.Companies).Where(x => x.Name == "IBM").ToList() – stovroz Feb 10 '12 at 18:22
@stovroz: Why do you use this cast in the first place? – BrokenGlass Feb 10 '12 at 18:28
@BrokenGlass. The cast is to simulate the db call being inside a GetCompanies() function of that return type. – stovroz Feb 10 '12 at 18:35
Can't you change that to return an IQueryable instead? That way, your query won't be run until you hit .ToList() – Felipe Castro Feb 10 '12 at 18:49
any difference if you turn MARS on in the conn string? – James Manning Feb 11 '12 at 03:59
@James, it does indeed. Many thanks. And sorry if your comment pre-dates Bear's answer, which I'm about to accept. – stovroz Feb 12 '12 at 14:05

Entity Framework LINQ SelectMany exception

2 Answers2