Why Does Entity Framework Throw 'There is already an open DataReader associated with this Command which must be closed first'?

Entity Framework's "Open DataReader" Error: A Common Pitfall

Modern entity frameworks simplify database interactions, but the error "There is already an open DataReader associated with this Command which must be closed first" remains a frequent headache. This article explains the root cause and provides a solution.

Consider this scenario: a query retrieves account data, then performs further processing:

var accounts = from account in context.Accounts
               from guranteer in account.Gurantors
               select new AccountsReport
               {
                   CreditRegistryId = account.CreditRegistryId,
                   AccountNumber = account.AccountNo,
                   DateOpened = account.DateOpened,
               };

return accounts.AsEnumerable()
                .Select((account, index) => new AccountsReport()
                    {
                        RecordNumber = FormattedRowNumber(account, index + 1),
                        CreditRegistryId = account.CreditRegistryId,
                        DateLastUpdated = DateLastUpdated(account.CreditRegistryId, account.AccountNumber),
                        AccountNumber = FormattedAccountNumber(account.AccountType, account.AccountNumber)
                    })
                .OrderBy(c=>c.FormattedRecordNumber)
                .ThenByDescending(c => c.StateChangeDate);

The DateLastUpdated method, shown below, adds another database query:

public DateTime DateLastUpdated(long creditorRegistryId, string accountNo)
{
    return (from h in context.AccountHistory
            where h.CreditorRegistryId == creditorRegistryId && h.AccountNo == accountNo
            select h.LastUpdated).Max();
}

The problem? The main query opens a DataReader. DateLastUpdated opens another without closing the first, leading to the error.

The Solution: Multiple Active Result Sets (MARS)

The simplest fix is to enable Multiple Active Result Sets (MARS) in your connection string:

Data Source=myServer;Initial Catalog=myDatabase;MultipleActiveResultSets=true

MARS allows the database to handle multiple open cursors simultaneously, preventing the conflict and resolving the "open DataReader" error. This avoids the need for manual DataReader closure, simplifying your code.

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