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:

<code class="language-csharp">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);</code>

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

<code class="language-csharp">public DateTime DateLastUpdated(long creditorRegistryId, string accountNo)
{
    return (from h in context.AccountHistory
            where h.CreditorRegistryId == creditorRegistryId && h.AccountNo == accountNo
            select h.LastUpdated).Max();
}</code>

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:

<code class="language-csharp">Data Source=myServer;Initial Catalog=myDatabase;MultipleActiveResultSets=true</code>

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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