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

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

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);

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

Error is:


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

Update:

stack trace added:

```
InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first.]
System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand command) +5008639
System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String method, SqlCommand command) +23
System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async) +144
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsync

This can happen if you execute a query while iterating over the results from another query. It is not clear from your example where this happens because the example is not complete.

One thing that can cause this is lazy loading triggered when iterating over the results of some query.

This can be easily solved by allowing MARS in your connection string. Add MultipleActiveResultSets=true to the provider part of your connection string (where Data Source, Initial Catalog, etc. are specified).

Stack Overflow Q#6062192, score: 1573