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)  {  var dateReported = (from h in context.AccountHistory  where h.CreditorRegistryId == creditorRegistryId && h.AccountNo == accountNo   select h.LastUpdated).Max();  return dateReported; } 

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

[EDIT]

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(); } 

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

Update:

[EDIT]

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, DbAsyncResult result) +87 System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method) +32 System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method) +141 System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior) +12 System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior) +10 System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior) +443 [EntityCommandExecutionException: An error occurred while executing the command definition. See the inner exception for details.] System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior) +479 System.Data.Objects.Internal.ObjectQueryExecutionPlan.Execute(ObjectContext context, ObjectParameterCollection parameterValues) +683 System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption) +119 System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator() +38 System.Linq.Enumerable.Single(IEnumerable`1 source) +114 System.Data.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__3(IEnumerable`1 sequence) +4 System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle(IEnumerable`1 query, Expression queryRoot) +29 System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute(Expression expression) +91 System.Data.Entity.Internal.Linq.DbQueryProvider.Execute(Expression expression) +69 System.Linq.Queryable.Max(IQueryable`1 source) +216 CreditRegistry.Repositories.CreditRegistryRepository.DateLastUpdated(Int64 creditorRegistryId, String accountNo) in D:\Freelance Work\SuperExpert\CreditRegistry\CreditRegistry\Repositories\CreditRegistryRepository.cs:1497 CreditRegistry.Repositories.CreditRegistryRepository.<AccountDetails>b__88(AccountsReport account, Int32 index) in D:\Freelance Work\SuperExpert\CreditRegistry\CreditRegistry\Repositories\CreditRegistryRepository.cs:1250 System.Linq.<SelectIterator>d__7`2.MoveNext() +198 System.Linq.Buffer`1..ctor(IEnumerable`1 source) +217 System.Linq.<GetEnumerator>d__0.MoveNext() +96