Question

I have this following piece of code:

public TimestampedRowStorage GetTimestampedRowStorage(string startTime, string endTime, long trendSettingID, int? period)
    {
        var timestampedList = (from t in dataContext.TrendRecords
                                     where t.TrendSetting_ID == trendSettingID
                                     select t).ToList();

        return new TimestampedRowStorage
        {
            TimestampedDictionary = timestampedList.ToDictionary(m => m.Timestamp,
                m => (from j in dataContext.TrendSignalRecords
                      where j.TrendRecord_ID == m.ID
                      select j).ToDictionary(p => p.TrendSignalSetting.Name,
                p => (double?)p.Value))
        };
    }

But I always get the following exception:

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

Here is the stack trace:

[MySqlException (0x80004005): There is
already an open DataReader associated
with this Connection which must be
closed first.]
MySql.Data.MySqlClient.MySqlCommand.CheckState()
+237 MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior
behavior) +146
MySql.Data.Entity.EFMySqlCommand.ExecuteDbDataReader(CommandBehavior
behavior) +47
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) +736
System.Data.Objects.ObjectQuery1.GetResults(Nullable1
forMergeOption) +149
System.Data.Objects.ObjectQuery1.Execute(MergeOption
mergeOption) +31
System.Data.Objects.DataClasses.EntityReference1.Load(MergeOption
mergeOption) +148
System.Data.Objects.DataClasses.RelatedEnd.Load()
+37 System.Data.Objects.DataClasses.RelatedEnd.DeferredLoad()
+8032198 System.Data.Objects.DataClasses.EntityReference1.get_Value()
+12 Nebula.Models.TrendSignalRecord.get_TrendSignalSetting()
in C:\Users\Bruno
Leonardo\documents\visual studio
2010\Projects\Nebula\Nebula\Models\SmgerDataModel.Designer.cs:2528
Nebula.Models.Trends.TrendRepository.<GetTimestampedRowStorage>b__b(TrendSignalRecord
p) in C:\Users\Bruno
Leonardo\documents\visual studio
2010\Projects\Nebula\Nebula\Models\Trends\TrendRepository.cs:229
System.Linq.Enumerable.ToDictionary(IEnumerable1
source, Func2 keySelector, Func2
elementSelector, IEqualityComparer1
comparer) +226
System.Linq.Enumerable.ToDictionary(IEnumerable1
source, Func2 keySelector, Func2
elementSelector) +54
Nebula.Models.Trends.TrendRepository.b__a(TrendRecord
m) in C:\Users\Bruno
Leonardo\documents\visual studio
2010\Projects\Nebula\Nebula\Models\Trends\TrendRepository.cs:227
System.Linq.Enumerable.ToDictionary(IEnumerable1
source, Func2 keySelector, Func2
elementSelector, IEqualityComparer1
comparer) +240
System.Linq.Enumerable.ToDictionary(IEnumerable1
source, Func2 keySelector, Func2
elementSelector) +53
Nebula.Models.Trends.TrendRepository.GetTimestampedRowStorage(String
startTime, String endTime, Int64
trendSettingID, Nullable1 period) in
C:\Users\Bruno
Leonardo\documents\visual studio
2010\Projects\Nebula\Nebula\Models\Trends\TrendRepository.cs:224
Nebula.Models.Trends.TrendRepository.GetTrendSettingContainer(String
startTime, String endTime, Int64
unitID, Int64 plantID, Int64
trendSettingID, GridSortOptions
gridSortOptions, Nullable1 page,
Nullable1 recordsPerPage, Nullable1
period, Int64[] trends, Nullable1
allTrends) in C:\Users\Bruno
Leonardo\documents\visual studio
2010\Projects\Nebula\Nebula\Models\Trends\TrendRepository.cs:206
Nebula.Controllers.GeneratingUnitController.TrendSettings(Int64
id, Int64 plantID, Int64
trendSettingID, String startTime,
String endTime, Nullable1 page,
Nullable1 recordsPerPage,
GridSortOptions options, Nullable1
period, Int64[] trends, Nullable1
allTrends) in C:\Users\Bruno
Leonardo\documents\visual studio
2010\Projects\Nebula\Nebula\Controllers\GeneratingUnitController.cs:148
lambda_method(Closure , ControllerBase
, Object[] ) +543

Can you guys help me out?

Answer 1

The error is probably because you are trying to access the database while you are accessing the database.
You should try to separate the two Linq expressions.
Maybe put something like this:

var TimestampedList = (from t in dataContext.TrendRecords
                                         where t.TrendSetting_ID == trendSettingID
                                         select t).ToList();
TimestampedDictionary = timestampedList.ToDictionary(m => m.Timestamp,
                    m => (from j in dataContext.TrendSignalRecords
                          where j.TrendRecord_ID == m.ID
                          select j).ToDictionary(p => p.TrendSignalSetting.Name,
                    p => (double?)p.Value))