Question

I have a piece of code that’s performing badly, and need to rewite it to introduce a proper where clause before starting the .ToList however, that’s where I’m getting stuck.

Currently the code looks lke this (roughly, I’ve taken some of the search criteria out to make it easier to display)

var Widgets = from b in _caspEntities.Widgets.Include("WidgetRegionLogs")
                    .Include("WidgetStatusLogs").Include("WidgetVoltageTests")
                        select b;

IEnumerable<Widget> results = Widgets.ToList();
if (comboBoxRegion.SelectedValue.ToString() != "0")
{
    results = from b in results
              where b.CurrentRegionLog != null && b.CurrentRegionLog.RegionId == int.Parse(comboBoxRegion.SelectedValue.ToString())
              select b;
}

if (comboBoxStatus.SelectedValue != null)
{
    results = from b in results
              where b.CurrentStatusLog != null && b.CurrentStatusLog.StatusId == comboBoxStatus.SelectedValue.ToString()
              select b;
}

if (txtCode.Text.Trim().Length > 0)
{
    results = from b in results
              where b.CodeNumber == txtCode.Text.Trim()
              select b;
}

dataGridViewWidget.DataSource = results.ToList();

I can write the SQL easily enough, essentially the model is simple, I have a Widget it has a RegionLog and a StatusLog, both of which store a history. The current region and status are retrieved from this by grouping by WidgetID and selecting the most recent Date Updated (and then going off to Region and Status tables to get the actual value).

So, I need to translate this into LINQ, but to be honest I don’t have a clue but am ken and willing to learn. In my head, I think I need to add some better where clauses, and then do the Widget.toList after I have applied the where clauses. I’m struggling with the CurrentRegionLog and CurrentStatusLog concepts as they are not populated until I run the IEnumerable.

If anyone can give some pointers, I’d be grateful,
Thanks

Edit – Added

    public BatteryRegionLog CurrentRegionLog
    {
        get { return _currentRegionLog; }
    }

    private BatteryRegionLog _currentRegionLog
    {
        get
        {
            if (this.BatteryRegionLogs.Count > 0)
            {
                BatteryRegionLog log = this.BatteryRegionLogs.OrderByDescending(item =>    item.LastModifiedDate).First();
                return log;
            }
            else
            {
                return null;
            }
        }
    }

Answer 1

You can compose the query like this:

    if (comboBoxRegion.SelectedValue.ToString() != "0")
    {
        var id = int.Parse(comboBoxRegion.SelectedValue.ToString()
        Widgets = from b in Widgets
                  let currentRegionLog = 
                        b.BatteryRegionLogs
                         .OrderByDescending(item => item.LastModifiedDate)
                         .FirstOrDefault()
                  where currentRegionLog.RegionId == id)
                  select b;
    }
    ... // Same for the other criteria.

    dataGridViewWidget.DataSource = Widgets.ToList();

The whole query is not executed before you do ToList(). As everything is translated to SQL you don’t need the null check b.CurrentRegionLog != null. SQL will evaluate b.CurrentRegionLog.RegionId == id just fine when there is no CurrentRegionLog.

Edit

Since CurrentRegionLog is a calculated property of your Widget class it cannot be translated into SQL. I made an effort to incorporate the code of calculated property into the query in a way that only the basic navigation property is used, so EF can translate it to SQL again.