连接字符串中缺少MultipleActiveResultSets导致错误

Question

I have a table with Subscribers. Each subscriber is following Targets since a certain date. New information about the targets is being accumulated in a table named TargetDatas.

I want to create a list of Subscribers that need to get a report, which contains the updates about the targets since the last report/or if no report has ever been issued since the date the target is being followed.

I tried for 2 days to figure out a linq query to get that list and failed. The only way I can get the list is like this:

List<Subscriber> outlistl = new List<Subscriber>();

foreach (Subscriber s in db.Subscribers)
{
    bool foundUpdate = false;

    foreach (TargetSubscriber ts in s.TargetsX)
    {
        //has this subscriber received a report in the past
        //if yes, since that date
        //otherwise, since the target is being watched
        DateTime fromDate;

        var latestReport = s.SubscriberReports.OrderByDescending(sr => sr.ToDate).FirstOrDefault();

        if (latestReport != null)
        {
            fromDate = latestReport.ToDate;
        }
        else
        {
            fromDate = ts.CreatedDate;
        }

        foreach (TargetData td in ts.Target.TargetDatas)
        {
            if (td.CreatedDate > fromDate)
            {
                outlistl.Add(s);
                foundUpdate = true;
                break;
            }
        }
        if (foundUpdate) { break; }
    }
    if (foundUpdate) { break; }
}

When I ran it the first time I got an error:

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

After some research I found that adding MultipleActiveResultSets=True; to my connectionstring got rid of that error and I get the results that I need.

There has to be a better way. Could someone please enlighten me on how this ought to be done, without me having set MultipleActiveResultsSets to true.

Answer 1

It's common practice to have MultipleActiveResultSets=True . It's often necessary for EF to be able to perform lazy loading of navigation properties of objects it is materializing.

But I think your query should look like this:

from s in db.Subscribers
from ts in s.TargetsX
let fromDate = s.SubscriberReports.Select(sr => (DateTime?)sr.ToDate)
                .OrderByDescending(d => d)
                .FirstOrDefault() ?? ts.CreatedDate
where ts.Target.TargetDatas.Any(td => td.CreatedDate > fromDate)
select s

I cast sr.ToDate to DateTime? to allow using the ?? operator. Now if there are no SubscriberReports the query for sr.ToDate will return a null date. (In fact it's only a trick to make the compiler happy, for the generated SQL query it wouldn't make any difference).