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I have been working a little bit with LINQ recently, and thanks to the help of some StackOverflowers I was able to get this statement working:

var traceJob = from jobDefinition in service.JobDefinitions where jobDefinition.Id == traceGuid select jobDefinition; if (traceJob != null && traceJob.Count() == 1) { traceJob.First().RunNow(); Console.WriteLine(traceJob.First().DisplayName + " Last Run Time: " + traceJob.First().LastRunTime); }

However, I am confused because the piece that makes it work is the if(traceJob.Count() ==1). If I remove that section, then I get an ObjectNullRef error saying that the enumeration of traceJob yielded no results.

Now, to my knowledge, an if statement checking the count should not actually alter the results of the Linq statement right? Can anyone explain to me why I am seeing this behavior?

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    Side Note: If you're simply checking for existance, replacing the call of traceJob.Count() == 1 to traceJob.Any() will perform better; as it returns immediately if an item is found rather than enumerating all items to get the count. Commented Nov 26, 2012 at 17:01

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No, it shouldn't. My guess is you have run into a case where the enumeration truly is empty, and by checking for a count > 0, First() does not fail.

As a side note, Any() may be a better check here as (depending on the underlying storage of your repository) it may be faster than Count():

if (traceJob != null && traceJob.Any()) { traceJob.First().RunNow(); Console.WriteLine(traceJob.First().DisplayName + " Last Run Time: " + traceJob.First().LastRunTime); }
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This code will enumerate through service.JobDefinitions 4 times, stopping each time on the first item where .Id == traceGuid. As this is looking for a single item in the loop a better answer would be one that only executes the loop once.
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var traceJob = (from jobDefinition in service.JobDefinitions where jobDefinition.Id == traceGuid select jobDefinition).SingleOrDefault();

You can use singleOrDefault to check for a single result. It will return the result that matches the where condition or null if no match is found. If more that one match for your query is found an exception is thrown.

This covers your tracejob == null as well as tracejob.count == 1 conditions.

MSDN Article

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SingleOrDefault does not throw an exception; it returns a default value (null for reference types) if there is not exactly one element.
I think you have to wrap the linq statement in brackets (like @NET3's answer below) or call the SingleOrDefault extension method on the collection variable. You're calling it on jobDefinition, which is just one item of the enumeration.
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I don't know the actual implementation of your "service" but usually linq queries actually populate their results only when requested. So Count() does change the state of traceJob, most probably populating internal collection. And it looks like First() doesn't populate internal collection or doesn't do it properly even though normally it should.

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Ok, that makes sense. So by calling the Count() check it is populating through the results so when I get to RunNow it has an object to run instead of being empty?
Yes. But First() should do the same so there should be no difference. Why it doesn't do it I don't know. It could depend on a way the service is implemented.
Interesting. Service is just from foreach(SPService service in centralAdmin.Farm.Services) So maybe it has to do with the way SharePoint objects behave.
To check it (but only to check) you can add ToList() statement to the end of you selection. Then traceJob would be a usual list and linq operations would be the ones implemented from linq to objects. Then there should be no difference whether you call Count() before the First() or not. This is my guess :-)
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Regarding to https://stackoverflow.com/a/1745716/1289709
I think using FirstOrDefault is more reasonable here.

Whenever you use SingleOrDefault, you clearly state that the query should result in at most a single result. On the other hand, when FirstOrDefault is used, the query can return any amount of results but you state that you only want the first one.

So change your code to something like this:

var traceJob = (from jobDefinition in service.JobDefinitions where jobDefinition.Id == traceGuid select jobDefinition).FirstOrDefault(); if (traceJob != null) { traceJob.RunNow(); Console.WriteLine(traceJob.DisplayName + " Last Run Time: " + traceJob.LastRunTime); }
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I think your problem arises from how linq statements are executed when they are called, not from where they are declared.

So your statement:

var traceJob = from jobDefinition in service.JobDefinitions where jobDefinition.Id == traceGuid select jobDefinition;

This is roughly functionally equivalent to:

IEnumerable<JobDefinition> GetJobDefinitions(YourService service, Guid traceGuid) { foreach(var jobDefinition in service.JobDefinitions) if(jobDefinition.Id == traceGuid) yield return jobDefinition; }

So when you call traceJob.Count() you're doing the equivalent of calling GetJobDefinitions(service, traceGuid).Count() and each time you call traceJob.First() you're hitting the loop again.

This probably isn't a problem if service.JobDefinitions can be called over and over again. However if the results change over time (for instance if jobs are added while you're executing) or if subsequent runs have different results you'll have problems.

In any case you may be better off executing the loop only once:

var traceJobs = from jobDefinition in service.JobDefinitions where jobDefinition.Id == traceGuid select jobDefinition; // This is where the loop above actually executes - if it's empty it will return null var firstJob = traceJobs.FirstorDefault(); if(firstJob != null) { firstJob.RunNow(); Console.WriteLine(firstJob.DisplayName + " Last Run Time: " + firstJob.LastRunTime); }

Alternatively you can force the loop to execute by converting it to a list or array:

var traceJobsExecuted = traceJobs.ToList();
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