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Check for null after FirstOrDefault? [duplicate]

This question already has an answer here:
selecting a property from FirstOrDefault in case FirstOrDefault returns null
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Closed 4 years ago.
I want to find the first member of a dictionary which satisfies certain condition and then get some properties from the found item. My concern is what if no item is found. Consider this code:
Dictionary<int, Class1> dict;
....
....
var foundPair = dict.Select(i => new { i }).FirstOrDefault(somePredicate);
SomeClass result = null;
if (foundPair != null)
result = foundPair.i.Value.SomeProp;
The result is what I'm looking for and this code looks obscure to me. Can we get the above functionality in a single linq chain? I mean something similar to this:
var result = protsDict.Select(i => new { i }).FirstOrDefault(somePredicate).SomeLinqChain(...).i.Value.SomeProps;
And the result should be SomeProps if an item is found and null if no item is found.

It could be rewritten, in various ways, as:
The key is: use Where
var result = protsDict.Where(somePredicate).SomeLinqChain(...) //some chain
.Where(someMore) //some more
.Where(i => i.Value != null) //or even...
.Select(i =>i.Value.SomeProps).FirstOrDefault();//then select; take or skip

You can use Where followed by Take(1) to isolate at most one element which satisfies the condition.
var foundPair = dict
.Select(i => new { i })
.Where(somePredicate)
.Take(1)
.Select(x => some_mapping(x)
...;
When done this way, the subsequent expressions will only be executed if the first element satisfying the predicate has been found, and only on that element. Should more elements satisfy the predicate, all but the first one would be ignored.
In that sense, this technique works the same as FirstOrDefault, only in a safe way.

Check if query to DB is empty

The action method in the controller receive a value, like a couple of letters. The code checks if the table contains any of this letters. I'm using this code for that task:
var result = db.People.Where(b => b.Name.ToUpper().Contains(filter.ToUpper()));
But how can I check if the result variable is empty or null, when there are not any matching letters? I tested this, but it's not working!
If(result == ""){
// Do something
}
I would like to use Viewbag to send a message that there was no match or perhaps do this check in the View. I'm using this with some AJAX and Partial Views and it's working perfect, but I just want show a message if there are not any matches. What is the best way to check if the result value is empty or null?

The simplest way would be by using !result.Any():
var result = db.People.Where(b => b.Name.ToUpper().Contains(filter.ToUpper()));
If(!result.Any()){
// Do something
}
From MSDN on IEnumerable:
Any()
Determines whether a sequence contains any elements.
Fits exactly what you need.

Try this, Using FirstOrDefault it will give you the first record from the result, else if no result is yielded from the query it returns default value that is null,
var result = db.People.Where(b => b.Name.ToUpper().Contains(filter.ToUpper())).FirstOrDefault();
If(result == null){
// Do something
}
Or if you want to use this result, to manipulate something in your code then you can use the ToList(). It will return you list of values and if the query didnt yield anything then the list count will be 0.
var result = db.People.Where(b => b.Name.ToUpper().Contains(filter.ToUpper())).ToList();
If(result.Count == 0){
// Do something
}

Your code is not working because the code is returning an object not a string if you want to return a string then you have to use the "Select" clause to select a specific field and if you want to check in the same code then modify :
var result = db.People.Where(b => b.Name.ToUpper().Contains(filter.ToUpper())).ToList();
if(result != null && result.Count > 0)
it will work for you.

For IEnumerable we can use Any(), your code will can be written as
if(!db.People.Where(b => b.Name.ToUpper().Contains(filter.ToUpper())).Any() {
// do some thing
}

LINQ optimization using single query on collection of integers

I have a list of integers as below
List<int> myCollection = new List<int> { 2625 };
I am checking below condition
if(myCollection.Count() == 1 && myCollection.Any(number=> number == 2625))
{
// Do something
}
How can I optimize my query so that I can include both conditions single query?
Note: MyCollection may contain multiple elements hence I have used Any().

One obvious optimization would be to use List instance properties:
if(myCollection.Count == 1 && myCollection[0] == 2625))
{
// Do something
}

Actually, here you have one query. Not two. Since your collection is a list, Count() will be resolved to Count property of list. That being said, nothing actually will happen here
myCollection.Count() == 1
except from getting the value of Count.
The only query happens here
myCollection.Any(number=> number == 2625)
Furthermore, since the first condition you check if the Count is 1, if it doesn't, then Any wouldn't be evaluated at all. (This happens because we make use of &&).

I'm not sure why you want to
Optimize this, but if there is a single element why use .Any().
You could simply do
if(myCollection.Count() == 1 && myCollection.Single() == 2625)
{
// Do something
}

Remove child list item when checking grandchild list using Linq

The following code shows how I am assigning data into IEnumerable<UnCompletedJobDetailsBO>.
There is a list (IEnumerable<UnCompletedJobDetailsBO>) that has another list (List<JobDetailsBO>), with that child list (List<JobDetailsBO>) having a list on it. But the AllocationDetailList only ever has one list item.
public IEnumerable<UnCompletedJobDetailsBO> GetControlDetails(DateTime startDate)
{
var controlDetails =
(from booking in db.BookingDetail
where booking.BookingDateTime >= startDate
orderby booking.DocketNo
select new UnCompletedJobDetailsBO()
{
CustomerName = booking.Customer.Name,
CompanyName = booking.CompanyDetail.Name,
JobList =
(from job in db.BookingJob.Where(x => x.BookingID == booking.BookingID) //get job list
select new JobDetailsBO()
{
JobID = job.JobID,
JobType = job.JobType,
ItemName = job.ItemName,
AllocationDetailList =
(from jobAllocationDetail in db.JobAllocation
join returnUnCollected in db.JobReturnUnCollected
on jobAllocationDetail.JobAllocationDetailID
equals returnUnCollected.JobAllocationDetailID
into returnJob
from returnUnCollected in returnJob.DefaultIfEmpty()
where (jobAllocationDetail.Booking.BookingID == booking.BookingID)
select new AllocationBO()
{
JobUnCollectedID = returnJob.JobUnCollectedID,
JobType = jobAllocationDetail.JobType,
CurrentStatus = jobAllocationDetail.CurrentStatus,
}).DefaultIfEmpty().ToList(),
}).DefaultIfEmpty().ToList(),
}).ToList();
return controlDetails;
}
I want to remove the JobList item if the inner list (AllocationDetailList) item satisfies the condition below. Sometimes AllocationDetailList may be null, so I check that also. But when I write below query, it does not remove that particular JobList item that satisfies the condition. Thanks in advance.
public List<UnCompletedJobDetailsBO> RemovePODFromSelectedList(
List<UnCompletedJobDetailsBO> unCompletedJobDetailsBO)
{
unCompletedJobDetailsBO
.SelectMany(y => y.JobList)
.ToList()
.RemoveAll(x => ((x.AllocationDetailList[0] != null) ?
x.AllocationDetailList[0].JobType == "D" &&
x.AllocationDetailList[0].JobUnCollectedID == null &&
x.AllocationDetailList[0].CurrentStatus == 5 :
x.AllocationDetailList.Count > 1));
return unCompletedJobDetailsBO;
}

Without a good, minimal, complete code example, I'm not sure that any performance concern can be addressed. It's hard enough to fully understand the question as it is, but without being able to actually test the code, to reproduce and observe a specific performance concern, it's hard to know for sure where your concern specifically lies, never mind how to fix it.
That said, from the code you posted, it is clear why items are not being removed from the list. The basic issue is that while the SelectMany() method does have the effect of allowing you to enumerate all of the elements from all of the different JobList objects as a single enumeration, the elements are enumerated as a new enumeration.
When you call ToList(), you are creating a whole new list from that new enumeration, and when you call RemoveAll(), you are only removing elements from that new list, not the lists from which they originally came.
You say you can get it to work with a for loop. I assume you mean something like this:
public List<UnCompletedJobDetailsBO> RemovePODFromSelectedList(
List<UnCompletedJobDetailsBO> unCompletedJobDetailsBO)
{
foreach (var item in unCompletedJobDetailsBO)
{
item.JobList.RemoveAll(x => ((x.AllocationDetailList[0] != null) ?
x.AllocationDetailList[0].JobType == "D" &&
x.AllocationDetailList[0].JobUnCollectedID == null &&
x.AllocationDetailList[0].CurrentStatus == 5 :
x.AllocationDetailList.Count > 1));
}
return unCompletedJobDetailsBO;
}
Note: there is no need to return unCompletedJobDetailsBO. That entire object is unchanged, never mind the variable. The only thing the code is modifying is each individual JobList object within the passed-in object's members. I.e. the above method could actually have a return type of void, and the return statement could be removed entirely.
It is possible you could speed the code up by removing the elements in a different way. The List<T>.RemoveAll() method is in fact reasonably efficient, with O(n) cost. But it still involves copying all of the data in the list after the first element that is removed (so that all the elements are shifted down in the list). If you have to have the list ordered, this may be as good as you can do, but if not, you could process the removal differently, or use a different data structure altogether, something unordered where removal of one or more elements costs less.
But again, without more details and without a good example to work with addressing that particular issue doesn't seem practical here.

The condition
x.AllocationDetailList[0] != null
will throw exception if there is no item in the AllocationDetailList. Instead you need to check
x.AllocationDetailList!=null && x.AllocationDetailList.Count>0.
Also .ToList() after SelectMany in your code will create a new list and items will be removed from that new list instead of unCompletedJobDetailsBO. You need to modify the remove function as below
unCompletedJobDetailsBO.ForEach(y => y.JobList.RemoveAll(x => ((x.AllocationDetailList != null && x.AllocationDetailList.Count>0)
?
x.AllocationDetailList[0].JobType == "D"
&& x.AllocationDetailList[0].JobUnCollectedID == null
&& x.AllocationDetailList[0].CurrentStatus == "5"
:
x.AllocationDetailList.Count > 1
)
));

Sequence contains no matching element

I have an asp.net application in which I am using linq for data manipulation. While running, I get the exception "Sequence contains no matching element".
if (_lstAcl.Documents.Count > 0)
{
for (i = 0; i <= _lstAcl.Documents.Count - 1; i++)
{
string id = _lstAcl.Documents[i].ID.ToString();
var documentRow = _dsACL.Documents.First(o => o.ID == id);
if (documentRow !=null)
{
_lstAcl.Documents[i].Read = documentRow.Read;
_lstAcl.Documents[i].ReadRule = documentRow.ReadRule;
_lstAcl.Documents[i].Create= documentRow.Create;
_lstAcl.Documents[i].CreateRule = documentRow.CreateRule;
_lstAcl.Documents[i].Update = documentRow.Update;
_lstAcl.Documents[i].UpdateRule = documentRow.UpdateRule;
_lstAcl.Documents[i].Delete = documentRow.Delete;
_lstAcl.Documents[i].DeleteRule = documentRow.DeleteRule;
}
}
}

Well, I'd expect it's this line that's throwing the exception:
var documentRow = _dsACL.Documents.First(o => o.ID == id)
First() will throw an exception if it can't find any matching elements. Given that you're testing for null immediately afterwards, it sounds like you want FirstOrDefault(), which returns the default value for the element type (which is null for reference types) if no matching items are found:
var documentRow = _dsACL.Documents.FirstOrDefault(o => o.ID == id)
Other options to consider in some situations are Single() (when you believe there's exactly one matching element) and SingleOrDefault() (when you believe there's exactly one or zero matching elements). I suspect that FirstOrDefault is the best option in this particular case, but it's worth knowing about the others anyway.
On the other hand, it looks like you might actually be better off with a join here in the first place. If you didn't care that it would do all matches (rather than just the first) you could use:
var query = from target in _lstAcl.Documents
join source in _dsAcl.Document
where source.ID.ToString() equals target.ID
select new { source, target };
foreach (var pair in query)
{
target.Read = source.Read;
target.ReadRule = source.ReadRule;
// etc
}
That's simpler and more efficient IMO.
Even if you do decide to keep the loop, I have a couple of suggestions:
Get rid of the outer if. You don't need it, as if Count is zero the for loop body will never execute
Use exclusive upper bounds in for loops - they're more idiomatic in C#:
for (i = 0; i < _lstAcl.Documents.Count; i++)
Eliminate common subexpressions:
var target = _lstAcl.Documents[i];
// Now use target for the rest of the loop body
Where possible use foreach instead of for to start with:
foreach (var target in _lstAcl.Documents)

Use FirstOrDefault. First will never return null - if it can't find a matching element it throws the exception you're seeing.
_dsACL.Documents.FirstOrDefault(o => o.ID == id);

From the MSDN library:
The First<TSource>(IEnumerable<TSource>) method throws an exception if source contains no elements. To instead return a default value when the source sequence is empty, use the FirstOrDefault method.

For those of you who faced this issue while creating a controller through the context menu, reopening Visual Studio as an administrator fixed it.

Maybe using Where() before First() can help you, as my problem has been solved in this case.
var documentRow = _dsACL.Documents.Where(o => o.ID == id).FirstOrDefault();