I am trying to check to see if any results were returned from an entity framework query before acting upon it, I'm trying to use the code :-
var shoppingCartSessions = from c in context.ShoppingCartSessions where c.UserId == gUserID select c;
if (shoppingCartSessions.First() != null)
{
}
However I get the error
Sequence contains no elements
I have checked around stack and found that I can replace .First with .FirstOrDefault however I wanted to check if this is the correct way to be checking for existence of elements. Is there a better way rather than trying to fetch the item and then checking it?
Use Any():
var shoppingCartSessions = from c in context.ShoppingCartSessions
where c.UserId == gUserID
select c;
if (shoppingCartSessions.Any())
{
//not empty
}
Have you tried checking .Count() > 0 ?
EDIT:
As stated by Mahmoud Gamal, using Any() should render slightly better performance, since it will execute an EXISTS query, rather than a COUNT() on the DB, and you ultimately don't care about the exact amount.
Related
var Sent = request.Transactions.First(x => x.isSent == "4");
When the above query runs it gets the complete list of a table row
when condition meets. I just want a specific column value
Date_Sent.
Can I also create a if statement to check if the Date_Sent is null
or not?
TheFirst returns an item of the collection's type. Simple access it's property.
var sent = request.Transactions.FirstOrDefault(x => x.isSent=="4")?.Data_Sent;
If(sent == null) { }
See 2 things:
I suggest you use FirstOrDefault instead. It will not throw an exception if no item in sequence match predicate.
Use the ?. to access the property. It is called Null Propagation
If you do want to use the .Select then thr nicer way without creating temporary anonymous objects or breaking it into separate commands is to use the query syntax instead like this:
var sent = (from item in request.Transactions
where item.isSent =="4"
select Sent_Data).FirstOrDefault()
By the way I'd recommend looking into naming conventions in C#
Is that possible in LINQ to write a nice one-liner to get a first matched element or if there's no match than get first element in the collection?
E.g. you have a collection of parrots and you want yellow parrot but if there's no yellow parrots - then any will do, something like this:
Parrots.MatchedOrFirst(x => x.Yellow == true)
I'm trying to avoid double-go to SQL Server and the ORM we use in this particular case is Dapper.
What about:
var matchedOrFirst = Parrots.FirstOrDefault(x => x.Yellow == true)
?? Parrots.FirstOrDefault();
Edit
For structs, this should work:
var matchedOrFirst = Parrots.Any(x => x.Yellow == true)
? Parrots.First(x => x.Yellow == true)
: Parrots.FirstOrDefault();
Edit: It was a linq to SQL solution
First building a handy extension
public static T MatchedOrFirstOrDefault<T>(this IQueryable<T> collection, System.Linq.Expressions.Expression<Func<T, Boolean>> predicate)
{
return (from item in collection.Where(predicate) select item)
.Concat((from item in collection select item).Take(1))
.ToList() // Convert to query result
.FirstOrDefault();
}
Using the code
var matchedOrFirst = Parrots.MatchedOrFirstOrDefault(x => x.Yellow);
If you want to avoid a 2nd SQL call and since requires branching logic, its unlikely that Dapper will know how to convert a LINQ query you come up with into appropriate SQL IIF, CASE, or whatever other SQL-specific functions you end up using.
I recommend you write a simple stored procedure to do that and call it from Dapper.
Depending on its usage though, if this page only has one or two queries on it already, and is located reasonably close (latency wise) to the server, a 2nd simple SELECT won't hurt the overall application that much. Unless it is in a loop or something, or your example is trivial compared to the actual query regarding the cost of the first SELECT.
I have a situation wherein two urls need to be compared, one coming in to the method and the other in the db. The one in the db may or may not be Url decoded and will have %20 for space and so on.
Here's the linq query in the method:
var Result = (from s in context.301Redirects
where s.IsDeleted == false && s.Status == true && HttpUtility.UrlDecode(s.OldURL).ToUpper() == HttpUtility.UrlDecode(OldURLPath).ToUpper()
select s.NewURL).FirstOrDefault();
return Result;
Inside the method I can UrlDecode the incoming param but what about the one in the db?
This one fails because EF will not recognize UrlDecode method and throw and exception saying so.
Is there a workaround?
You can bring the collection in memory, then perform the UrlDecode at that point by evaluating the query. Try:
var Result = (from s in context.301Redirects
where s.IsDeleted == false && s.Status == true)
.AsEnumerable()
.FirstOrDefault(HttpUtility.UrlDecode(s.OldURL).ToUpper() == HttpUtility.UrlDecode(OldURLPath).ToUpper();
return Result;
If your database table is huge though, I'd consider creating some sort of TRIGGER/update script to either URL encode or decode all records, that way you don't have to check at all, you just encode/decode the parameter passed in.
Do your adjustments before the query so that you have absolute values for comparisons in the query that align with the particular formats, and always check against the ones in the database, rather than transform them - so, this potentially includes using OR in your query, instead of pulling the whole thing into memory a la ToList.
I am new to Linq and C#, I've spent a very long time trying to solve this very simple problem.
I have the following code:
var currcard = from c in db.CreditCards where c.Id == cid select c;
Where db is a database context and credit cards is a table in that database.
This has the possible result of returning several records, which I then iterate through using a foreach. When there are records to be returned, everything works beautifully.
Sometimes, however, no records are returned, and I get the 'Sequence contains no elements' error. I don't want to use FirstOrDefault or SingleOrDefault, because I want ALL the records if they are returned. However, I cannot for the life of me figure out what equivalent to these functions I would use when I need null if NO records are returned.
I would not be surprised if the solution weren't simple. Sometimes c# seems overly complex.
Edit: In the end I discovered that the error was being thrown by the SaveChanges method on the context, not the assigning statement or iteration block. Using the Any keyword led me in the end to discovering my problem, so even though it turned out to be a separate issue, I've marked that answer as correct. But thanks to everyone for your assistance.
You can iterate over it without issues:
foreach (var r in currcard) // won't throw
Likewise, you can check beforehand if there are any items in the list:
if (currcard.Any()) // won't throw
If the sequence is empty and you iterate over it, nothing should happen, which is the semantic you usually want (and what you're looking for as well). What exactly is the iteration code you're using? A foreach shouldn't have any problems with an empty sequence.
The Any() method will be true if the sequence contains elements. When working with databases it's usually a lot more efficient that using Count() > 0
var currcard = from c in CreditCards where c.Id == cid select c;
if (currcard.Any()) // do not be tempted to use currcard.Count() > 0
{
// something with currcard; e.g.
System.Console.WriteLine("Currcard");
foreach(var card in currcard) // should do nothing if currcard has no elements
{
}
}
I have a rather ugly service job that runs through a legacy database and compares it to our production database:
if (vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number) != null) {
var oldDbContractItem = vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number).First();
// check to see if there were changes
if (oldDbContractItem.DateStamp != vendorContractItem.Date_Stamp)
{
oldDbContractItem.Update(vendorContractItem);
}
}
I will get an error on var oldDbContratItem, "Sequence contains no elements", yet I just did a != null check. This must be simple, what's going on?
If I could teach people just one thing about LINQ it's this: the value of a query expression is an object that represents the query, not the results of the query. Fundamentally that's your problem; you're treating the query as its results. A query isn't a result any more than a restaurant is a club sandwich. A restaurant is a device which produces club sandwiches; a query is a device that produces results.
This is the test against null you are doing:
vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number) != null
That will always be true; That will always return at least an empty sequence... never a null.
You might be meaning to test that its length is greater than 0?
There's an easier way, though, IMO. Call FirstOrDefault() instead of First() and leave out the pre-test completely. Then instead, test if the result of FirstOrDefault() is null.
var oldDbContractItem = vendorContract.Item
.Where(x => x.ItemNumber == contractItem.Item_Number).FirstOrDefault();
if(oldDbContractItem != null) //would be null if there are no items
{
// check to see if there were changes
if (oldDbContractItem.DateStamp != vendorContractItem.Date_Stamp)
{
oldDbContractItem.Update(vendorContractItem);
}
}
}
Because your query returned a container, it just happened to be empty, the null check is on the return not what the return contains.
try this instead...
if (vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number).Any())
{
.....
}
Don't run the query twice. It's inefficient and may introduce a race condition into your code. Also, your logic is much better supported by either using IEnumerator<T> directly, or with a foreach loop.
Use either:
var result = vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number).GetEnumerator();
if (result.MoveNext) {
var oldDbContractItem = result.Current;
// ...
}
or
foreach (var oldDbContractItem in vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number)) {
// ...
break;
}
A sequence with no elements is still an object.
Where can return a non-null value, but still resolve to a sequence containing no elements. In your failing statement you are using something else, which is Where().First(). That will be null if the sequence from Where is indeed empty.
There is difference between null and an empty sequence (e.g. something similar to new List<T>()). What you want to do is to use Any() to check whether the sequence contains any elements, or rather replace First() with FirstOrDefault(), which returns null if the sequence contains no element. (If you use that, make sure that null is not a valid value that could be returned by First()).
After using a fix from the accepted answer, you can further debug the LINQ situation (if need be) by using the following steps. LINQ is an exciting technology but takes a while to wraps ones' head around it - a bit of a paradigm shift I would say. Eric's answer hit the nail on the head because he likely helped build the stuff!
Debugging Steps for LINQ
To deal with the issue of no results from the second statement change .First() to .FirstOrDefault() If nothing is found it will return the default value of the data type - if the data type is a class it will return a null value. Your second statement should then work with a null check too and without error.
Then you can debug your LINQ statement to find out why it's doing what it does.
if using LINQ to SQL, Intellisense in Visual Studio 2010 will show you the SQL generated when you hover over a query variable (not the result of a query). If you need the visualizer for VS 2008 it's here
Similarily if you're using LINQ to Entity Framework, you can get the generated SQL using the visualizer plugin here.
I always take the generated SQL from these tools, paste it directly into a query window and run it there. It will show you the empty set you're getting back and if that's a problem you can further debug it in this manner of visualizing the generated statements.