I have an exchange rate table. I need to get current rate and previous rate and then compare results.
I can get first record using FirstOrDefault.
When I am using ElementAtOrDefault, this error shows "The query operator 'ElementAtOrDefault' is not supported". How can I get the second record?
You can try this:
var query=data.Skip(1).Take(1);
Take() returns null if the element is not there (so is equivalent to FirstOrDefault()).
If you'd rather an exception was thrown (cos the second element is not there) like First() then use:
Skip(1).Single()
Try this simple implementation this might be useful
var query= (from p in db.Person where p.Person == person_id select p);
var firstResult = query.First();
var secondResult = query.Skip(1).Take(1).Single();
var thirdResult = query.Skip(2).Take(1).Single();
There's a detail not explicitly mentioned here: FirstOrDefault returns an Element, Skip(1).Take(1) returns a set of 1 element; i.e. the type returned by Skip(1).Take(1) is IEnumerable, while FirstOrDefault is not.
If you use
.Take(2)
you will get the first and second.
EDIT- If you need both the first and second, then the above will be more efficient than runing your query twice:
.Take(1)
.Skip(1).Take(1)
Collection.ElementAt(1)
will fetch the second record in the collection.
Select top 2, then select second element.
As John Anderson already pointed out Skip(1).Take(1) returns an IEnumerable of size 1.
In order to get the 2nd entry itself or else a null (i.e. no exceptions), I'm using the following:
var secondEntry = collection?.Skip(1)?.FirstOrDefault();
It may not work in your specific case, but may be useful for use cases of other.
Use .Skip(1) method
Related
LINQ newbie here.
I have a long LINQ query, called it MYLONGQUERY, that returns a collection of certain class instances. If the list is not empty, I want to return a property (MYPROPERTY) of the first instance; otherwise it returns some default value (DEFAULTPROPERTY). So the query looks like this
(0 != MYLONGQUERY.count()) ? MYLONGQUERY.FirstOrDefault().MYPROPERTY: DEFAULTPROPERTY
This works fine. However, I don't like the fact that I have to repeat MYLONGQUERY before and after "?". I have been trying Let and Into, but have not been able to get those to work. And it has to be Method Syntax, not Query Syntax. Suggestions? Appreciate it.
You have to select the property first, then you can specify the default-value with DefaultIfEmpty:
var prop = MYLONGQUERY
.Select(x => x.MYPROPERTY)
.DefaultIfEmpty(DEFAULTPROPERTY) // new default-value
.First(); // never exception
everyone! ))
Here is code.
var existingEntities = (from record in globalOne.serviceContext.records_out
where record.timestamp.Date == DateTime.Now.Date
select record ).ToList();
It doesn't work.
Another code:
var existingEntities = (from record in globalOne.serviceContext.records_out
where record.timestamp.Day == DateTime.Now.Day
select record ).ToList();
It does work.
So, problem id in next string:
where record.timestamp.**Date** == DateTime.Now.Date
also won't do
where record.timestamp.Date.Equals(DateTime.Now.Date)
But why? I have no clue. "Timestamp" field is dateTime field in MS SQL SERVER.
And - there is NO records in table.
And I almost forgot - what does it mean - "doesn't work".
App just will not reach the breakpoint after that query(first), without any error, without anything.
Thanks.
You can call record.timestamp.Date because EF can't convert it to required expression tree (then convert it to sql command). In fact EF supports limited number of functions and properties, But for DateTime, EF has some good Canonical functions. You can use them in your case, e.g you can use Day(),Month(),Year() functions to solve your problem (see the link).
I am new to LINQ queries and to EF too, I usually work with MySQL and I can't guess how to write really simples queries.
I'd like to select all results from a table. So, I used like this:
ZXContainer db = new ZXContainer();
ViewBag.ZXproperties = db.ZXproperties.All();
But I see that I have to write something inside All(---).
Could someone guide me in how could I do that? And if someone has any good link for references too, I thank so much.
All() is an boolean evaluation performed on all of the elements in a collection (though immediately returns false when it reaches an element where the evaluation is false), for example, you want to make sure that all of said ZXproperties have a certain field set as true:
bool isTrue = db.ZXproperties.All(z => z.SomeFieldName == true);
Which will either make isTrue true or false. LINQ is typically lazy-loading, so if you're calling db.ZXproperties directly, you have access to all of the objects as is, but it isn't quite what you're looking for. You can either load all of the objects at the variable assignment with an .ToList():
ViewBag.ZXproperties = db.ZXproperties.ToList();
or you can use the below expression:
ViewBag.ZXproperties = from s in db.ZXproperties
select s;
Which is really no different than saying:
ViewBag.ZXproperties = db.ZXproperties;
The advantage of .ToList() is that if you are wanting to do multiple calls on this ViewBag.ZXproperties, it will only require the initial database call when it is assigning the variable. Alternatively, if you do any form of queryable action on the data, such as .Where(), you'll have another query performed, which is less than ideal if you already have the data to work with.
To select everything, just skip the .All(...), as ZXproperties allready is a collection.
ZXContainer db = new ZXContainer();
ViewBag.ZXproperties = db.ZXproperties;
You might want (or sometimes even need) to call .ToList() on this collection before use...
You don't use All. Just type
ViewBag.ZXproperties = db.ZXproperties;
or
ViewBag.ZXproperties = db.ZXproperties.ToList();
The All method is used to determine if all items of collection match some condition.
If you just want all of the items, you can just use it directly:
ViewBag.ZXproperties = db.ZXproperties;
If you want this evaluated immediately, you can convert it to a list:
ViewBag.ZXproperties = db.ZXproperties.ToList();
This will force it to be pulled across the wire immediately.
You can use this:
var result = db.ZXproperties.ToList();
For more information on linq see 101 linq sample.
All is some checking on all items and argument in it, called lambda expression.
I have created a structure and list.
public struct CarMake
{
public string name;
public string id;
}
I added structure objects to this (carMakers) and am trying to query
string selCar = from c in carMakers
where c.name == selectedCarMfgName
select c.id;
I am getting an error near select statement- cannont implicity convert IEnumerable to string. I know that query returns only one value, that's why I have like that.
thanks !
string selCar = (from c in carMakers
where c.name == selectedCarMfgName
select c.id).SingleOrDefault();
Your query returns a collection (with one element). You should use Single() (or SingleOrDefault()) to get that one item. If the query can return more than one result, you should look into First() ( or FirstOrDefault())
Pay attention to the error message. It probably says something like
"cannot implicitly convert IEnumerable<string> to string."
The results of a query of a sequence is another sequence, an IEnumerable<T>. You may know that you expect only one result, but that's not what the query does. To obtain only one result, you can optionally include another extension method on the end.
yourQuery.First();
yourQuery.FirstOrDefault();
yourQuery.Single();
yourQuery.SingleOrDefault();
The difference in these is that the First* variations can work with sequenes with many elements, whereas the Single* variations will throw exceptions if more than one element is present. The *OrDefault variations support the concept of no matching elements, and returns the default value for the type (null in the case of classes, a default value (such as 0 for int) for structs).
Use the version that conforms to your expectation. If you expect one and only one match, prefer Single. If you only care about one out of arbitrarily many, prefer First.
carMakers.Add(new CarMake() { name = "Audi", id = "1234" });
string selCar =(from c in carMakers
where c.name == "Audi"
select c.id).FirstOrDefault();
Output- 1234
I would refactor my query slightly:
var selCar = carMakers.Single(c => c.name == selectedCarMfgName).id;
This assumes you know that the car is in the list. If not, use SingleOrDefault and check the return before getting the id.
I've not done too much with LINQ but because you are selecting into a string you may need to use FirstOrDefault as your statement could return back more than one value yet your string can only hold one.
First will return null value I think if nothing is found but FirstOrDefault will return you a blank string.
I have this query:
var iterator = criteria.binaryAssetBranchNodeIds.GetEnumerator();
iterator.MoveNext();
var binaryAssetStructures = from bas in db.BinaryAssetStructures
where bas.BinaryAssetStructureId == iterator.Current
select bas;
When I iterate over the binaryAssetStructureIds with a foreach loop no problems occur. When I try this
var binaryAssetStructure = binaryAssetStructures.ElementAt(0);
I get following error:
Unable to cast object of type 'System.Linq.Expressions.MethodCallExpression' to type 'SubSonic.Linq.Structure.ProjectionExpression'
First() for example does work... What am I missing here...
I don't know SubSonic at all, but FWIW a similar issue exists with the Entity Framework. In that case it boils down to the fact that there's no direct translation of ElementAt to SQL.
First() can be easily translated to SELECT TOP 1 FROM ... ORDER BY ..., but the same is not easily expressed for ElementAt.
You could argue that e.g. ElementAt(5) should be translated to SELECT TOP 5 FROM ... ORDER BY ... and then the first four elements simply discarded, but that doesn't work very well if you ask for ElementAt(100000).
In EF, you can partialle overcome this issue forcing the expression to be evaluated first, which can be done with calls to AsEnumerable, ToList or ToArray.
For example
var binaryAssetStructure = binaryAssetStructures.AsEnumerable().ElementAt(0);
I hope this helps although not explicitly directed at SubSonic.