LINQ extension methods - Any() vs. Where() vs. Exists()
C#LinqC# Problem Overview
Unfortunately the names of these methods make terrible search terms, and I've been unable to find a good resource that explains the difference between these methods--as in when to use each.
Thanks.
Edit:
The sort of query that I'm trying to fully understand is something like this:
context.Authors.Where(a => a.Books.Any(b => b.BookID == bookID)).ToList();
And thanks to all who've answered.
C# Solutions
Solution 1 - C#
Where
returns a new sequence of items matching the predicate.
Any
returns a Boolean value; there's a version with a predicate (in which case it returns whether or not any items match) and a version without (in which case it returns whether the query-so-far contains any items).
I'm not sure about Exists
- it's not a LINQ standard query operator. If there's a version for the Entity Framework, perhaps it checks for existence based on a key - a sort of specialized form of Any
? (There's an Exists
method in List<T>
which is similar to Any(predicate)
but that predates LINQ.)
Solution 2 - C#
>> context.Authors.Where(a => a.Books.Any(b => b.BookID == bookID)).ToList();
a.Books
is the list of books by that author. The property is automatically created by Linq-to-Sql, provided you have a foreign-key relationship set up.
So, a.Books.Any(b => b.BookID == bookID)
translates to "Do any of the books by this author have an ID of bookID", which makes the complete expression "Who are the authors of the book with id bookID?"
That could also be written something like
from a in context.Authors
join b in context.Books on a.AuthorId equal b.AuthorID
where b.BookID == bookID
select a;
UPDATE:
Any()
as far as I know, only returns a bool
. Its effective implementation is:
public Any(this IEnumerable<T> coll, Func<T, bool> predicate)
{
foreach(T t in coll)
{
if (predicte(t))
return true;
}
return false;
}
Solution 3 - C#
Just so you can find it next time, here is how you search for the enumerable Linq extensions. The methods are static methods of Enumerable, thus Enumerable.Any, Enumerable.Where and Enumerable.Exists.
- google.com/search?q=Enumerable.Any
- google.com/search?q=Enumerable.Where
google.com/search?q=Enumerable.Exists
As the third returns no usable result, I found that you meant List.Exists, thus:
I also recommend hookedonlinq.com as this is has very comprehensive and clear guides, as well clear explanations of the behavior of Linq methods in relation to deferness and lazyness.
Solution 4 - C#
Any - boolean function that returns true when any of object in list satisfies condition set in function parameters. For example:
List<string> strings = LoadList();
boolean hasNonEmptyObject = strings.Any(s=>string.IsNullOrEmpty(s));
Where - function that returns list with all objects in list that satisfy condition set in function parameters. For example:
IEnumerable<string> nonEmptyStrings = strings.Where(s=> !string.IsNullOrEmpty(s));
Exists - basically the same as any but it's not generic - it's defined in List
Solution 5 - C#
Any()
returns true if any of the elements in a collection meet your predicate's criteria. (Any()
does not iterate through the entire collection, as it returns upon the first match.)
Where()
returns an enumerable of all elements in a collection that meet your predicate's criteria.
Exists()
does the same thing as Any()
except it's just an older implementation that was there on the List
back before LINQ.
Solution 6 - C#
IEnumerable introduces quite a number of extensions to it which helps you to pass your own delegate and invoking the resultant from the IEnumerable back. Most of them are by nature of type Func
The Func
In case of
Where - Func
So if you have 1,5,3,6,7 as IEnumerable and you write .where(r => r<5) it will return a new IEnumerable of 1,3.
Any - Func
Exists - Predicate
Solution 7 - C#
foreach (var item in model.Where(x => !model2.Any(y => y.ID == x.ID)).ToList())
{
enter code here
}
same work you also can do with Contains
secondly Where
is give you new list of values.
thirdly using Exist
is not a good practice, you can achieve your target from Any
and contains
like
EmployeeDetail _E = Db.EmployeeDetails.where(x=>x.Id==1).FirstOrDefault();
Hope this will clear your confusion.