LINQ extension methods - Any vs Where vs Exists

Question

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   gt  a Books Any b   gt  b BookID    bookID   ToList      And thanks to all who ve answered

User · Answer

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    google com search q List Exists   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

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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 lt T gt  which is similar to Any predicate  but that predates LINQ

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Any - boolean function that returns true when any of object in list satisfies condition set in function parameters  For example   List lt string gt  strings   LoadList    boolean hasNonEmptyObject   strings Any s  gt string IsNullOrEmpty s      Where - function that returns list with all objects in list that satisfy condition set in function parameters  For example   IEnumerable lt string gt  nonEmptyStrings   strings Where s  gt   string IsNullOrEmpty s      Exists - basically the same as any but it s not generic - it s defined in List class  while Any is defined on IEnumerable interface

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foreach  var item in model Where x   gt   model2 Any y   gt  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  gt x Id  1  FirstOrDefault      Hope this will clear your confusion

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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 takes an argument T and returns TResult    In case of   Where - Func    So it takes IEnumerable of T and Returns a bool  The where will ultimately returns the IEnumerable of T s for which Func returns true    So if you have 1 5 3 6 7 as IEnumerable and you write  where r    r lt 5  it will return a new IEnumerable of 1 3    Any - Func basically is similar in signature but returns true only when any of the criteria returns true for the IEnumerable  In our case  it will return true as there are few elements present with r lt 5    Exists  - Predicate on the other hand will return true only when any one of the predicate returns true   So in our case if you pass  Exists r    5  will return true as 5 is an element present in IEnumerable

User · Answer

Any   returns true if any of the elements in a collection meet your predicate s criteria   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 IList back before Linq

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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   gt  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 lt T gt  coll  Func lt T  bool gt  predicate          foreach T t in coll                  if  predicte t               return true              return false

[c#] LINQ extension methods - Any() vs. Where() vs. Exists()

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