Proper Linq where clauses

Question

I write a fair amount of linq in my day to day life  but mostly simple statements   I have noticed that when using where clauses  there are many ways to write them and each have the same results as far as I can tell   For example   from x in Collection   where x Age    10   where x Name     Fido    where x Fat    true   select x    Appears to be equivalent to this at least as far as the results are concerned   from x in Collection   where x Age    10  amp  amp          x Name     Fido   amp  amp          x Fat    true   select x    So is there really a difference other than syntax   If so  what is the preferred style and why

User · Answer

The first one will be implemented   Collection Where x   gt  x Age    10             Where x   gt  x Name     Fido      applied to the result of the previous            Where x   gt  x Fat    true        applied to the result of the previous   As opposed to the much simpler  and far fasterpresumably faster       all in one fell swoop Collection Where x   gt  x Age    10  amp  amp  x Name     Fido   amp  amp  x Fat    true

User · Answer

Looking under the hood  the two statements will be transformed into different query representations  Depending on the QueryProvider of Collection  this might be optimized away or not   When this is a linq-to-object call  multiple where clauses will lead to a chain of IEnumerables that read from each other  Using the single-clause form will help performance here   When the underlying provider translates it into a SQL statement  the chances are good that both variants will create the same statement

User · Answer

when i run   from c in Customers where c CustomerID    1 where c CustomerID    2 where c CustomerID    3 select c   and   from c in Customers where c CustomerID    1  amp  amp  c CustomerID    2  amp  amp  c CustomerID    3 select c customer table in linqpad   against my Customer table it output the same sql query  -- Region Parameters DECLARE  p0 Int   1 DECLARE  p1 Int   2 DECLARE  p2 Int   3 -- EndRegion SELECT  t0   CustomerID    t0   CustomerName  FROM  Customers  AS  t0  WHERE   t0   CustomerID     p0  AND   t0   CustomerID     p1  AND   t0   CustomerID     p2    so in translation to sql there is no difference and you already have seen in other answers how they will be converted to lambda expressions

User · Answer

EDIT  LINQ to Objects doesn t behave how I d expected it to  You may well be interested in the blog post I ve just written about this       They re different in terms of what will be called - the first is equivalent to   Collection Where x   gt  x Age    10             Where x   gt  x Name     Fido              Where x   gt  x Fat    true    wheras the latter is equivalent to   Collection Where x   gt  x Age    10  amp  amp                         x Name     Fido   amp  amp                        x Fat    true    Now what difference that actually makes depends on the implementation of Where being called  If it s a SQL-based provider  I d expect the two to end up creating the same SQL  If it s in LINQ to Objects  the second will have fewer levels of indirection  there ll be just two iterators involved instead of four   Whether those levels of indirection are significant in terms of speed is a different matter   Typically I would use several where clauses if they feel like they re representing significantly different conditions  e g  one is to do with one part of an object  and one is completely separate  and one where clause when various conditions are closely related  e g  a particular value is greater than a minimum and less than a maximum   Basically it s worth considering readability before any slight performance difference

User · Answer

The second one would be more efficient as it just has one predicate to evaluate against each item in the collection where as in the first one  it s applying the first predicate to all items first and the result  which is narrowed down at this point  is used for the second predicate and so on  The results get narrowed down every pass but still it involves multiple passes   Also the chaining  first method  will work only if you are ANDing your predicates  Something like this x Age    10    x Fat    true will not work with your first method

[c#] Proper Linq where clauses

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