c# - Filtering in-memory collection: does IQueryable give better performance? -

let's have in-memory collection(persons) on have apply filter condition.

var distinctproductkeys = persons.where(d => d.productsdictionary != null)                                  .selectmany(d => d.productsdictionary.keys)                                  .distinct(); 

vs. if use iqueryable before applying filters

var distinctproductkeys = persons.asqueryable()                                  .where(d => d.productsdictionary != null)                                  .selectmany(d => d.productsdictionary.keys)                                  .distinct(); 

i have read once have collection in-memory, both iqueryable , ienumerable gives same performance.. running both queries observed iqueryable version more fast. can tell me why?

i don't know how testing performance main thing remember when working iqueryable , ienumerable both give deferred execution.

the difference iqueryable interface allows linq-to-sql (linq.-to-anything really) work. if further refine query on iqueryable, query executed in database, if possible.

for ienumerable case, linq-to-object, meaning objects matching original query have loaded memory database.

in code:

iqueryable<customer> custs = ...; // later on... var goldcustomers = custs.where(c => c.isgold); 

that code execute sql select gold customers. following code, on other hand, execute original query in database, filtering out non-gold customers in memory:

ienumerable<customer> custs = ...; // later on... var goldcustomers = custs.where(c => c.isgold); 

this quite important difference, , working on iqueryable can in many cases save returning many rows database. prime example doing paging: if use take , skip on iqueryable, number of rows requested; doing on ienumerable cause of rows loaded in memory.