c# - Returning IEnumerable<T> vs. IQueryable<T> -

what difference between returning iqueryable<t> vs. ienumerable<t>?

iqueryable<customer> custs = c in db.customers c.city == "<city>" select c;  ienumerable<customer> custs = c in db.customers c.city == "<city>" select c; 

will both deferred execution , when should 1 preferred on other?

yes, both give deferred execution.

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

for ienumerable<t> 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<t> 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<t> cause of rows loaded in memory.