Question

Assume that there are two queries running on a memory list;

First query (employing extension methods):

var temp = listX.Where(q => q.SomeProperty == someValue);

Second query:

var temp = from o in listX
              where o.SomeProperty == someValue
              select o;

Is there a difference between two queries in terms of performance; and if there is, why?

Answer 1

No, there is no difference at all. The compiler internally transforms the second version to the first one.

The C# specification (§7.6.12) states:

The C# language does not specify the execution semantics of query
expressions. Rather, query expressions are translated into invocations
of methods that adhere to the query expression pattern (§7.16.3).
Specifically, query expressions are translated into invocations of
methods named Where, Select, SelectMany, Join, GroupJoin, OrderBy,
OrderByDescending, ThenBy, ThenByDescending, GroupBy, and Cast.These
methods are expected to have particular signatures and result types,
as described in §7.16.3. These methods can be instance methods of the
object being queried or extension methods that are external to the
object, and they implement the actual execution of the query.

The translation from query expressions to method invocations is a
syntactic mapping that occurs before any type binding or overload
resolution has been performed. The translation is guaranteed to be
syntactically correct, but it is not guaranteed to produce
semantically correct C# code. Following translation of query
expressions, the resulting method invocations are processed as regular
method invocations, and this may in turn uncover errors, for example
if the methods do not exist, if arguments have wrong types, or if the
methods are generic and type inference fails.