Ienumerable vs List

gistfile1.txt

IEnumerable describes behavior, while List is an implementation of that behavior. When you use IEnumerable, you give the compiler a chance
to defer work until later, possibly optimizing along the way. If you use ToList() you force the compiler to reify the results right away.
Whenever I'm "stacking" LINQ expressions, I use IEnumerable, because by only specifying the behavior I give LINQ a chance to defer evaluation and possibly optimize the program. Remember how LINQ doesn't generate the SQL to query the database until you enumerate it.
i.e deffered execution

- Non-Generic collections - These are the collections that can hold elements of different data types. It holds all elements as object type.
So it includes overhead of type conversions.

- Generic collections - These are the collections that can hold data of same type and we can decide what type of data that collections can hold.

Boxing:
int i = 123;
// The following line boxes i.
object o = i;
Unboxing:
o = 123;
i = (int)o;  // unboxing