When using the Linq methods .Any(<predicate>) and .All(<predicate>) on an IList, are the predicates applied to the list in a strict sequence?

Question

I have a class SomeObject which has a function Evaluate() which returns a bool. If I had an IList<SomeObject> like this:

IList<SomeObject> parameters;
parameters.Add( objA );
parameters.Add( objC );
parameters.Add( objB );

And I apply a Linq expression like Any or All

var res = parameters.Any( p => p.Evaluate() );

what order would the predicate p.Evaluate() be evaluated in? Would it be

• objA then objB then objC (assuming the SomeObject is sortable and objA < objB < objC )
• objA then objC then objB (the order they were Add ed to the IList<> )

Or is the order of evaluation not something that can be relied upon?

Answer 1

LINQ operates on IEnumerable<T> instances, so the order in which operations are applied is pretty much the order the underlying enumerator returns them, by definition.

However, IList<T> does not have any guarantee in which order items are enumerated. In the vast majority of cases (and probably for all framework implementations of IList<T> ) it will be the index order of items in the list, but the interface doesn't seem to guarantee that, so an implementation could do whatever it wants.