Is there any way to negate a Predicate?

Question

I want to do something like this:

List list1 = ...
List list2 = ...
Predicate condition = ...

...

list2.         


        

Answer 1

You could use a lambda expression to define an anonymous delegate inplace that is the result of negating the result of the predicate:

list.RemoveAll(x => !condition(x));    

Another option:

static Predicate Negate(Predicate predicate) {
     return x => !predicate(x);
}

Usage:

// list is List some T
// predicate is Predicate some T
list.RemoveAll(Negate(predicate));

The reason that list.RemoveAll(!condition) does not work is that there is no ! operator defined on delegates. This is why you must define a new delegate in terms of condition as shown above.