Question

I want to do something like this:

List<SomeClass> list1 = ...
List<SomeClass> list2 = ...
Predicate<SomeClass> condition = ...

...

list2.RemoveAll (!condition);

...

list2.AddRange (list1.FindAll (condition));

However, this results in a compiler error, as ! can’t be applied to Predicate<SomeClass>. Is there any way to do this?

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

Usage:

// list is List<T> some T
// predicate is Predicate<T> 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.