How to negate a delegate?

Question

This is my code, heavily abbreviated for simplicity

Func<Product, bool> selector;
...
selector = p => p.IsNew;
...
if(negative) // not selector
  selector = x => !selector(x); // This is wrong (causes infinite loop)
  // How do you do negate it? The result should be p => !p.IsNew

...
IEnumerable<Product> products = MyContext.Products.Where(selector);

Answer 1

You can do this with a helper method:

public static Predicate<T> Negate<T>(this Predicate<T> predicate) {
    return t => !predicate(t);
}

(or, replace Predicate with Func<T, bool>).

Then:

selector = selector.Negate();

Your stack overflow issue is pretty obvious; you're defining selector in terms of itself¹. The helper method avoids that problem.

¹: That is, this will clearly cause a stack overflow too:

public bool M() { return !M(); }

Believe it or not, you're doing exactly the same thing.