Is there any way to negate a Predicate?

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.

Answer 2

This is actually possible, but maybe in a slightly different form than you're used to. In .NET, lambda expressions can either be interpreted as delegates OR as expression trees. It is relatively straightforward to perform a NOT operation on an expression tree.

Here is a sample using your code as a starting point:

namespace Sample {     using System;     using System.Collections.Generic;     using System.Linq.Expressions;      internal class ExpressionSample     {         private static Expression<TDelegate> Negate<TDelegate>(Expression<TDelegate> expression)         {             return Expression.Lambda<TDelegate>(Expression.Not(expression.Body), expression.Parameters);         }          private static void Main()         {             // Match any string of length 2 or more characters             Expression<Predicate<string>> expression = (s) => s.Length > 1;              // Logical negation, i.e. match string of length 1 or fewer characters             Expression<Predicate<string>> negatedExpression = ExpressionSample.Negate(expression);              // Compile expressions to predicates             Predicate<string> predicate = expression.Compile();             Predicate<string> negativePredicate = negatedExpression.Compile();              List<string> list1 = new List<string> { string.Empty, "an item", "x", "another item" };             List<string> list2 = new List<string> { "yet another item", "still another item", "y", string.Empty };              list2.RemoveAll(negativePredicate);             list2.AddRange(list1.FindAll(predicate));              list2.ForEach((s) => Console.WriteLine(s));         }     } }