C# in Depth

Generic collection classes in .NET 2.097■Remove all elements in the list matching a given predicate (RemoveAll).■ Perform a given action on each element on the list (ForEach). 10We’ve already seen the ConvertAll method in listing 3.2, but there are two more delegatetypes that are very important for this extra functionality: Predicate andAction, which have the following signatures:public delegate bool Predicate (T obj)public delegate void Action (T obj)A predicate is a way of testing whether a value matches a criterion. For instance, youcould have a predicate that tested for strings having a length greater than 5, or onethat tested whether an integer was even. An action does exactly what you might expectit to—performs an action with the specified value. You might print the value to theconsole, add it to another collection—whatever you want.For simple examples, most of the methods listed here are easily achieved with aforeach loop. However, using a delegate allows the behavior to come from somewhereother than the immediate code in the foreach loop. With the improvements todelegates in C# 2, it can also be a bit simpler than the loop.Listing 3.13 shows the last two methods—ForEach and RemoveAll—in action. Wetake a list of the integers from 2 to 100, remove multiples of 2, then multiples of 3, andso forth up to 10, finally listing the numbers. You may well recognize this as a slightvariation on the “Sieve of Eratosthenes” method of finding prime numbers. I’ve usedthe streamlined method of creating delegates to make the example more realistic.Even though we haven’t covered the syntax yet (you can peep ahead to chapter 5 ifyou want to get the details), it should be fairly obvious what’s going on here.Listing 3.13Printing primes using RemoveAll and ForEach from ListList candidates = new List();for (int i=2; i