Delegate-Based APIs
Generics and Anonymous Methods/Delegates make up a powerful pair that can be used to create elegant query APIs. Here are some that we just added to the .NET Framework’s Collection libraries. I used List<T> to illustrate the APIs but most of those were also added to System.Array.
namespace System {
public delegate void Action<T>(T obj);
public delegate bool Predicate<T>(T obj);
public delegate U Converter<T,U>(T from);
public delegate int Comparison<T>(T x, T y);
}
public class List<T> : … {
public int FindIndex(Predicate<T> match);
public int FindIndex(int index, Predicate<T> match);
public int FindIndex(int index, int count, Predicate<T> match);
public int FindLastIndex(Predicate<T> match);
public int FindLastIndex(int index, Predicate<T> match);
public int FindLastIndex(int index, int count, Predicate<T> match);
public List<T> FindAll(Predicate<T> match);
public T Find(Predicate<T> match);
public T FindLast(Predicate match);
public bool Exists(Predicate<T> match);
public bool TrueForAll(Predicate<T> match);
public int RemoveAll(Predicate<T> match);
public void ForEach(Action<T> action);
public void Sort(Comparison<T> comparison);
public List<U> ConvertAll<U>(Converter<T,U> converter);
}
Finding Even Integers in List<T>
List<int> integers = new List<int>();
For(int i=1; i<=10; i++) integers.Add(i);
List<int> even = integers.FindAll(delegate(int i){
return i%2==0;
});
public class Order {
public Order(int number, string item) { … }
public int Number { get { return number; } }
public string Item { get { return item; } }
…
}
List<Order> orders = new List<Order>();
int orderNumber = 10;
Order order = orders.Find(delegate(Order o){
return o.Number==orderNumber;
});
List<int> integers = new List<int>();
for(int i=1; i<=10; i++) integers.Add(i);
int sum;
integers.ForEach(delegate(int i){ sum+=i; });
List<Order> orders = new List<Order>();
orders.Add(new Order(10,”Milk”));
orders.Add(new Order(5,”Cheese”));
orders.Sort(delegate(Order x, Order y){
return Comparer<int>.Default.Compare(x.Number,y.Number);
});
List<Order> orders = new List<Order>();
orders.Add(new Order(10,”Milk”));
orders.Add(new Order(5,”Cheese”));
List<int> numbers = orders.ConvertAll(delegate(Order x){
return o.Number;
});
