闭包 Closure

http://en.wikipedia.org/wiki/Closure_(computer_science)

In computer science, a closure (also lexical closure or function closure) is a function or reference to a function together with a referencing environment—a table storing a reference to each of the non-local variables (also called free variables) of that function.^[1] A closure—unlike a plainfunction pointer—allows a function to access those non-local variables even when invoked outside of its immediate lexical scope.

The concept of closures was developed in the 1960s and was first fully implemented in 1975 as a language feature in the Scheme programming language to support lexically scoped first-class functions. The explicit use of closures is associated with functional programming languages such asLisp and ML, as traditional imperative languages such as Algol, C and Pascal did not support returning nested functions as results of higher-order functions and thus did not require supporting closures either. Many modern garbage-collected imperative languages (such as Smalltalk, the first object-oriented language featuring closures,^[2] C#, but notably not Java^[3]) support closures.

Python:

def counter():
    x = 0
    def increment(y):
        nonlocal x
        x += y
        print(x)
    return increment

C#:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ClosureDemo
{
    public delegate int DelFun ();
    
    class Program
    {
        public static DelFun A(int y){
            int x = y;
            //DelFun d1 = delegate() { return x++; };
            //return d1;
            return delegate() { return x++; };
        }

        static void Main(string[] args)
        {
            DelFun f1 = A(10);
            DelFun f2 = A(20);
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine("f1:" + f1());
                Console.WriteLine("f2:" + f2());
            }

                Console.ReadKey();
        }
    }
}