Func<int, int> GetMultiplier(int a)
{
     return delegate(int b) { return a * b; } ;
}
//...
var fn2 = GetMultiplier(2);
var fn3 = GetMultiplier(3);
Console.WriteLine(fn2(2));  //outputs 4
Console.WriteLine(fn2(3));  //outputs 6
Console.WriteLine(fn3(2));  //outputs 6
Console.WriteLine(fn3(3));  //outputs 9

闭包是传递到创建闭包的函数外部的匿名函数。 它维护它所使用的函数中的任何变量。

Func<int, int> GetMultiplier(int a)
{
     return delegate(int b) { return a * b; } ;
}
//...
var fn2 = GetMultiplier(2);
var fn3 = GetMultiplier(3);
Console.WriteLine(fn2(2));  //outputs 4
Console.WriteLine(fn2(3));  //outputs 6
Console.WriteLine(fn3(2));  //outputs 6
Console.WriteLine(fn3(3));  //outputs 9

闭包是传递到创建闭包的函数外部的匿名函数。 它维护它所使用的函数中的任何变量。

闭包是在函数中定义的函数，可以访问它的局部变量以及它的父变量。

public string GetByName(string name)
{
    List<things> theThings = new List<things>();
    return  theThings.Find<things>(t => t.Name == name)[0];
}

find方法中的函数。

t => t.Name == name

可以访问其作用域中的变量t，以及父作用域中的变量名。即使它是由find方法作为委托从另一个作用域执行的。

我有一篇关于这个主题的文章。(书中有很多例子。)

从本质上讲，闭包是一段可以在以后执行的代码，但它维护了它第一次创建时的环境——即它仍然可以使用创建它的方法的局部变量等，即使该方法已经执行完毕。

闭包的一般特性是通过匿名方法和lambda表达式在c#中实现的。

下面是一个使用匿名方法的例子:

using System;

class Test
{
    static void Main()
    {
        Action action = CreateAction();
        action();
        action();
    }

    static Action CreateAction()
    {
        int counter = 0;
        return delegate
        {
            // Yes, it could be done in one statement; 
            // but it is clearer like this.
            counter++;
            Console.WriteLine("counter={0}", counter);
        };
    }
}

输出:

counter=1
counter=2

在这里，我们可以看到CreateAction返回的操作仍然可以访问counter变量，并且确实可以增加它，即使CreateAction本身已经完成。

如果你有兴趣了解c#如何实现闭包，请阅读“I know the answer (its 42) blog”。

编译器在后台生成一个类来封装匿名方法和变量j

[CompilerGenerated]
private sealed class <>c__DisplayClass2
{
    public <>c__DisplayClass2();
    public void <fillFunc>b__0()
    {
       Console.Write("{0} ", this.j);
    }
    public int j;
}

对于函数:

static void fillFunc(int count) {
    for (int i = 0; i < count; i++)
    {
        int j = i;
        funcArr[i] = delegate()
                     {
                         Console.Write("{0} ", j);
                     };
    } 
}

把它变成:

private static void fillFunc(int count)
{
    for (int i = 0; i < count; i++)
    {
        Program.<>c__DisplayClass1 class1 = new Program.<>c__DisplayClass1();
        class1.j = i;
        Program.funcArr[i] = new Func(class1.<fillFunc>b__0);
    }
}

Closures are chunks of code that reference a variable outside themselves, (from below them on the stack), that might be called or executed later, (like when an event or delegate is defined, and could get called at some indefinite future point in time)... Because the outside variable that the chunk of code references may gone out of scope (and would otherwise have been lost), the fact that it is referenced by the chunk of code (called a closure) tells the runtime to "hold" that variable in scope until it is no longer needed by the closure chunk of code...