11.2 委托的声明

委托是一种类：

static void Main(string[] args)
{
    Type t = typeof(Action);
    Console.WriteLine(t.IsClass);
}

委托是类，所以声明位置是和 class 处于同一个级别。但 C# 允许嵌套声明类（一个类里面可以声明另一个类），所以有时也会有 delegate 在 class 内部声明的情况。

实例：

    public delegate double Calc(double x, double y);
    class Program
    {
        static void Main(string[] args)
        {
            Calculator calculator = new Calculator();
            Calc calc1 = new Calc(calculator.Add);
            Calc calc2 = new Calc(calculator.Sub); 
            Calc calc3 = new Calc(calculator.Mul);
            Calc calc4 = new Calc(calculator.Div);
            double a = 100;
            double b = 200;
            double c = 0;
            c = calc1.Invoke(a, b);
            Console.WriteLine(c);
            c = calc2.Invoke(a, b);
            Console.WriteLine(c);
            c = calc3.Invoke(a, b);
            Console.WriteLine(c);
            c = calc4.Invoke(a, b);
            Console.WriteLine(c);
        }
    }
    class Calculator
    {
        public double Add(double x,double y)
        {
            return x + y;
        }
        public double Sub(double x, double y)
        {
            return x - y;
        }
        public double Mul(double x, double y)
        {
            return x * y;
        }
        public double Div(double x, double y)
        {
            return x / y;
        }
    }