委托(2.多播委托)

多播委托的理解

上节介绍的委托都是只包含一个方法的调用，其实委托可以包含N个方法的，调用时候以加入的顺序来进行。 可以使用 +=来将一个方法加入委托 -=从委托中删除一个方法的引用。

多播委托的注意事项之方法返回值

多播委托中，如方法有返回值的话，只能接收到最后一个方法调用的返回值

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

namespace ConsoleApplication29
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<double, double> func = MathOperations.Square;
            func += MathOperations.MultiplyByTwo;
            Console.WriteLine(func(10)); //20
        }
    }
    public class MathOperations
    {
        public static double MultiplyByTwo(double value)
        {
            return value * 2;
        }
        public static double Square(double value)
        {
            return value * value;
        }
    }
}

那么只能改写算术运算中的方法了

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

namespace ConsoleApplication29
{
    class Program
    {
        static void Main(string[] args)
        {
            Action<double> action = MathOperations.Square;
            action += MathOperations.MultiplyByTwo;
            action(10);
        }
    }
    public class MathOperations
    {
        public static void MultiplyByTwo(double value)
        {
             Console.WriteLine(value*2);
        }
        public static void Square(double value)
        {
            Console.WriteLine(value * value);
        }
    }
}

还有一种解决方式，就是使用GetInvocationList方法来获得多播委托的调用序列

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

namespace ConsoleApplication29
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<double, double> func = MathOperations.Square;
            func += MathOperations.MultiplyByTwo;

            foreach (Func<double, double> item in func.GetInvocationList())
            {
                Console.WriteLine(item(10)); 
            }
        }
    }
    public class MathOperations
    {
        public static double MultiplyByTwo(double value)
        {
            return value * 2;
        }
        public static double Square(double value)
        {
            return value * value;
        }
    }
}

多播委托的注意事项之方法异常

我们将第一个求平方的方法抛出一个异常，发现委托将不能在继续执行下面的方法了。

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

namespace ConsoleApplication29
{
    class Program
    {
        static void Main(string[] args)
        {
            Action<double> action = MathOperations.Square;
            action += MathOperations.MultiplyByTwo;
            try
            {
                action(10);  //100  Square exception.
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
    }
    public class MathOperations
    {
        public static void MultiplyByTwo(double value)
        {
             Console.WriteLine(value*2);
        }
        public static void Square(double value)
        {
            Console.WriteLine(value * value);
            throw new Exception("Square exception.");
        }
    }
}

那么将如何继续下去呢？使用委托中的GetInvocationList来获得多播委托的调用列表

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

namespace ConsoleApplication29
{
    class Program
    {
        static void Main(string[] args)
        {
            Action<double> action = MathOperations.Square;
            action += MathOperations.MultiplyByTwo;

            foreach (Action<double> item in action.GetInvocationList())
            {
                try
                {
                    item(10);  //100  Square exception 20.
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.Message);
                } 
            }

        }
    }
    public class MathOperations
    {
        public static void MultiplyByTwo(double value)
        {
             Console.WriteLine(value*2);
        }
        public static void Square(double value)
        {
            Console.WriteLine(value * value);
            throw new Exception("Square exception.");
        }
    }
}