全局变量，局部变量，和使用ref

class Program
    {
       
        static void showDouble(ref int val)
        {
            val *= 2;
            Console.WriteLine("Val double = {0}", val);
        }

        static void Main(string[] args)
        {
            int val = 5;
            Console.WriteLine("val = {0}", val);
            showDouble(ref val);
            Console.WriteLine("val = {0}", val);

        }
    }

result:
val = 5
val double = 10
val = 10     
#########################################################################################################
class Program
    {
        static int val;
        static void showDouble(int val)
        {
            val *= 2;
            Console.WriteLine("Val double = {0}", val);
        }

        static void Main(string[] args)
        {
            val = 5;
            Console.WriteLine("val = {0}", val);
            showDouble(val);
            Console.WriteLine("val = {0}", val);

        }
    }

result:
val = 5
val double = 10
val = 5                                     可以理解成全局变量val的值没有改变，ShowDouble里定义的val接受了全局变量val的值传递，所以改变的只有ShowDouble里定义的局部变量val。

#########################################################################################################
class Program
    {
        static int val;
        static void showDouble(ref int val)
        {
            val *= 2;
            Console.WriteLine("Val double = {0}", val);
        }

        static void Main(string[] args)
        {
            val = 5;
            Console.WriteLine("val = {0}", val);
            showDouble(ref val);
            Console.WriteLine("val = {0}", val);

        }
    }
result:
val = 5
val double = 10
val = 10                            ShowDouble函数的参数使用的ref关键字，所以是引用传递，所以全局变量val的值也改变了。

#########################################################################################################
class Program
    {
        static int val;
        static void showDouble()
        {
            val *= 2;
            Console.WriteLine("Val double = {0}", val);
        }

        static void Main(string[] args)
        {
            val = 5;
            Console.WriteLine("val = {0}", val);
            showDouble();
            Console.WriteLine("val = {0}", val);

        }
    }

result:
val = 5
val double = 10
val = 10                                          在函数ShowDouble里没有定义val，所以函数使用的是全局变量val，在函数内改变全局变量val的值后，在函数外它的值也改变了

#########################################################################################################
class Program
    {
        static int val;
        static void showDouble(int val)
        {
            val *= 2;
            Console.WriteLine("Val double = {0}", val);
        }

        static void Main(string[] args)
        {
            int val = 5;
            Console.WriteLine("val = {0}", val);
            showDouble(val);
            Console.WriteLine("val = {0}", val);

        }
    }

result:
val = 5
val double = 10
val = 5                        程序提示"Program.val" is never used.在Main中定义的局部变量val屏蔽了全局变量val。

#########################################################################################################

#########################################################################################################
class Program
    {
        static int val;
        static void showDouble()
        {
            val *= 2;
            Console.WriteLine("Val double = {0}", val);
        }

        static void Main(string[] args)
        {
            int val = 5;
            Console.WriteLine("val = {0}", val);
            showDouble();
            Console.WriteLine("val = {0}", val);

        }
    }

result:
val = 5
val double = 0
val = 10                                     这里很有意思的，我觉得可以理解成在函数ShowDouble里由于没有定义val，所以在函数中使用的是全局变量，如果删掉static int val，程序就会报错，说函数中的val没有定义。函数外面的val使用的是局部函数int val = 5,这里因为有局部变量所以把全局变量屏蔽了。