One 、switch sentence

stay switch In the sentence , If there isn't any case The expression matches the switch value , Then control is passed to the following optional default The statement after the tag . without default label , Then control passes to switch outside .

switch (key){
       
	case 1:       
			Console.WriteLine("1");       
			break;   
	case 2:       
			Console.WriteLine("2");       
			break;   
	default:       
			Console.WriteLine("Sorry, no one matches!");       
			break;
		}

Two 、 Generic code

The reason why it is used default keyword , It's because you don't know whether the generic type parameter is a value type or a reference type , The initial value of the object is . Consider the following code ：

class TestDefault<T>
    {
    
        public T foo()
        {
    
            // Here is uncertainty T Is it a value type or a reference type ,int i = null （？）or class i = null (?)
            T t = null; 
            return t;
        }
    }

If we use int Type to bind generic parameters , that T Namely int type , Then the line of the comment becomes int t = null; Obviously, this is meaningless . To solve this problem , Introduced default keyword ：

Just to give T This unknown type of parameter is assigned a default value

class TestDefault<T>
    {
    
        public T foo()
        {
    
                return default(T);
        }
    }