What does default mean in C#

Default in C# is generally used in switch statements or generic code. In switch statements, default is the default situation for processing non-case matching. There are two situations when using default in generics. In reference types, the generic type is initialized to null, and in value types, the generic type is initialized to 0

default Key Words can be used in switch statements or generic code.

switch statement: Specify the default label.

Generic code: Specify the default value of the type parameter. Null for reference types, zero for value types

switch statement

In a switch statement, if no case expression matches the switch value, control is passed For statements following the optional default label. If there is no default label, control is passed outside the switch.

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

Generic code

In generic classes and generic methods, how to assign default values ​​to parameterizations when the following situations are not known in advance Type T:

T is a reference type or a value type.

If T is a value type, whether it is a numeric value or a structure.

Given a variable t of parameterized type T, the statement t = null is valid only when T is a reference type; the statement t = 0 can be used normally only when T is a numeric type rather than a structure. . The solution is to use the default keyword, which returns null for reference types and zero for numeric types. For structures, this keyword returns each structure member initialized to zero or empty, depending on whether the structures are value or reference types.

public class GenericClass
{
   public T Function(arg)
    {
        T temp = default(T);
       if (arg != null) temp = arg.Favs;
        return temp;
    }
   //...
}

注意：不仅可以在定义泛型类或者泛型方法的代码中可以使用default关键字，甚至在任何地方都可以使用该关键字
如下面代码。
static void Main(string[] args)
{
   Console.WriteLine(default(int));
   Console.WriteLine(default(string));
   Console.WriteLine(default(Color));
}

Output:

行1： 0
行2：       
行3： Color [Empty]

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