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<T>
{
public T Function(arg)
{
T temp = default(T);
if (arg != null) temp = arg.Favs;
return temp;
}
//...
}
注意：不僅可以在定義泛型類或者泛型方法的代碼中可以使用default關(guān)鍵字，甚至在任何地方都可以使用該關(guān)鍵字
如下面代碼。
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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