类型约束（类和结构）

通过使用相应的约束 class 或 struct，可以指定 type 参数是应该是引用类型还是值类型。如果使用这些约束，则必须先定义它们，然后才能列出所有其他约束（例如父类型或 new()）。

// TRef must be a reference type, the use of Int32, Single, etc. is invalid.
// Interfaces are valid, as they are reference types
class AcceptsRefType<TRef>
    where TRef : class
{
    // TStruct must be a value type.
    public void AcceptStruct<TStruct>()
        where TStruct : struct
    {
    }

    // If multiple constraints are used along with class/struct
    // then the class or struct constraint MUST be specified first
    public void Foo<TComparableClass>()
        where TComparableClass : class, IComparable
    {
    }
}