型別約束（類和結構）

通過使用相應的約束 class 或 struct，可以指定 type 引數是應該是引用型別還是值型別。如果使用這些約束，則必須先定義它們，然後才能列出所有其他約束（例如父型別或 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
    {
    }
}