C++/CLI-Question: Is there an equivalent to the C# “is” keyword or do I have to use reflection?

Question

I\'ve read somewhere on MSDN that the equivalent to C#\'s \"is\" keyword would be dynamic_cast, but that\'s not really equivalent: It doesn\'t work with value types or with

Answer 1

While a simple workaround would be to use safe_cast(x) and catch System::InvalidCastException^, this has the obvious overhead of exception handling (unrolling the stack and all the related fun) when the type doesn't match.

I tried to come up with a different approach. While I wouldn't exactly call it simple, it does its job without using exceptions.

#using 

namespace detail
{
    generic  ref class is_instance_of_managed_helper sealed abstract
    {
    public:
        static initonly System::Func^ is_instance_of = build();

    private:
        static System::Func^ build()
        {
            using System::Linq::Expressions::Expression;
            auto param = Expression::Parameter(System::Object::typeid);
            return Expression::Lambda^>(
                Expression::TypeIs(param, T::typeid),
                param)->Compile();
        }
    };

    template  struct is_instance_of_helper
    {
        static bool is_instance_of(System::Object^ obj)
        {
            return is_instance_of_managed_helper::is_instance_of(obj);
        }
    };

    template  struct is_instance_of_helper
    {
        static bool is_instance_of(System::Object^ obj)
        {
            return dynamic_cast(obj) != nullptr;
        }
    };
}

template  bool is_instance_of(System::Object^ obj)
{
    return detail::is_instance_of_helper::is_instance_of(obj);
}

A bit of explanation:

• is_instance_of_managed_helper is a managed class which generates a function at runtime giving the equivalent of C#'s is operator. It uses Expression::TypeIs to achieve this in a simple way. One such function will be generated once for every T.

• template struct is_instance_of_helper is a template struct which simply uses the above solution. This is the general case.

• template struct is_instance_of_helper is a partial specialization of the above struct which uses dynamic_cast for managed handle types. This way, we'll spare ourselves the trouble of generating code at runtime when T can simply be used with dynamic_cast.

• template bool is_instance_of(System::Object^ obj) is the final helper function which will choose the template to use.