Question

I have a class defined as:

    public ref class MyParameters
    {
    private:
        property Decimal^ tickValue;
    public:
        MyParameters(Decimal^ tickValue){
              this->tickValue = tickValue;
          }
        std::string GetTickValue();

   };


    std::string MyParameters::GetTickValue()
    {
        String^ test = this->tickValue->ToString(); <--Invalid Cast here
        return marshal_as<std::string>(test);
    }

If I breakpoint on that line and paste this->tickValue->ToString() into the quick watch it works fine!

I did notice that my Constructor signature was changed to MyParameters(ValueType^ tickValue)??

Answer 1

The is no concept of “typed reference to value type” in .Net.* And since Decimal^ is exactly that, it’s not possible to compile that properly. As you noticed, what the compiler does is to instead make the parameter ValueType^, which is the closest thing to Decimal^ in the .Net type system. It also adds some metadata, so that when you call the constructor from C++/CLI, only boxed Decimal can be passed to it.

But other languages don’t understand that metadata, all they see is ValueType. That means that you can call the constructor from, say, C# like this and it works fine:

new MyParameters(42)

Notice that I passed in an int, not decimal. And this int is saved in the tickValue field (which is actually also ValueType^). And only when trying to actually use its value as Decimal^, you get InvalidCastException.

The easiest way to fix this is not to use references to value types. That is, instead of Decimal^, use just Decimal and everything should work fine.

* Strictly speaking, there is TypedReference, but that’s not meant for normal use.