Question

I have a C++/CLI wrapper class to interop between C# and native C++. I’m getting a strange error related with System.Nullable. I understand that, for basic types, System.Nullable<T> is equivalent to T?. So I do this:

C#:

public int? RoboticsArmRotation {
   get { return mRobotics.ArmRotation; }
}

C++/CLI, interface:

virtual property System::Nullable<int>^ ArmRotation{ System::Nullable<int>^ get() = 0; }

C++/CLI, concrete class:

virtual property System::Nullable<int>^ ArmRotation {
    System::Nullable<int>^ get() {
        boost::optional<int> value = m_pNativeInstance->getArmRotation();
        return value.is_initialized()? gcnew System::Nullable<int>(value.get()) : gcnew System::Nullable<int>();
    }
}

But I get the title’s compile error. Casting to int? solves it, but what bugs me is that it’s saying System.ValueType when I defined my nullable as a reference. Can I leave the cast and move on, or am I doing something wrong?

Answer 1

You’re using Nullable<int>^ which is a reference. Since references with a value type aren’t directly supported by the runtime, the actual type at the IL level is ValueType that’s tagged with Nullable<int>^ in a way that C++/CLI supports, but C# doesn’t. For C# it’s just typed as System.ValueType.

This makes even less sense for nullables than for normal value types, since nullables get boxed as their underlying type.

I’d recommend not declaring the field as a reference in C++ either, but to use a simple Nullable<int>.