Overloading subscript operator in a C++/WinRT runtimeclass - c++

Question

I am new authoring C++/WinRT runtime classes, and I was requested to overload the subscript operator ([ ]) for a runtime class Parameter. The class Parameter contains an IMap<hstring, IInspectable> that stores a set of parameters indexed by the parameter name.
Here is the code:
namespace winrt::Parameter::implementation
{
using namespace winrt;
using namespace Windows::Foundation::Collections;
struct Parameter : ParameterT<Parameter>
{
private:
IMap<hstring, IInspectable> m_Parameters{ nullptr };
public:
Parameter() { m_Parameters = single_threaded_map<hstring, IInspectable>(); }
~Parameter() {};
IInspectable& operator[](const hstring& key)
{
if (!m_Parameters.HasKey(key))
{
m_Parameters.Insert(key, {});
}
return m_Parameters.Lookup(key);
}
IInspectable const& operator[](const hstring& key) const
{
if (!m_Parameters.HasKey(key))
{
m_Parameters.Insert(key, {});
}
return m_Parameters.Lookup(key);
}
};
}
namespace winrt::Parameter::factory_implementation
{
struct Parameter : ParameterT<Parameter, implementation::Parameter>
{
};
}
The code above compiles with no errors, but I get the below error when I try to consume the Parameter code in a UWP C# test App:
Parameter parameter = new Parameter();
const string V = "test";
parameter[V] = "any test value";
string test = parameter[V];
Error CS0021 Cannot apply indexing with [ ] to an expression of type 'Parameter'
Does anyone have any ideas on what I am missing here?
The problem seems to be with the C++/WinRT version because this other similar code using standard C++/17 works as expected.
class Parameter
{
private:
std::map<std::string, std::any> m_Parameter{};
public:
std::any& operator[](const std::string& name);
};
std::any& Parameter::operator[](const std::string& key)
{
if (m_Parameter.find(key) == m_Parameter.end())
{
m_Parameter.insert({ key, {} });
}
return m_Parameter.at(key);
}
int main()
{
Parameter parameter{};
parameter["test"] = 'b';
parameter["other test"] = std::string("other test value");
std::cout << std::any_cast<std::string>(parameter["other test"]) << '\n';
std::cout << std::any_cast<char>(parameter["test"]) << '\n';
return 0;
}