<TextBlock  FontSize="{Binding FontSize}"... />

float GetFontSize() const{
	NsString fontSizeString = "20pt";
	float fontSize;
	//Convert from NsString to float
	return fontSize;
}

class ViewModel: public BaseComponent
{
public:
  ViewModel()
  {
    mFontSizeConverter = NsCreateComponent<TypeConverter>(NsSymbol("Converter<Length>"));
  }

  void UpdateFontSize(const char* size)
  {
    Ptr<BaseComponent> result;
    if (mFontSizeConverter->TryConvertFromString(size, result))
    {
      mFontSize = Boxing::Unbox<float>(result);
    }
  }
  ...
  
private:
  Ptr<TypeConverter> mFontSizeConverter;
  float mFontSize;
};