unable to cast from CodeModel to Microsoft.VisualStudio.VCCodeModel

I'm trying to run the following code(to discover all variable names in a vc++ project):

                    VCCodeModel model = _applicationObject.Solution.Item(1).CodeModel as VCCodeModel;

                    foreach (VCCodeVariable elem in model.Variables)
                    {
                        string s = elem.DisplayName;

                        // ...
                    }

But it fails to work as the conversion does not succeed and returns null, and I'm running this code from a c++ project.

What's the problem here?