Covariance based generic Master Managemnet Service

///////////////////////  Model classes /////////////////////////////////////////////////////////////
public class _ModelBaseMaster
{
    public int id{get;set;}
    public string name{get;set;}
}
public class Master1: _ModelBaseMaster{}

////////////////////////// Generic repository for DAL /////////////////////////////////////////////
public class RepositoryMaster<T> where T : _ModelBaseMaster
    {
        // This is the generic repository for master tables which will handle CRUD & fetch
        public RepositoryMaster(MyDBContext context){...}
        ...
    }

///////////////////////// Service Interface and its implementation /////////////////////////////////
public interface IMasterService<out T> : IBaseService where T: _IModelBaseMaster
{
    List<T> FetchAll();
    void Add(T mObj);
    ...
}

public class MasterService<T> : _ServiceBase, IMasterService<T> where T : _ModelBaseMaster
{
    IRepositoryMaster<T> mstrRepo;
    public MasterService(IRepositoryMaster<T> mRepository)
        {
            mstrRepo = mRepository;
        }
    public IEnumerable<T> FetchAll()
    {
        IQueryable<T> data = mstrRepo.FetchAll();
        // I can do - List<_ModelBaseMaster> result = data.Cast<_ModelBaseMaster>().ToList();
        ...
        return data.Cast<T>().ToList();
    }
    public void Add(object mObject)
    {
        var mObj = ConvertParentToChild<T>(mObject);
        mstrRepo.Insert(mObj); // can it be this simple - and work for ALL master tables ?
    }
    ...
    // The unconventional method to convert 'Parent' to 'Child'
    // Use at your own risk (i.e. ensure that child do not have any extra properties)
    // https://www.dotnetperls.com/generic-method
        public static T ConvertParentToChild<T>(object parentInstance)
        {
            var serializedParent = JsonConvert.SerializeObject(parentInstance);
            return JsonConvert.DeserializeObject<T>(serializedParent);
        }

}

/////////////////////////// usage in MVC controller's action /////////////////////////////////////

// Now I can initialize as -

IMasterService<_ModelBaseMaster> mstrService = new RepositoryMaster<Master1>(context);

// And finally -

IEnumerable<_ModelBaseMaster> genericList = mstrService.FetchAll();

//- this will prevent me from lengthy switch case for each master table