public List<SelectListItem> GetSelectedListFromEntity<T>(Expression<Func<T, object>> property) where T : BaseEntity<int>
    {
         var result = _repository.Query<T>().Select(p => new SelectListItem()
         {
           Text = property, //? (in simple case it looks like:  p.Name + p.Category)
           Value = p.Id.ToString(),
         ).ToList();

         return result;
    }

var result = GetSelectedListFromEntity<Product>(p => p.Name + p.Category);

Expression<Func<T, string>> displayText

public List<SelectListItem> GetSelectedListFromEntity<T>(Expression<Func<T, string>> displayText) where T : BaseEntity<int> {
     var result = _repository.Query<T>().Select(p => new SelectListItem() {
       Text = displayText.Compile()(p),
       Value = p.Id.ToString(),
     }).ToList();

     return result;
}

public string Display<T>(Expression<Func<T, object>> e)
{
    var toStr = e.ToString();
    return toStr.Substring(toStr.IndexOf("=>")+3).TrimStart('(').TrimEnd(')');
}

Expression<Func<Product, object>> p = t=> t.Name+t.Category;
Expression<Func<Product, object>> p1 = t=> t.Name;