The extension method is not visible

I am thinking to create some Extension method for the thing below.

So I plan to see

 @item.Roles.ConvertToString(...

I created also the extancion method itself. But It is not appearing in place I need.

 @item.Roles.  Nothing what I need... :(

Any clue?

P.S.

 public static class MyExtensions
    {
        public static string ConvertToString(this ICollection<IdentityUserRole> identityUserRole)
        {
            var result = string.Empty;

            return result;
        }
    }


You need to import the namespace containing your extension method using the @using directive.