Report Builder: get all items in .Value object

Hello everybody,

i have a report with multiple rows in a dataset.

In one field of the Report I have the expression:

=Fields!Category1.Value

Now I want to join all Values in this field like:

=Join(Fields!Category1.Value , ",")

But this is not working. But =Count(Fields!Category1.Value) works perfectly.

What can I do to get all values in one string.

Thanks in advance.

Best regards,

Marc