Display results seperated by comma's

Hi

Just wondering if there was a way to display results from seperate rows in one text box where the results are seperated by comma's?

=First(Fields!ClientId.Value, "DataSet1")

The example above will just show one ClientId however I would like to list all of them all like below

C001,C002,C546,F546,H232

Am I after Lookup/LookupSet?

Any help appreciated.

Cheers

Cheers Chris