This is mainly a note for myself because keep forgetting how the code works..if you have an SSRS Report with a drop-down that the user can select all, and you want to display in the report what parameters the user selected, BUT you don't want to list 1000s of selections if the user selected the 'All' option, you can display 'All' instead of each item to specifically because it takes up too much room in the report.
=IIF( Parameters!YourMultivalueParam.Count = countrows("DatasetName"), "All", Join(Parameters!YourMultivalueParam.Label,", ") )
