displaying parameter value in textbox

i have a ssrs report.in this report i have a parameter name which is not multi valued parameter(acc. to condition) and which select value either ALL or singly other values .in this parameter i have values -

ALL and other 20 names.i want to display these values in text box.

when i select single name it is displayed in textbox but when i select ALL it is showing error as i cannot use JOIN function whithin this parameter.

how to display all names when ALL is selected.

PLZ Help!!!