ssrs store textbox value in parameter code example
Example: display parameter value in textbox ssrs
=IIF(Parameters!YourParameter.Count=Count(Fields!DataSetFieldName.Value, "Name OF Your DataSet"),"ALL",Join(Parameters! YourParameter.Value,","))
=IIF(Parameters!YourParameter.Count=Count(Fields!DataSetFieldName.Value, "Name OF Your DataSet"),"ALL",Join(Parameters! YourParameter.Value,","))