multi selection drop down creation in excel cell from code

Question

I created the dropdown with some values in one of the columns of excel sheet up to 100 records.there, the user has to select either one or multiple values as the user like.how can I provide a facility to select multiple items.if he selects multiple items, they will come like comma separated.  please help me guys...in advance thank you. i can't provide any sample code.please help me