How to select all the option in the multipledropdown field by default and also provide "select all" option in dropdown

Question

Hi All, 
 
 i want to know , how to configure the below logic 
 BY default all the option should be selected. and also in dropdown i can have the option to deselect all /select all along with all the options. 
 Please help me how i can go about this

Stefan Helzle · Answer

To "select" all options, you will have to write all available choice values into the record field in which you store the selected values. 
 A "Select All / Select None" has to do the same. Either copy all choice values into that record field, or empty that record field.