dropdown excel code example

Example: excel sheet with drop down list

var options = new List<string>{"Option1","Option2","Option3"};
var validOptions = $"\"{String.Join(",", options)}\"";
ws.Cell(1,1).DataValidation.List(validOptions, true);

//validOptions contains :
"Option1,Option2,Option3"

//Your own code should change to :

.List("\"one,two,three\"", true);