In actual the ASP.Net DropDownList control does not support multiple column names for its DataTextField property that provides the functionality to map the field of database table whose value appear as text in the dropdownlist menu. If you want to display the values retrieved from the multiple columns of database table then you cannot specify the names of multiple columns to the DataTextField property instead you have to build your own logic to provide the functionality.
In this sample we have used the Employees table of Northwind SQL database to show you the possible output that can be generated for displaying the multiple column values as text value of list item of dropdownlist control. We have used the SqlDataReader object to read the forward-only stream of SQL records to add the ListItems dynamically by specifying the combined string type value to display the FirstName and LastName of each employee as a list item of dropdownlist control. You can also modify the SQL query by concatenating the values of multiple fields and assigning a single alias for the new customized field that can be specified as a value for DataTextField property of dropdownlist control to display the text items.
See the sample below to understand the functionality of displaying the values of multiple columns as text value of list items of ASP.Net DropDownList control: