Multiselected DDL and textbox in asp.net.

To achieve this functionality, you can use JavaScript in the onchange event of the dropdown list.

1. Define your dropdown list and textbox in the ASP.NET markup

2.  

3. Implement the JavaScript function to handle the dropdown change:

4. function handleDropdownChange() {
       var selectedItems = $('#ddlItems').val();
       var predefinedEmails = {
           'ID2': '[email protected]',
           'ID4': '[email protected]'
       };
   
       var emailsToInsert = selectedItems.filter(item => predefinedEmails[item]);
       var manualEmails = selectedItems.filter(item => !predefinedEmails[item]);
   
       var finalEmails = emailsToInsert.map(item => predefinedEmails[item]).concat(manualEmails);
   
       $('#txtEmail').val(finalEmails.join(', '));
   }
   

5. Ensure that the JavaScript function updates the textbox with the appropriate email IDs based on the selected dropdown items.

6. Handle the removal of items from the dropdown list by adjusting the textbox value accordingly in the handleDropdownChange function.