1. Create a Partial View for the Modal: Create a partial view named _ConfirmationModal.cshtml that contains the HTML for your confirmation modal:

   <!-- _ConfirmationModal.cshtml -->
   <div id="confirmationModal" class="modal fade" tabindex="-1" role="dialog">
       <div class="modal-dialog" role="document">
           <div class="modal-content">
               <div class="modal-header">
                   <h5 class="modal-title">Confirmation</h5>
                   <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                       <span aria-hidden="true">&times;</span>
                   </button>
               </div>
               <div class="modal-body">
                   <p id="confirmationMessage"></p>
               </div>
               <div class="modal-footer">
                   <button type="button" class="btn btn-secondary" data-dismiss="modal">Cancel</button>
                   <button type="button" class="btn btn-primary" id="confirmButton">Confirm</button>
               </div>
           </div>
       </div>
   </div>

2. Add JavaScript to Show Modal and Handle Confirmation: Add JavaScript to show the modal and handle the user's confirmation. You can include this script in your layout file so it's available on all pages:

   <script>
   function showConfirmationModal(message, confirmCallback) {
       $('#confirmationMessage').text(message);
       $('#confirmationModal').modal('show');
   
       $('#confirmButton').off('click').on('click', function() {
           $('#confirmationModal').modal('hide');
           if (confirmCallback) {
               confirmCallback();
           }
       });
   }
   </script>

3. Use the Modal in Your Views: You can use the showConfirmationModal function from anywhere in your views or scripts to display the confirmation modal. For example:

   <button onclick="showConfirmationModal('Are you sure you want to delete this item?', function() { deleteItem(); })">Delete</button>

With these steps, you have a reusable confirmation modal that can be used from any page in your ASP.NET MVC or ASP.NET Razor Pages application.