public sealed class ApplicationClosingEvent : ArcGIS.Core.Events.AsyncPresentationEvent<CancelEventArgs> 

{
  // There are two ways to prevent ArcGIS Pro from closing
  // 1. Override the CanUnload method on your add-in's module and return false.
  // 2. Subscribe to the ApplicationClosing event and cancel the event when you receive it

  // 1. Override the CanUnload method on your add-in's module and return false.
  // Called by Framework when ArcGIS Pro is closing
  {
    //return false to ~cancel~ Application close
  }
  {
    // 2. Subscribe to the ApplicationClosing event and cancel the event when you receive it
    // Replace this line:
    // ArcGIS.Desktop.Framework.Events.ApplicationClosingEvent.Subscribe(() => { });

    // With the following, which matches the required delegate signature:
    ArcGIS.Desktop.Framework.Events.ApplicationClosingEvent.Subscribe((args) =>
    {
      //Do something here, e.g. prompt user to save work
      return Task.CompletedTask;
    }
    );


    // in the Application Closing event handler set the Cancel property of the event args to true to prevent Pro from closing
  }
}
// Called by Framework when ArcGIS Pro is closing
{
  ArcGIS.Desktop.Framework.Events.ApplicationClosingEvent.Subscribe((args) =>
  {
    //Do something here, e.g. prompt user to save work
    return Task.CompletedTask;
  }
    );
}