Create a shared project with all Win App common files (8.1 and 10)

Finally use the dependency injected Resource Service

WinAppCommon/Common/MessageDialogHelper.cs

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+using System;
+using System.Threading.Tasks;
+using Windows.UI.Popups;
+
+namespace ModernKeePass.Common
+{
+    public static class MessageDialogHelper
+    {
+        public static async Task ShowActionDialog(string title, string contentText, string actionButtonText, string cancelButtonText, UICommandInvokedHandler actionCommand, UICommandInvokedHandler cancelCommand)
+        {
+            // Create the message dialog and set its content
+            var messageDialog = CreateBasicDialog(title, contentText, cancelButtonText, cancelCommand);
+
+            // Add commands and set their callbacks; both buttons use the same callback function instead of inline event handlers
+            messageDialog.Commands.Add(new UICommand(actionButtonText, actionCommand));
+            
+            // Show the message dialog
+            await messageDialog.ShowAsync().AsTask().ConfigureAwait(false);
+        }
+        
+        public static async Task ShowErrorDialog(Exception exception)
+        {
+            if (exception == null) return;
+            // Create the message dialog and set its content
+            var messageDialog = CreateBasicDialog(exception.Message, exception.StackTrace, "OK");
+            
+            // Show the message dialog
+            await messageDialog.ShowAsync().AsTask().ConfigureAwait(false);
+        }
+
+        public static async Task ShowNotificationDialog(string title, string message)
+        {
+            var dialog = CreateBasicDialog(title, message, "OK");
+
+            // Show the message dialog
+            await dialog.ShowAsync().AsTask().ConfigureAwait(false);
+        }
+
+        private static MessageDialog CreateBasicDialog(string title, string message, string dismissActionText, UICommandInvokedHandler cancelCommand = null)
+        {
+            // Create the message dialog and set its content
+            var messageDialog = new MessageDialog(message, title);
+
+            // Add commands and set their callbacks
+            messageDialog.Commands.Add(new UICommand(dismissActionText, cancelCommand));
+
+            // Set the command that will be invoked by default
+            messageDialog.DefaultCommandIndex = 1;
+
+            // Set the command to be invoked when escape is pressed
+            messageDialog.CancelCommandIndex = 1;
+
+            return messageDialog;
+        }
+    }
+}