diff --git a/SourceGen/WpfGui/EditProjectProperties.xaml b/SourceGen/WpfGui/EditProjectProperties.xaml
index a79b256..b99db63 100644
--- a/SourceGen/WpfGui/EditProjectProperties.xaml
+++ b/SourceGen/WpfGui/EditProjectProperties.xaml
@@ -25,7 +25,7 @@ limitations under the License.
         Title="Edit Project Properties"
         Width="640" Height="400" ResizeMode="NoResize"
         ShowInTaskbar="False" WindowStartupLocation="CenterOwner"
-        Loaded="Window_Loaded">
+        Loaded="Window_Loaded" Closing="Window_Closing">
 
     <Window.Resources>
         <!-- don't center the column headers -->
@@ -33,6 +33,9 @@ limitations under the License.
             <Setter Property="HorizontalContentAlignment" Value="Left" />
         </Style>
 
+        <system:String x:Key="str_ConfirmDiscardChanges">Some changes have not been applied.  Discard them?</system:String>
+        <system:String x:Key="str_ConfirmDiscardChangesCaption">Discard Changes?</system:String>
+
         <!-- strings for combo boxes -->
         <system:String x:Key="str_6502">MOS 6502</system:String>
         <system:String x:Key="str_65C02">WDC W65C02S</system:String>
diff --git a/SourceGen/WpfGui/EditProjectProperties.xaml.cs b/SourceGen/WpfGui/EditProjectProperties.xaml.cs
index efb64d0..07cf2c7 100644
--- a/SourceGen/WpfGui/EditProjectProperties.xaml.cs
+++ b/SourceGen/WpfGui/EditProjectProperties.xaml.cs
@@ -149,6 +149,18 @@ namespace SourceGen.WpfGui {
             UpdateControls();
         }
 
+        private void Window_Closing(object sender, CancelEventArgs e) {
+            if (IsDirty) {
+                string msg = (string)FindResource("str_ConfirmDiscardChanges");
+                string caption = (string)FindResource("str_ConfirmDiscardChangesCaption");
+                MessageBoxResult result = MessageBox.Show(msg, caption, MessageBoxButton.OKCancel,
+                    MessageBoxImage.Question);
+                if (result == MessageBoxResult.Cancel) {
+                    e.Cancel = true;
+                }
+            }
+        }
+
         private void ApplyButton_Click(object sender, RoutedEventArgs e) {
             NewProps = new ProjectProperties(mWorkProps);
             IsDirty = false;
@@ -160,6 +172,7 @@ namespace SourceGen.WpfGui {
             // insufficient.  Might be best to just let the UndoableChange stuff figure out
             // that nothing changed.
             NewProps = new ProjectProperties(mWorkProps);
+            IsDirty = false;
             DialogResult = true;
 
             //GridView view = (GridView)projectSymbolsListView.View;
diff --git a/SourceGen/WpfGui/MainWindow.xaml.cs b/SourceGen/WpfGui/MainWindow.xaml.cs
index 509d2f0..97ced27 100644
--- a/SourceGen/WpfGui/MainWindow.xaml.cs
+++ b/SourceGen/WpfGui/MainWindow.xaml.cs
@@ -341,7 +341,6 @@ namespace SourceGen.WpfGui {
             }
             if (!mMainCtrl.WindowClosing()) {
                 e.Cancel = true;
-                return;
             }
         }