diff --git a/QuickLook.Plugin/QuickLook.Plugin.ArchiveViewer/Converters.cs b/QuickLook.Plugin/QuickLook.Plugin.ArchiveViewer/Converters.cs
index c52d175..949d2be 100644
--- a/QuickLook.Plugin/QuickLook.Plugin.ArchiveViewer/Converters.cs
+++ b/QuickLook.Plugin/QuickLook.Plugin.ArchiveViewer/Converters.cs
@@ -23,12 +23,11 @@ using System.Windows.Data;
 
 namespace QuickLook.Plugin.ArchiveViewer;
 
-public class Percent100ToVisibilityVisibleConverter : DependencyObject, IValueConverter
+public sealed class Percent100ToVisibilityVisibleConverter : DependencyObject, IValueConverter
 {
     public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
     {
-        if (value == null)
-            value = 0;
+        value ??= 0;
 
         var percent = (double)value;
         return Math.Abs(percent - 100) < 0.00001 ? Visibility.Visible : Visibility.Collapsed;
@@ -40,12 +39,11 @@ public class Percent100ToVisibilityVisibleConverter : DependencyObject, IValueCo
     }
 }
 
-public class Percent100ToVisibilityCollapsedConverter : DependencyObject, IValueConverter
+public sealed class Percent100ToVisibilityCollapsedConverter : DependencyObject, IValueConverter
 {
     public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
     {
-        if (value == null)
-            value = 0;
+        value ??= 0;
 
         var percent = (double)value;
         return Math.Abs(percent - 100) < 0.00001 ? Visibility.Collapsed : Visibility.Visible;
@@ -57,7 +55,7 @@ public class Percent100ToVisibilityCollapsedConverter : DependencyObject, IValue
     }
 }
 
-public class LevelToIndentConverter : DependencyObject, IMultiValueConverter
+public sealed class LevelToIndentConverter : DependencyObject, IMultiValueConverter
 {
     public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
     {
@@ -75,7 +73,7 @@ public class LevelToIndentConverter : DependencyObject, IMultiValueConverter
     }
 }
 
-public class LevelToBooleanConverter : DependencyObject, IValueConverter
+public sealed class LevelToBooleanConverter : DependencyObject, IValueConverter
 {
     public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
     {
@@ -90,7 +88,7 @@ public class LevelToBooleanConverter : DependencyObject, IValueConverter
     }
 }
 
-public class BooleanToAsteriskConverter : DependencyObject, IValueConverter
+public sealed class BooleanToAsteriskConverter : DependencyObject, IValueConverter
 {
     public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
     {
@@ -105,7 +103,7 @@ public class BooleanToAsteriskConverter : DependencyObject, IValueConverter
     }
 }
 
-public class SizePrettyPrintConverter : DependencyObject, IMultiValueConverter
+public sealed class SizePrettyPrintConverter : DependencyObject, IMultiValueConverter
 {
     public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
     {
@@ -121,7 +119,7 @@ public class SizePrettyPrintConverter : DependencyObject, IMultiValueConverter
     }
 }
 
-public class DatePrintConverter : DependencyObject, IMultiValueConverter
+public sealed class DatePrintConverter : DependencyObject, IMultiValueConverter
 {
     public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
     {
@@ -137,7 +135,7 @@ public class DatePrintConverter : DependencyObject, IMultiValueConverter
     }
 }
 
-public class FileExtToIconConverter : DependencyObject, IMultiValueConverter
+public sealed class FileExtToIconConverter : DependencyObject, IMultiValueConverter
 {
     public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
     {