Fixed field declarations which should be readonly.

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -52,7 +52,7 @@ namespace Avalonia.Controls.Primitives
         internal Calendar Owner { get; set; }
         internal CalendarDayButton CurrentButton { get; set; }
 
-        public static StyledProperty<IBrush> HeaderBackgroundProperty = Calendar.HeaderBackgroundProperty.AddOwner<CalendarItem>();
+        public static readonly StyledProperty<IBrush> HeaderBackgroundProperty = Calendar.HeaderBackgroundProperty.AddOwner<CalendarItem>();
         public IBrush HeaderBackground
         {
             get { return GetValue(HeaderBackgroundProperty); }

src/Avalonia.Controls/Primitives/AdornerLayer.cs

@@ -13,7 +13,7 @@ namespace Avalonia.Controls.Primitives
     // TODO: Need to track position of adorned elements and move the adorner if they move.
     public class AdornerLayer : Panel, ICustomSimpleHitTest
     {
-        public static AttachedProperty<Visual> AdornedElementProperty =
+        public static readonly AttachedProperty<Visual> AdornedElementProperty =
             AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, Visual>("AdornedElement");
 
         private static readonly AttachedProperty<AdornedElementInfo> s_adornedElementInfoProperty =

src/Avalonia.Controls/Viewbox.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// The stretch property
         /// </summary>
-        public static AvaloniaProperty<Stretch> StretchProperty =
+        public static readonly AvaloniaProperty<Stretch> StretchProperty =
                 AvaloniaProperty.RegisterDirect<Viewbox, Stretch>(nameof(Stretch),
                     v => v.Stretch, (c, v) => c.Stretch = v, Stretch.Uniform);
 

src/Avalonia.Input/Cursors.cs

@@ -47,7 +47,7 @@ namespace Avalonia.Input
 
     public class Cursor
     {
-        public static Cursor Default = new Cursor(StandardCursorType.Arrow);
+        public static readonly Cursor Default = new Cursor(StandardCursorType.Arrow);
 
         internal Cursor(IPlatformHandle platformCursor)
         {

src/Avalonia.Input/DataFormats.cs

@@ -5,11 +5,11 @@
         /// <summary>
         /// Dataformat for plaintext
         /// </summary>
-        public static string Text = nameof(Text);
+        public static readonly string Text = nameof(Text);
 
         /// <summary>
         /// Dataformat for one or more filenames
         /// </summary>
-        public static string FileNames = nameof(FileNames);
+        public static readonly string FileNames = nameof(FileNames);
     }
-}
+}

src/Avalonia.Input/DragDrop.cs

@@ -9,21 +9,21 @@ namespace Avalonia.Input
         /// <summary>
         /// Event which is raised, when a drag-and-drop operation enters the element.
         /// </summary>
-        public static RoutedEvent<DragEventArgs> DragEnterEvent = RoutedEvent.Register<DragEventArgs>("DragEnter", RoutingStrategies.Bubble, typeof(DragDrop));
+        public static readonly RoutedEvent<DragEventArgs> DragEnterEvent = RoutedEvent.Register<DragEventArgs>("DragEnter", RoutingStrategies.Bubble, typeof(DragDrop));
         /// <summary>
         /// Event which is raised, when a drag-and-drop operation leaves the element.
         /// </summary>
-        public static RoutedEvent<RoutedEventArgs> DragLeaveEvent = RoutedEvent.Register<RoutedEventArgs>("DragLeave", RoutingStrategies.Bubble, typeof(DragDrop));
+        public static readonly RoutedEvent<RoutedEventArgs> DragLeaveEvent = RoutedEvent.Register<RoutedEventArgs>("DragLeave", RoutingStrategies.Bubble, typeof(DragDrop));
         /// <summary>
         /// Event which is raised, when a drag-and-drop operation is updated while over the element.
         /// </summary>
-        public static RoutedEvent<DragEventArgs> DragOverEvent = RoutedEvent.Register<DragEventArgs>("DragOver", RoutingStrategies.Bubble, typeof(DragDrop));
+        public static readonly RoutedEvent<DragEventArgs> DragOverEvent = RoutedEvent.Register<DragEventArgs>("DragOver", RoutingStrategies.Bubble, typeof(DragDrop));
         /// <summary>
         /// Event which is raised, when a drag-and-drop operation should complete over the element.
         /// </summary>
-        public static RoutedEvent<DragEventArgs> DropEvent = RoutedEvent.Register<DragEventArgs>("Drop", RoutingStrategies.Bubble, typeof(DragDrop));
+        public static readonly RoutedEvent<DragEventArgs> DropEvent = RoutedEvent.Register<DragEventArgs>("Drop", RoutingStrategies.Bubble, typeof(DragDrop));
 
-        public static AvaloniaProperty<bool> AllowDropProperty = AvaloniaProperty.RegisterAttached<Interactive, bool>("AllowDrop", typeof(DragDrop), inherits: true);
+        public static readonly AvaloniaProperty<bool> AllowDropProperty = AvaloniaProperty.RegisterAttached<Interactive, bool>("AllowDrop", typeof(DragDrop), inherits: true);
 
         /// <summary>
         /// Gets a value indicating whether the given element can be used as the target of a drag-and-drop operation. 

src/Avalonia.Visuals/Media/GradientStop.cs

@@ -11,13 +11,13 @@ namespace Avalonia.Media
         /// <summary>
         /// Describes the <see cref="Offset"/> property.
         /// </summary>
-        public static StyledProperty<double> OffsetProperty =
+        public static readonly StyledProperty<double> OffsetProperty =
             AvaloniaProperty.Register<GradientStop, double>(nameof(Offset));
 
         /// <summary>
         /// Describes the <see cref="Color"/> property.
         /// </summary>
-        public static StyledProperty<Color> ColorProperty =
+        public static readonly StyledProperty<Color> ColorProperty =
             AvaloniaProperty.Register<GradientStop, Color>(nameof(Color));
 
         /// <summary>

src/Avalonia.Visuals/Media/Typeface.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Media
     /// </summary>
     public class Typeface
     {
-        public static Typeface Default = new Typeface(FontFamily.Default);
+        public static readonly Typeface Default = new Typeface(FontFamily.Default);
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Typeface"/> class.