Enable NRT in Avalonia.Interactivity.

src/Avalonia.Interactivity/Avalonia.Interactivity.csproj

@@ -1,6 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
+    <Nullable>Enable</Nullable>
+    <WarningsAsErrors>CS8600;CS8602;CS8603</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\Avalonia.Animation\Avalonia.Animation.csproj" />
@@ -9,6 +11,4 @@
     <ProjectReference Include="..\Avalonia.Visuals\Avalonia.Visuals.csproj" />
   </ItemGroup>
   <Import Project="..\..\build\Rx.props" />
-</Project>
-
-
+</Project>

src/Avalonia.Interactivity/EventSubscription.cs

@@ -9,12 +9,24 @@ namespace Avalonia.Interactivity
 
     internal class EventSubscription
     {
-        public HandlerInvokeSignature InvokeAdapter { get; set; }
+        public EventSubscription(
+            Delegate handler,
+            RoutingStrategies routes,
+            bool handledEventsToo,
+            HandlerInvokeSignature? invokeAdapter = null)
+        {
+            Handler = handler;
+            Routes = routes;
+            HandledEventsToo = handledEventsToo;
+            InvokeAdapter = invokeAdapter;
+        }
 
-        public Delegate Handler { get; set; }
+        public HandlerInvokeSignature? InvokeAdapter { get; }
 
-        public RoutingStrategies Routes { get; set; }
+        public Delegate Handler { get; }
 
-        public bool AlsoIfHandled { get; set; }
+        public RoutingStrategies Routes { get; }
+
+        public bool HandledEventsToo { get; }
     }
 }

src/Avalonia.Interactivity/IInteractive.cs

@@ -13,7 +13,7 @@ namespace Avalonia.Interactivity
         /// <summary>
         /// Gets the interactive parent of the object for bubbling and tunneling events.
         /// </summary>
-        IInteractive InteractiveParent { get; }
+        IInteractive? InteractiveParent { get; }
 
         /// <summary>
         /// Adds a handler for the specified routed event.

src/Avalonia.Interactivity/Interactive.cs

@@ -15,16 +15,16 @@ namespace Avalonia.Interactivity
     /// </summary>
     public class Interactive : Layoutable, IInteractive
     {
-        private Dictionary<RoutedEvent, List<EventSubscription>> _eventHandlers;
+        private Dictionary<RoutedEvent, List<EventSubscription>>? _eventHandlers;
 
         private static readonly Dictionary<Type, HandlerInvokeSignature> s_invokeHandlerCache = new Dictionary<Type, HandlerInvokeSignature>();
 
         /// <summary>
         /// Gets the interactive parent of the object for bubbling and tunneling events.
         /// </summary>
-        IInteractive IInteractive.InteractiveParent => ((IVisual)this).VisualParent as IInteractive;
+        IInteractive? IInteractive.InteractiveParent => ((IVisual)this).VisualParent as IInteractive;
 
-        private Dictionary<RoutedEvent, List<EventSubscription>> EventHandlers => _eventHandlers ?? (_eventHandlers = new Dictionary<RoutedEvent, List<EventSubscription>>());
+        private Dictionary<RoutedEvent, List<EventSubscription>> EventHandlers => _eventHandlers ??= new Dictionary<RoutedEvent, List<EventSubscription>>();
 
         /// <summary>
         /// Adds a handler for the specified routed event.
@@ -40,16 +40,10 @@ namespace Avalonia.Interactivity
             RoutingStrategies routes = RoutingStrategies.Direct | RoutingStrategies.Bubble,
             bool handledEventsToo = false)
         {
-            Contract.Requires<ArgumentNullException>(routedEvent != null);
-            Contract.Requires<ArgumentNullException>(handler != null);
-
-            var subscription = new EventSubscription
-            {
-                Handler = handler,
-                Routes = routes,
-                AlsoIfHandled = handledEventsToo,
-            };
+            routedEvent = routedEvent ?? throw new ArgumentNullException(nameof(routedEvent));
+            handler = handler ?? throw new ArgumentNullException(nameof(handler));
 
+            var subscription = new EventSubscription(handler, routes, handledEventsToo);
             return AddEventSubscription(routedEvent, subscription);
         }
 
@@ -68,12 +62,12 @@ namespace Avalonia.Interactivity
             RoutingStrategies routes = RoutingStrategies.Direct | RoutingStrategies.Bubble,
             bool handledEventsToo = false) where TEventArgs : RoutedEventArgs
         {
-            Contract.Requires<ArgumentNullException>(routedEvent != null);
-            Contract.Requires<ArgumentNullException>(handler != null);
+            routedEvent = routedEvent ?? throw new ArgumentNullException(nameof(routedEvent));
+            handler = handler ?? throw new ArgumentNullException(nameof(handler));
 
             // EventHandler delegate is not covariant, this forces us to create small wrapper
             // that will cast our type erased instance and invoke it.
-            Type eventArgsType = routedEvent.EventArgsType;
+            var eventArgsType = routedEvent.EventArgsType;
 
             if (!s_invokeHandlerCache.TryGetValue(eventArgsType, out var invokeAdapter))
             {
@@ -90,14 +84,7 @@ namespace Avalonia.Interactivity
                 s_invokeHandlerCache.Add(eventArgsType, invokeAdapter);
             }
 
-            var subscription = new EventSubscription
-            {
-                InvokeAdapter = invokeAdapter,
-                Handler = handler,
-                Routes = routes,
-                AlsoIfHandled = handledEventsToo,
-            };
-
+            var subscription = new EventSubscription(handler, routes, handledEventsToo, invokeAdapter);
             return AddEventSubscription(routedEvent, subscription);
         }
 
@@ -108,12 +95,11 @@ namespace Avalonia.Interactivity
         /// <param name="handler">The handler.</param>
         public void RemoveHandler(RoutedEvent routedEvent, Delegate handler)
         {
-            Contract.Requires<ArgumentNullException>(routedEvent != null);
-            Contract.Requires<ArgumentNullException>(handler != null);
-
-            List<EventSubscription> subscriptions = null;
+            routedEvent = routedEvent ?? throw new ArgumentNullException(nameof(routedEvent));
+            handler = handler ?? throw new ArgumentNullException(nameof(handler));
 
-            if (_eventHandlers?.TryGetValue(routedEvent, out subscriptions) == true)
+            if (_eventHandlers is object &&
+                _eventHandlers.TryGetValue(routedEvent, out var subscriptions) == true)
             {
                 subscriptions.RemoveAll(x => x.Handler == handler);
             }
@@ -137,9 +123,14 @@ namespace Avalonia.Interactivity
         /// <param name="e">The event args.</param>
         public void RaiseEvent(RoutedEventArgs e)
         {
-            Contract.Requires<ArgumentNullException>(e != null);
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
-            e.Source = e.Source ?? this;
+            if (e.RoutedEvent == null)
+            {
+                throw new ArgumentException("Cannot raise an event whose RoutedEvent is null.");
+            }
+
+            e.Source ??= this;
 
             if (e.RoutedEvent.RoutingStrategies == RoutingStrategies.Direct)
             {
@@ -167,7 +158,7 @@ namespace Avalonia.Interactivity
         /// <param name="e">The event args.</param>
         private void BubbleEvent(RoutedEventArgs e)
         {
-            Contract.Requires<ArgumentNullException>(e != null);
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
             e.Route = RoutingStrategies.Bubble;
 
@@ -182,7 +173,7 @@ namespace Avalonia.Interactivity
         /// <param name="e">The event args.</param>
         private void TunnelEvent(RoutedEventArgs e)
         {
-            Contract.Requires<ArgumentNullException>(e != null);
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
             e.Route = RoutingStrategies.Tunnel;
 
@@ -197,18 +188,17 @@ namespace Avalonia.Interactivity
         /// <param name="e">The event args.</param>
         private void RaiseEventImpl(RoutedEventArgs e)
         {
-            Contract.Requires<ArgumentNullException>(e != null);
-
-            e.RoutedEvent.InvokeRaised(this, e);
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
-            List<EventSubscription> subscriptions = null;
+            e.RoutedEvent!.InvokeRaised(this, e);
 
-            if (_eventHandlers?.TryGetValue(e.RoutedEvent, out subscriptions) == true)
+            if (_eventHandlers is object && 
+                _eventHandlers.TryGetValue(e.RoutedEvent, out var subscriptions) == true)
             {
                 foreach (var sub in subscriptions.ToList())
                 {
                     bool correctRoute = (e.Route & sub.Routes) != 0;
-                    bool notFinished = !e.Handled || sub.AlsoIfHandled;
+                    bool notFinished = !e.Handled || sub.HandledEventsToo;
 
                     if (correctRoute && notFinished)
                     {
@@ -313,7 +303,7 @@ namespace Avalonia.Interactivity
             {
                 _preTraverse.Execute(target, _args);
 
-                IInteractive parent = target.InteractiveParent;
+                var parent = target.InteractiveParent;
 
                 if (parent != null)
                 {

src/Avalonia.Interactivity/InteractiveExtensions.cs

@@ -2,8 +2,6 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
 using System.Reactive.Linq;
 
 namespace Avalonia.Interactivity
@@ -30,6 +28,9 @@ namespace Avalonia.Interactivity
             bool handledEventsToo = false)
                 where TEventArgs : RoutedEventArgs
         {
+            o = o ?? throw new ArgumentNullException(nameof(o));
+            routedEvent = routedEvent ?? throw new ArgumentNullException(nameof(routedEvent));
+
             return Observable.Create<TEventArgs>(x => o.AddHandler(
                 routedEvent, 
                 (_, e) => x.OnNext(e), 

src/Avalonia.Interactivity/RoutedEvent.cs

@@ -25,10 +25,14 @@ namespace Avalonia.Interactivity
             Type eventArgsType,
             Type ownerType)
         {
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentNullException>(eventArgsType != null);
-            Contract.Requires<ArgumentNullException>(ownerType != null);
-            Contract.Requires<InvalidCastException>(typeof(RoutedEventArgs).IsAssignableFrom(eventArgsType));
+            name = name ?? throw new ArgumentNullException(nameof(name));
+            eventArgsType = eventArgsType ?? throw new ArgumentNullException(nameof(name));
+            ownerType = ownerType ?? throw new ArgumentNullException(nameof(name));
+
+            if (!typeof(RoutedEventArgs).IsAssignableFrom(eventArgsType))
+            {
+                throw new InvalidCastException("eventArgsType must be derived from RoutedEventArgs.");
+            }
 
             EventArgsType = eventArgsType;
             Name = name;
@@ -52,7 +56,7 @@ namespace Avalonia.Interactivity
             RoutingStrategies routingStrategy)
                 where TEventArgs : RoutedEventArgs
         {
-            Contract.Requires<ArgumentNullException>(name != null);
+            name = name ?? throw new ArgumentNullException(nameof(name));
 
             var routedEvent = new RoutedEvent<TEventArgs>(name, routingStrategy, typeof(TOwner));
             RoutedEventRegistry.Instance.Register(typeof(TOwner), routedEvent);
@@ -65,7 +69,7 @@ namespace Avalonia.Interactivity
             Type ownerType)
                 where TEventArgs : RoutedEventArgs
         {
-            Contract.Requires<ArgumentNullException>(name != null);
+            name = name ?? throw new ArgumentNullException(nameof(name));
 
             var routedEvent = new RoutedEvent<TEventArgs>(name, routingStrategy, ownerType);
             RoutedEventRegistry.Instance.Register(ownerType, routedEvent);
@@ -108,8 +112,6 @@ namespace Avalonia.Interactivity
         public RoutedEvent(string name, RoutingStrategies routingStrategies, Type ownerType)
             : base(name, routingStrategies, typeof(TEventArgs), ownerType)
         {
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentNullException>(ownerType != null);
         }
 
         [Obsolete("Use overload taking Action<TTarget, TEventArgs>.")]

src/Avalonia.Interactivity/RoutedEventArgs.cs

@@ -11,12 +11,12 @@ namespace Avalonia.Interactivity
         {
         }
 
-        public RoutedEventArgs(RoutedEvent routedEvent)
+        public RoutedEventArgs(RoutedEvent? routedEvent)
         {
             RoutedEvent = routedEvent;
         }
 
-        public RoutedEventArgs(RoutedEvent routedEvent, IInteractive source)
+        public RoutedEventArgs(RoutedEvent? routedEvent, IInteractive? source)
         {
             RoutedEvent = routedEvent;
             Source = source;
@@ -24,10 +24,10 @@ namespace Avalonia.Interactivity
 
         public bool Handled { get; set; }
 
-        public RoutedEvent RoutedEvent { get; set; }
+        public RoutedEvent? RoutedEvent { get; set; }
 
         public RoutingStrategies Route { get; set; }
 
-        public IInteractive Source { get; set; }
+        public IInteractive? Source { get; set; }
     }
 }

src/Avalonia.Interactivity/RoutedEventRegistry.cs

@@ -32,8 +32,8 @@ namespace Avalonia.Interactivity
         /// </remarks>
         public void Register(Type type, RoutedEvent @event)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-            Contract.Requires<ArgumentNullException>(@event != null);
+            type = type ?? throw new ArgumentNullException(nameof(type));
+            @event = @event ?? throw new ArgumentNullException(nameof(@event));
 
             if (!_registeredRoutedEvents.TryGetValue(type, out var list))
             {
@@ -66,7 +66,7 @@ namespace Avalonia.Interactivity
         /// <returns>All routed events registered with the provided type.</returns>
         public IReadOnlyList<RoutedEvent> GetRegistered(Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
+            type = type ?? throw new ArgumentNullException(nameof(type));
 
             if (_registeredRoutedEvents.TryGetValue(type, out var events))
             {