diff --git a/samples/ControlCatalog/Pages/DataGridPage.xaml b/samples/ControlCatalog/Pages/DataGridPage.xaml
index 6817d0698e..323eaa3463 100644
--- a/samples/ControlCatalog/Pages/DataGridPage.xaml
+++ b/samples/ControlCatalog/Pages/DataGridPage.xaml
@@ -1,5 +1,5 @@
 <UserControl xmlns="https://github.com/avaloniaui"
-             xmlns:local="clr-namespace:ControlCatalog.Models;assembly=ControlCatalog"
+             xmlns:local="using:ControlCatalog.Models"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:Class="ControlCatalog.Pages.DataGridPage">
   <UserControl.Resources>
@@ -26,7 +26,8 @@
         <DataGrid Name="dataGrid1" Margin="12" CanUserResizeColumns="True" CanUserReorderColumns="True" CanUserSortColumns="True" HeadersVisibility="All">
           <DataGrid.Columns>
             <DataGridTextColumn Header="Country" Binding="{Binding Name}" Width="6*" />
-            <DataGridTextColumn Header="Region" Binding="{Binding Region}" Width="4*" />
+            <!-- CompiledBinding example of usage. -->
+            <DataGridTextColumn Header="Region" Binding="{CompiledBinding Region}" Width="4*" x:DataType="local:Country" />
             <DataGridTextColumn Header="Population" Binding="{Binding Population}" Width="3*" />
             <DataGridTextColumn Header="Area" Binding="{Binding Area}" Width="3*" />
             <DataGridTextColumn Header="GDP" Binding="{Binding GDP}" Width="3*" CellStyleClasses="gdp" />
diff --git a/samples/ControlCatalog/Pages/DataGridPage.xaml.cs b/samples/ControlCatalog/Pages/DataGridPage.xaml.cs
index 2a30f4d91b..dc5cc49a90 100644
--- a/samples/ControlCatalog/Pages/DataGridPage.xaml.cs
+++ b/samples/ControlCatalog/Pages/DataGridPage.xaml.cs
@@ -24,8 +24,10 @@ namespace ControlCatalog.Pages
             dg1.LoadingRow += Dg1_LoadingRow;
             dg1.Sorting += (s, a) =>
             {
-                var property = ((a.Column as DataGridBoundColumn)?.Binding as Binding).Path;
-                if (property == dataGridSortDescription.PropertyPath
+                var binding = (a.Column as DataGridBoundColumn)?.Binding as Binding;
+
+                if (binding?.Path is string property
+                    && property == dataGridSortDescription.PropertyPath
                     && !collectionView1.SortDescriptions.Contains(dataGridSortDescription))
                 {
                     collectionView1.SortDescriptions.Add(dataGridSortDescription);
diff --git a/src/Avalonia.Controls.DataGrid/DataGridBoundColumn.cs b/src/Avalonia.Controls.DataGrid/DataGridBoundColumn.cs
index 1e72a07760..90401a00a2 100644
--- a/src/Avalonia.Controls.DataGrid/DataGridBoundColumn.cs
+++ b/src/Avalonia.Controls.DataGrid/DataGridBoundColumn.cs
@@ -10,7 +10,8 @@ using System.Reactive.Disposables;
 using System.Reactive.Subjects;
 using Avalonia.Reactive;
 using System.Diagnostics;
-using Avalonia.Controls.Utils; 
+using Avalonia.Controls.Utils;
+using Avalonia.Markup.Xaml.MarkupExtensions;
 
 namespace Avalonia.Controls
 {
@@ -47,14 +48,15 @@ namespace Avalonia.Controls
 
                     if (_binding != null)
                     {
-                        if(_binding is Avalonia.Data.Binding binding)
+                        if(_binding is BindingBase binding)
                         {
                             if (binding.Mode == BindingMode.OneWayToSource)
                             {
                                 throw new InvalidOperationException("DataGridColumn doesn't support BindingMode.OneWayToSource. Use BindingMode.TwoWay instead.");
                             }
 
-                            if (!String.IsNullOrEmpty(binding.Path) && binding.Mode == BindingMode.Default)
+                            var path = (binding as Binding)?.Path ?? (binding as CompiledBindingExtension)?.Path.ToString();
+                            if (!string.IsNullOrEmpty(path) && binding.Mode == BindingMode.Default)
                             {
                                 binding.Mode = BindingMode.TwoWay;
                             } 
@@ -136,13 +138,16 @@ namespace Avalonia.Controls
         internal void SetHeaderFromBinding()
         {
             if (OwningGrid != null && OwningGrid.DataConnection.DataType != null
-                && Header == null && Binding != null && Binding is Binding binding
-                && !String.IsNullOrWhiteSpace(binding.Path))
+                && Header == null && Binding != null && Binding is BindingBase binding)
             {
-                string header = OwningGrid.DataConnection.DataType.GetDisplayName(binding.Path);
-                if (header != null)
+                var path = (binding as Binding)?.Path ?? (binding as CompiledBindingExtension)?.Path.ToString();
+                if (!string.IsNullOrWhiteSpace(path))
                 {
-                    Header = header;
+                    var header = OwningGrid.DataConnection.DataType.GetDisplayName(path);
+                    if (header != null)
+                    {
+                        Header = header;
+                    }
                 }
             }
         }
diff --git a/src/Avalonia.Controls.DataGrid/DataGridColumn.cs b/src/Avalonia.Controls.DataGrid/DataGridColumn.cs
index 92ddd4e736..407d6ff058 100644
--- a/src/Avalonia.Controls.DataGrid/DataGridColumn.cs
+++ b/src/Avalonia.Controls.DataGrid/DataGridColumn.cs
@@ -12,6 +12,7 @@ using System;
 using System.Linq;
 using System.Diagnostics;
 using Avalonia.Controls.Utils;
+using Avalonia.Markup.Xaml.MarkupExtensions;
 
 namespace Avalonia.Controls
 {
@@ -1033,13 +1034,16 @@ namespace Avalonia.Controls
 
             if (String.IsNullOrEmpty(result))
             {
-
-                if(this is DataGridBoundColumn boundColumn && 
-                    boundColumn.Binding != null &&
-                    boundColumn.Binding is Binding binding &&
-                    binding.Path != null)
+                if (this is DataGridBoundColumn boundColumn)
                 {
-                    result = binding.Path;
+                    if (boundColumn.Binding is Binding binding)
+                    {
+                        result = binding.Path;
+                    }
+                    else if (boundColumn.Binding is CompiledBindingExtension compiledBinding)
+                    {
+                        result = compiledBinding.Path.ToString();
+                    }
                 }
             }
 
diff --git a/src/Avalonia.Controls.DataGrid/DataGridColumns.cs b/src/Avalonia.Controls.DataGrid/DataGridColumns.cs
index 46bcd0d347..a4577ee952 100644
--- a/src/Avalonia.Controls.DataGrid/DataGridColumns.cs
+++ b/src/Avalonia.Controls.DataGrid/DataGridColumns.cs
@@ -5,6 +5,7 @@
 
 using Avalonia.Controls.Utils;
 using Avalonia.Data;
+using Avalonia.Markup.Xaml.MarkupExtensions;
 using Avalonia.Utilities;
 using System;
 using System.Collections.Generic;
@@ -141,9 +142,9 @@ namespace Avalonia.Controls
             Debug.Assert(dataGridColumn != null);
 
             if (dataGridColumn is DataGridBoundColumn dataGridBoundColumn && 
-                dataGridBoundColumn.Binding is Binding binding)
+                dataGridBoundColumn.Binding is BindingBase binding)
             {
-                string path = binding.Path;
+                var path = (binding as Binding)?.Path ?? (binding as CompiledBindingExtension)?.Path.ToString();
 
                 if (string.IsNullOrWhiteSpace(path))
                 {
diff --git a/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs b/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs
index fe4e9dd94a..b6cdb77f81 100644
--- a/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs
+++ b/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs
@@ -29,7 +29,7 @@ namespace Avalonia.ReactiveUI
 
         /// <inheritdoc/>
         public bool ExecuteHook(
-            object source, object target,
+            object? source, object target,
             Func<IObservedChange<object, object>[]> getCurrentViewModelProperties,
             Func<IObservedChange<object, object>[]> getCurrentViewProperties,
             BindingDirection direction)
diff --git a/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj b/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
index 624c5772a9..2792ae1c91 100644
--- a/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
+++ b/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
@@ -3,6 +3,8 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <PackageId>Avalonia.ReactiveUI</PackageId>
     <SignAssembly>false</SignAssembly>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\packages\Avalonia\Avalonia.csproj" />
diff --git a/src/Avalonia.ReactiveUI/ReactiveUserControl.cs b/src/Avalonia.ReactiveUI/ReactiveUserControl.cs
index f4824ade0a..4987349162 100644
--- a/src/Avalonia.ReactiveUI/ReactiveUserControl.cs
+++ b/src/Avalonia.ReactiveUI/ReactiveUserControl.cs
@@ -17,8 +17,8 @@ namespace Avalonia.ReactiveUI
     /// <typeparam name="TViewModel">ViewModel type.</typeparam>
     public class ReactiveUserControl<TViewModel> : UserControl, IViewFor<TViewModel> where TViewModel : class
     {
-        public static readonly StyledProperty<TViewModel> ViewModelProperty = AvaloniaProperty
-            .Register<ReactiveUserControl<TViewModel>, TViewModel>(nameof(ViewModel));
+        public static readonly StyledProperty<TViewModel?> ViewModelProperty = AvaloniaProperty
+            .Register<ReactiveUserControl<TViewModel>, TViewModel?>(nameof(ViewModel));
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ReactiveUserControl{TViewModel}"/> class.
@@ -34,16 +34,16 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// The ViewModel.
         /// </summary>
-        public TViewModel ViewModel
+        public TViewModel? ViewModel
         {
             get => GetValue(ViewModelProperty);
             set => SetValue(ViewModelProperty, value);
         }
 
-        object IViewFor.ViewModel
+        object? IViewFor.ViewModel
         {
             get => ViewModel;
-            set => ViewModel = (TViewModel)value;
+            set => ViewModel = (TViewModel?)value;
         }
 
         protected override void OnDataContextChanged(EventArgs e)
@@ -51,7 +51,7 @@ namespace Avalonia.ReactiveUI
             ViewModel = DataContext as TViewModel;
         }
 
-        private void OnViewModelChanged(object value)
+        private void OnViewModelChanged(object? value)
         {
             if (value == null)
             {
diff --git a/src/Avalonia.ReactiveUI/ReactiveWindow.cs b/src/Avalonia.ReactiveUI/ReactiveWindow.cs
index 758a807bfc..726fb3d661 100644
--- a/src/Avalonia.ReactiveUI/ReactiveWindow.cs
+++ b/src/Avalonia.ReactiveUI/ReactiveWindow.cs
@@ -17,8 +17,8 @@ namespace Avalonia.ReactiveUI
     /// <typeparam name="TViewModel">ViewModel type.</typeparam>
     public class ReactiveWindow<TViewModel> : Window, IViewFor<TViewModel> where TViewModel : class
     {
-        public static readonly StyledProperty<TViewModel> ViewModelProperty = AvaloniaProperty
-            .Register<ReactiveWindow<TViewModel>, TViewModel>(nameof(ViewModel));
+        public static readonly StyledProperty<TViewModel?> ViewModelProperty = AvaloniaProperty
+            .Register<ReactiveWindow<TViewModel>, TViewModel?>(nameof(ViewModel));
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ReactiveWindow{TViewModel}"/> class.
@@ -35,19 +35,19 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// The ViewModel.
         /// </summary>
-        public TViewModel ViewModel
+        public TViewModel? ViewModel
         {
             get => GetValue(ViewModelProperty);
             set => SetValue(ViewModelProperty, value);
         }
 
-        object IViewFor.ViewModel
+        object? IViewFor.ViewModel
         {
             get => ViewModel;
-            set => ViewModel = (TViewModel)value;
+            set => ViewModel = (TViewModel?)value;
         }
 
-        private void OnDataContextChanged(object value)
+        private void OnDataContextChanged(object? value)
         {
             if (value is TViewModel viewModel)
             {
@@ -59,7 +59,7 @@ namespace Avalonia.ReactiveUI
             }
         }
 
-        private void OnViewModelChanged(object value)
+        private void OnViewModelChanged(object? value)
         {
             if (value == null)
             {
diff --git a/src/Avalonia.ReactiveUI/RoutedViewHost.cs b/src/Avalonia.ReactiveUI/RoutedViewHost.cs
index 421633cd58..63456bc13a 100644
--- a/src/Avalonia.ReactiveUI/RoutedViewHost.cs
+++ b/src/Avalonia.ReactiveUI/RoutedViewHost.cs
@@ -55,8 +55,8 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="Router"/> property.
         /// </summary>
-        public static readonly StyledProperty<RoutingState> RouterProperty =
-            AvaloniaProperty.Register<RoutedViewHost, RoutingState>(nameof(Router));
+        public static readonly StyledProperty<RoutingState?> RouterProperty =
+            AvaloniaProperty.Register<RoutedViewHost, RoutingState?>(nameof(Router));
     
         /// <summary>
         /// Initializes a new instance of the <see cref="RoutedViewHost"/> class.
@@ -67,12 +67,12 @@ namespace Avalonia.ReactiveUI
             {
                 var routerRemoved = this
                     .WhenAnyValue(x => x.Router)
-                    .Where(router => router == null)
-                    .Cast<object>();
+                    .Where(router => router == null)!
+                    .Cast<object?>();
 
                 this.WhenAnyValue(x => x.Router)
                     .Where(router => router != null)
-                    .SelectMany(router => router.CurrentViewModel)
+                    .SelectMany(router => router!.CurrentViewModel)
                     .Merge(routerRemoved)
                     .Subscribe(NavigateToViewModel)
                     .DisposeWith(disposables);
@@ -82,7 +82,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the <see cref="RoutingState"/> of the view model stack.
         /// </summary>
-        public RoutingState Router
+        public RoutingState? Router
         {
             get => GetValue(RouterProperty);
             set => SetValue(RouterProperty, value);
@@ -91,13 +91,13 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the ReactiveUI view locator used by this router.
         /// </summary>
-        public IViewLocator ViewLocator { get; set; }
+        public IViewLocator? ViewLocator { get; set; }
     
         /// <summary>
         /// Invoked when ReactiveUI router navigates to a view model.
         /// </summary>
         /// <param name="viewModel">ViewModel to which the user navigates.</param>
-        private void NavigateToViewModel(object viewModel)
+        private void NavigateToViewModel(object? viewModel)
         {
             if (Router == null)
             {
diff --git a/src/Avalonia.ReactiveUI/TransitioningContentControl.cs b/src/Avalonia.ReactiveUI/TransitioningContentControl.cs
index cc1613ce19..9685ecbe91 100644
--- a/src/Avalonia.ReactiveUI/TransitioningContentControl.cs
+++ b/src/Avalonia.ReactiveUI/TransitioningContentControl.cs
@@ -13,20 +13,20 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="PageTransition"/> property.
         /// </summary>
-        public static readonly StyledProperty<IPageTransition> PageTransitionProperty =
-            AvaloniaProperty.Register<TransitioningContentControl, IPageTransition>(nameof(PageTransition),
+        public static readonly StyledProperty<IPageTransition?> PageTransitionProperty =
+            AvaloniaProperty.Register<TransitioningContentControl, IPageTransition?>(nameof(PageTransition),
                 new CrossFade(TimeSpan.FromSeconds(0.5)));
 
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="DefaultContent"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> DefaultContentProperty =
-            AvaloniaProperty.Register<TransitioningContentControl, object>(nameof(DefaultContent));
+        public static readonly StyledProperty<object?> DefaultContentProperty =
+            AvaloniaProperty.Register<TransitioningContentControl, object?>(nameof(DefaultContent));
         
         /// <summary>
         /// Gets or sets the animation played when content appears and disappears.
         /// </summary>
-        public IPageTransition PageTransition
+        public IPageTransition? PageTransition
         {
             get => GetValue(PageTransitionProperty);
             set => SetValue(PageTransitionProperty, value);
@@ -35,7 +35,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the content displayed whenever there is no page currently routed.
         /// </summary>
-        public object DefaultContent
+        public object? DefaultContent
         {
             get => GetValue(DefaultContentProperty);
             set => SetValue(DefaultContentProperty, value);
@@ -44,7 +44,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the content with animation.
         /// </summary>
-        public new object Content
+        public new object? Content
         {
             get => base.Content;
             set => UpdateContentWithTransition(value);
@@ -60,7 +60,7 @@ namespace Avalonia.ReactiveUI
         /// Updates the content with transitions.
         /// </summary>
         /// <param name="content">New content to set.</param>
-        private async void UpdateContentWithTransition(object content)
+        private async void UpdateContentWithTransition(object? content)
         {
             if (PageTransition != null)
                 await PageTransition.Start(this, null, true);
diff --git a/src/Avalonia.ReactiveUI/ViewModelViewHost.cs b/src/Avalonia.ReactiveUI/ViewModelViewHost.cs
index 0169b268ca..c88323d674 100644
--- a/src/Avalonia.ReactiveUI/ViewModelViewHost.cs
+++ b/src/Avalonia.ReactiveUI/ViewModelViewHost.cs
@@ -15,8 +15,8 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="ViewModel"/> property.
         /// </summary>
-        public static readonly AvaloniaProperty<object> ViewModelProperty =
-            AvaloniaProperty.Register<ViewModelViewHost, object>(nameof(ViewModel));
+        public static readonly AvaloniaProperty<object?> ViewModelProperty =
+            AvaloniaProperty.Register<ViewModelViewHost, object?>(nameof(ViewModel));
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ViewModelViewHost"/> class.
@@ -34,7 +34,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the ViewModel to display.
         /// </summary>
-        public object ViewModel
+        public object? ViewModel
         {
             get => GetValue(ViewModelProperty);
             set => SetValue(ViewModelProperty, value);
@@ -43,13 +43,13 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the view locator.
         /// </summary>
-        public IViewLocator ViewLocator { get; set; }
+        public IViewLocator? ViewLocator { get; set; }
 
         /// <summary>
         /// Invoked when ReactiveUI router navigates to a view model.
         /// </summary>
         /// <param name="viewModel">ViewModel to which the user navigates.</param>
-        private void NavigateToViewModel(object viewModel)
+        private void NavigateToViewModel(object? viewModel)
         {
             if (viewModel == null)
             {
@@ -74,4 +74,4 @@ namespace Avalonia.ReactiveUI
             Content = viewInstance;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/CompiledBindingPath.cs b/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/CompiledBindingPath.cs
index f6636664c1..11489c39aa 100644
--- a/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/CompiledBindingPath.cs
+++ b/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/CompiledBindingPath.cs
@@ -72,7 +72,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
         internal object RawSource { get; }
 
         public override string ToString()
-            => string.Concat(_elements.Select(e => e.ToString()));
+            => string.Concat(_elements);
     }
 
     public class CompiledBindingPathBuilder
@@ -88,7 +88,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
 
         public CompiledBindingPathBuilder Property(IPropertyInfo info, Func<WeakReference<object>, IPropertyInfo, IPropertyAccessor> accessorFactory)
         {
-            _elements.Add(new PropertyElement(info, accessorFactory));
+            _elements.Add(new PropertyElement(info, accessorFactory, _elements.Count == 0));
             return this;
         }
 
@@ -161,10 +161,13 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
 
     internal class PropertyElement : ICompiledBindingPathElement
     {
-        public PropertyElement(IPropertyInfo property, Func<WeakReference<object>, IPropertyInfo, IPropertyAccessor> accessorFactory)
+        private readonly bool _isFirstElement;
+
+        public PropertyElement(IPropertyInfo property, Func<WeakReference<object>, IPropertyInfo, IPropertyAccessor> accessorFactory, bool isFirstElement)
         {
             Property = property;
             AccessorFactory = accessorFactory;
+            _isFirstElement = isFirstElement;
         }
 
         public IPropertyInfo Property { get; }
@@ -172,7 +175,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
         public Func<WeakReference<object>, IPropertyInfo, IPropertyAccessor> AccessorFactory { get; }
 
         public override string ToString()
-            => $".{Property.Name}";
+            => _isFirstElement ? Property.Name : $".{Property.Name}";
     }
 
     internal interface IStronglyTypedStreamElement : ICompiledBindingPathElement
diff --git a/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs b/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
index 287d2c49a5..c137926e4c 100644
--- a/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
+++ b/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
@@ -1438,15 +1438,15 @@ namespace Avalonia.Win32.Interop
         }
 
         [DllImport("ntdll")]
-        private static extern int RtlGetVersion(out RTL_OSVERSIONINFOEX lpVersionInformation);
+        private static extern int RtlGetVersion(ref RTL_OSVERSIONINFOEX lpVersionInformation);
 
         internal static Version RtlGetVersion()
         {
             RTL_OSVERSIONINFOEX v = new RTL_OSVERSIONINFOEX();
             v.dwOSVersionInfoSize = (uint)Marshal.SizeOf(v);
-            if (RtlGetVersion(out v) == 0)
+            if (RtlGetVersion(ref v) == 0)
             {
-                return new Version((int)v.dwMajorVersion, (int)v.dwMinorVersion, (int)v.dwBuildNumber, (int)v.dwPlatformId);
+                return new Version((int)v.dwMajorVersion, (int)v.dwMinorVersion, (int)v.dwBuildNumber);
             }
             else
             {