diff --git a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
index 89e2c096ee..ce104b3609 100644
--- a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
+++ b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
@@ -2,6 +2,7 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <RootNamespace>Avalonia</RootNamespace>
+    <Nullable>Enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <None Remove="Markup\Parsers\Nodes\ExpressionGrammer" />
diff --git a/src/Markup/Avalonia.Markup/Data/Binding.cs b/src/Markup/Avalonia.Markup/Data/Binding.cs
index bf43730481..554c27b85a 100644
--- a/src/Markup/Avalonia.Markup/Data/Binding.cs
+++ b/src/Markup/Avalonia.Markup/Data/Binding.cs
@@ -39,17 +39,17 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the name of the element to use as the binding source.
         /// </summary>
-        public string ElementName { get; set; }
+        public string? ElementName { get; set; }
 
         /// <summary>
         /// Gets or sets the relative source for the binding.
         /// </summary>
-        public RelativeSource RelativeSource { get; set; }
+        public RelativeSource? RelativeSource { get; set; }
 
         /// <summary>
         /// Gets or sets the source for the binding.
         /// </summary>
-        public object Source { get; set; }
+        public object? Source { get; set; }
 
         /// <summary>
         /// Gets or sets the binding path.
@@ -59,20 +59,21 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets a function used to resolve types from names in the binding path.
         /// </summary>
-        public Func<string, string, Type> TypeResolver { get; set; }
+        public Func<string, string, Type>? TypeResolver { get; set; }
 
-        protected override ExpressionObserver CreateExpressionObserver(IAvaloniaObject target, AvaloniaProperty targetProperty, object anchor, bool enableDataValidation)
+        protected override ExpressionObserver CreateExpressionObserver(IAvaloniaObject target, AvaloniaProperty targetProperty, object? anchor, bool enableDataValidation)
         {
-            Contract.Requires<ArgumentNullException>(target != null);
+            Contract.Requires<ArgumentNullException>(target is not null);
             anchor = anchor ?? DefaultAnchor?.Target;
             
             enableDataValidation = enableDataValidation && Priority == BindingPriority.LocalValue;
 
-            INameScope nameScope = null;
+            INameScope? nameScope = null;
             NameScope?.TryGetTarget(out nameScope);
 
             var (node, mode) = ExpressionObserverBuilder.Parse(Path, enableDataValidation, TypeResolver, nameScope);
 
+#nullable disable
             if (ElementName != null)
             {
                 return CreateElementObserver(
@@ -138,5 +139,6 @@ namespace Avalonia.Data
                 throw new NotSupportedException();
             }
         }
+#nullable enable
     }
 }
diff --git a/src/Markup/Avalonia.Markup/Data/BindingBase.cs b/src/Markup/Avalonia.Markup/Data/BindingBase.cs
index 3dbc83a7df..b900a043d7 100644
--- a/src/Markup/Avalonia.Markup/Data/BindingBase.cs
+++ b/src/Markup/Avalonia.Markup/Data/BindingBase.cs
@@ -38,22 +38,22 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the <see cref="IValueConverter"/> to use.
         /// </summary>
-        public IValueConverter Converter { get; set; }
+        public IValueConverter? Converter { get; set; }
 
         /// <summary>
         /// Gets or sets a parameter to pass to <see cref="Converter"/>.
         /// </summary>
-        public object ConverterParameter { get; set; }
+        public object? ConverterParameter { get; set; }
 
         /// <summary>
         /// Gets or sets the value to use when the binding is unable to produce a value.
         /// </summary>
-        public object FallbackValue { get; set; }
+        public object? FallbackValue { get; set; }
 
         /// <summary>
         /// Gets or sets the value to use when the binding result is null.
         /// </summary>
-        public object TargetNullValue { get; set; }
+        public object? TargetNullValue { get; set; }
 
         /// <summary>
         /// Gets or sets the binding mode.
@@ -68,23 +68,23 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the string format.
         /// </summary>
-        public string StringFormat { get; set; }
+        public string? StringFormat { get; set; }
 
-        public WeakReference DefaultAnchor { get; set; }
+        public WeakReference? DefaultAnchor { get; set; }
 
-        public WeakReference<INameScope> NameScope { get; set; }
+        public WeakReference<INameScope>? NameScope { get; set; }
 
         protected abstract ExpressionObserver CreateExpressionObserver(
             IAvaloniaObject target,
             AvaloniaProperty targetProperty,
-            object anchor,
+            object? anchor,
             bool enableDataValidation);
 
         /// <inheritdoc/>
         public InstancedBinding Initiate(
             IAvaloniaObject target,
             AvaloniaProperty targetProperty,
-            object anchor = null,
+            object? anchor = null,
             bool enableDataValidation = false)
         {
             Contract.Requires<ArgumentNullException>(target != null);
@@ -133,18 +133,13 @@ namespace Avalonia.Data
             IAvaloniaObject target,
             ExpressionNode node,
             bool targetIsDataContext,
-            object anchor)
+            object? anchor)
         {
             Contract.Requires<ArgumentNullException>(target != null);
 
             if (!(target is IDataContextProvider))
             {
-                target = anchor as IDataContextProvider;
-
-                if (target == null)
-                {
-                    throw new InvalidOperationException("Cannot find a DataContext to bind to.");
-                }
+                target = anchor as IDataContextProvider ?? throw new InvalidOperationException("Cannot find a DataContext to bind to.");
             }
 
             if (!targetIsDataContext)
@@ -173,8 +168,7 @@ namespace Avalonia.Data
         {
             Contract.Requires<ArgumentNullException>(target != null);
 
-            NameScope.TryGetTarget(out var scope);
-            if (scope == null)
+            if (NameScope is null || !NameScope.TryGetTarget(out var scope) || scope is null)
                 throw new InvalidOperationException("Name scope is null or was already collected");
             var result = new ExpressionObserver(
                 NameScopeLocator.Track(scope, elementName),
@@ -190,7 +184,7 @@ namespace Avalonia.Data
         {
             Contract.Requires<ArgumentNullException>(target != null);
 
-            IObservable<object> controlLocator;
+            IObservable<object?> controlLocator;
 
             switch (relativeSource.Tree)
             {
@@ -229,7 +223,7 @@ namespace Avalonia.Data
             IAvaloniaObject target,
             ExpressionNode node)
         {
-            Contract.Requires<ArgumentNullException>(target != null);
+            Contract.Requires<ArgumentNullException>(target is not null);
 
             var result = new ExpressionObserver(
                 () => target.GetValue(StyledElement.TemplatedParentProperty),
@@ -240,7 +234,7 @@ namespace Avalonia.Data
             return result;
         }
 
-        protected IObservable<object> GetParentDataContext(IAvaloniaObject target)
+        protected IObservable<object?> GetParentDataContext(IAvaloniaObject target)
         {
             // The DataContext is based on the visual parent and not the logical parent: this may
             // seem counter intuitive considering the fact that property inheritance works on the logical
@@ -252,7 +246,7 @@ namespace Avalonia.Data
                 .Select(x =>
                 {
                     return (x as IAvaloniaObject)?.GetObservable(StyledElement.DataContextProperty) ??
-                           Observable.Return((object)null);
+                           Observable.Return((object?)null);
                 }).Switch();
         }
 
diff --git a/src/Markup/Avalonia.Markup/Data/MultiBinding.cs b/src/Markup/Avalonia.Markup/Data/MultiBinding.cs
index cbc5f414f2..1a33536fa3 100644
--- a/src/Markup/Avalonia.Markup/Data/MultiBinding.cs
+++ b/src/Markup/Avalonia.Markup/Data/MultiBinding.cs
@@ -22,12 +22,12 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the <see cref="IMultiValueConverter"/> to use.
         /// </summary>
-        public IMultiValueConverter Converter { get; set; }
+        public IMultiValueConverter? Converter { get; set; }
 
         /// <summary>
         /// Gets or sets a parameter to pass to <see cref="Converter"/>.
         /// </summary>
-        public object ConverterParameter { get; set; }
+        public object? ConverterParameter { get; set; }
 
         /// <summary>
         /// Gets or sets the value to use when the binding is unable to produce a value.
@@ -52,12 +52,12 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the relative source for the binding.
         /// </summary>
-        public RelativeSource RelativeSource { get; set; }
+        public RelativeSource? RelativeSource { get; set; }
 
         /// <summary>
         /// Gets or sets the string format.
         /// </summary>
-        public string StringFormat { get; set; }
+        public string? StringFormat { get; set; }
 
         public MultiBinding()
         {
@@ -69,7 +69,7 @@ namespace Avalonia.Data
         public InstancedBinding Initiate(
             IAvaloniaObject target,
             AvaloniaProperty targetProperty,
-            object anchor = null,
+            object? anchor = null,
             bool enableDataValidation = false)
         {
             var targetType = targetProperty?.PropertyType ?? typeof(object);
@@ -105,7 +105,7 @@ namespace Avalonia.Data
             }
         }
 
-        private object ConvertValue(IList<object> values, Type targetType, IMultiValueConverter converter)
+        private object ConvertValue(IList<object> values, Type targetType, IMultiValueConverter? converter)
         {
             for (var i = 0; i < values.Count; ++i)
             {
diff --git a/src/Markup/Avalonia.Markup/Data/RelativeSource.cs b/src/Markup/Avalonia.Markup/Data/RelativeSource.cs
index e3d2dd4aaa..037175a137 100644
--- a/src/Markup/Avalonia.Markup/Data/RelativeSource.cs
+++ b/src/Markup/Avalonia.Markup/Data/RelativeSource.cs
@@ -94,7 +94,7 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets the type of ancestor to look for when in <see cref="RelativeSourceMode.FindAncestor"/>  mode.
         /// </summary>
-        public Type AncestorType { get; set; }
+        public Type? AncestorType { get; set; }
 
         /// <summary>
         /// Gets or sets a value that describes the type of relative source lookup.
diff --git a/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs b/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs
index 83f02c52aa..b6c723d68c 100644
--- a/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs
+++ b/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs
@@ -17,8 +17,8 @@ namespace Avalonia.Data
         ISetterValue
     {
         private bool _isSetterValue;
-        private IStyledElement _target;
-        private Type _targetType;
+        private IStyledElement _target = default!;
+        private Type? _targetType;
 
         public TemplateBinding()
         {
@@ -33,7 +33,7 @@ namespace Avalonia.Data
         public InstancedBinding Initiate(
             IAvaloniaObject target,
             AvaloniaProperty targetProperty,
-            object anchor = null,
+            object? anchor = null,
             bool enableDataValidation = false)
         {
             // Usually each `TemplateBinding` will only be instantiated once; in this case we can
@@ -68,12 +68,12 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the <see cref="IValueConverter"/> to use.
         /// </summary>
-        public IValueConverter Converter { get; set; }
+        public IValueConverter? Converter { get; set; }
 
         /// <summary>
         /// Gets or sets a parameter to pass to <see cref="Converter"/>.
         /// </summary>
-        public object ConverterParameter { get; set; }
+        public object? ConverterParameter { get; set; }
 
         /// <summary>
         /// Gets or sets the binding mode.
@@ -83,7 +83,7 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the name of the source property on the templated parent.
         /// </summary>
-        public AvaloniaProperty Property { get; set; }
+        public AvaloniaProperty? Property { get; set; }
 
         /// <inheritdoc/>
         public string Description => "TemplateBinding: " + Property;
@@ -164,10 +164,7 @@ namespace Avalonia.Data
         {
             if (e.Property == StyledElement.TemplatedParentProperty)
             {
-                var oldValue = (IAvaloniaObject)e.OldValue;
-                var newValue = (IAvaloniaObject)e.OldValue;
-
-                if (oldValue != null)
+                if (e.OldValue is IAvaloniaObject oldValue)
                 {
                     oldValue.PropertyChanged -= TemplatedParentPropertyChanged;
                 }
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/BindingExpressionGrammar.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/BindingExpressionGrammar.cs
index 7c362e24cc..4da6a7f3ae 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/BindingExpressionGrammar.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/BindingExpressionGrammar.cs
@@ -271,8 +271,8 @@ namespace Avalonia.Markup.Parsers
             }
             else if (mode.SequenceEqual("parent".AsSpan()))
             {
-                string ancestorNamespace = null;
-                string ancestorType = null;
+                string? ancestorNamespace = null;
+                string? ancestorType = null;
                 var ancestorLevel = 0;
                 if (PeekOpenBracket(ref r))
                 {
@@ -424,19 +424,19 @@ namespace Avalonia.Markup.Parsers
 
         public class PropertyNameNode : INode
         {
-            public string PropertyName { get; set; }
+            public string PropertyName { get; set; } = string.Empty;
         }
 
         public class AttachedPropertyNameNode : INode
         {
-            public string Namespace { get; set; }
-            public string TypeName { get; set; }
-            public string PropertyName { get; set; }
+            public string Namespace { get; set; } = string.Empty;
+            public string TypeName { get; set; } = string.Empty;
+            public string PropertyName { get; set; } = string.Empty;
         }
 
         public class IndexerNode : INode
         {
-            public IList<string> Arguments { get; set; }
+            public IList<string> Arguments { get; set; } = Array.Empty<string>();
         }
 
         public class NotNode : INode, ITransformNode { }
@@ -447,20 +447,20 @@ namespace Avalonia.Markup.Parsers
 
         public class NameNode : INode
         {
-            public string Name { get; set; }
+            public string Name { get; set; } = string.Empty;
         }
 
         public class AncestorNode : INode
         {
-            public string Namespace { get; set; }
-            public string TypeName { get; set; }
+            public string? Namespace { get; set; }
+            public string? TypeName { get; set; }
             public int Level { get; set; }
         }
 
         public class TypeCastNode : INode
         {
-            public string Namespace { get; set; }
-            public string TypeName { get; set; }
+            public string Namespace { get; set; } = string.Empty;
+            public string TypeName { get; set; } = string.Empty;
         }
     }
 }
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionObserverBuilder.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionObserverBuilder.cs
index f957bcab1e..e615923ff0 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionObserverBuilder.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionObserverBuilder.cs
@@ -8,8 +8,8 @@ namespace Avalonia.Markup.Parsers
 {
     public static class ExpressionObserverBuilder
     {
-        internal static (ExpressionNode Node, SourceMode Mode) Parse(string expression, bool enableValidation = false, Func<string, string, Type> typeResolver = null,
-            INameScope nameScope = null)
+        internal static (ExpressionNode? Node, SourceMode Mode) Parse(string expression, bool enableValidation = false, Func<string, string, Type>? typeResolver = null,
+            INameScope? nameScope = null)
         {
             if (string.IsNullOrWhiteSpace(expression))
             {
@@ -32,8 +32,8 @@ namespace Avalonia.Markup.Parsers
             object root,
             string expression,
             bool enableDataValidation = false,
-            string description = null,
-            Func<string, string, Type> typeResolver = null)
+            string? description = null,
+            Func<string, string, Type>? typeResolver = null)
         {
             return new ExpressionObserver(
                 root,
@@ -45,8 +45,8 @@ namespace Avalonia.Markup.Parsers
             IObservable<object> rootObservable,
             string expression,
             bool enableDataValidation = false,
-            string description = null,
-            Func<string, string, Type> typeResolver = null)
+            string? description = null,
+            Func<string, string, Type>? typeResolver = null)
         {
             Contract.Requires<ArgumentNullException>(rootObservable != null);
             return new ExpressionObserver(
@@ -61,8 +61,8 @@ namespace Avalonia.Markup.Parsers
             string expression,
             IObservable<Unit> update,
             bool enableDataValidation = false,
-            string description = null,
-            Func<string, string, Type> typeResolver = null)
+            string? description = null,
+            Func<string, string, Type>? typeResolver = null)
         {
             Contract.Requires<ArgumentNullException>(rootGetter != null);
 
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionParser.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionParser.cs
index 558130e23f..0a6f3f82ba 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionParser.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionParser.cs
@@ -11,25 +11,25 @@ namespace Avalonia.Markup.Parsers
     internal class ExpressionParser
     {
         private readonly bool _enableValidation;
-        private readonly Func<string, string, Type> _typeResolver;
-        private readonly INameScope _nameScope;
+        private readonly Func<string, string, Type>? _typeResolver;
+        private readonly INameScope? _nameScope;
 
-        public ExpressionParser(bool enableValidation, Func<string, string, Type> typeResolver, INameScope nameScope)
+        public ExpressionParser(bool enableValidation, Func<string, string, Type>? typeResolver, INameScope? nameScope)
         {
             _typeResolver = typeResolver;
             _nameScope = nameScope;
             _enableValidation = enableValidation;
         }
 
-        public (ExpressionNode Node, SourceMode Mode) Parse(ref CharacterReader r)
+        public (ExpressionNode? Node, SourceMode Mode) Parse(ref CharacterReader r)
         {
-            ExpressionNode rootNode = null;
-            ExpressionNode node = null;
+            ExpressionNode? rootNode = null;
+            ExpressionNode? node = null;
             var (astNodes, mode) = BindingExpressionGrammar.Parse(ref r);
 
             foreach (var astNode in astNodes)
             {
-                ExpressionNode nextNode = null;
+                ExpressionNode? nextNode = null;
                 switch (astNode)
                 {
                     case BindingExpressionGrammar.EmptyExpressionNode _:
@@ -57,13 +57,13 @@ namespace Avalonia.Markup.Parsers
                         nextNode = ParseFindAncestor(ancestor);
                         break;
                     case BindingExpressionGrammar.NameNode elementName:
-                        nextNode = new ElementNameNode(_nameScope, elementName.Name);
+                        nextNode = new ElementNameNode(_nameScope ?? throw new NotSupportedException("Invalid element name binding with null name scope!"), elementName.Name);
                         break;
                     case BindingExpressionGrammar.TypeCastNode typeCast:
                         nextNode = ParseTypeCastNode(typeCast);
                         break;
                 }
-                if (rootNode is null)
+                if (node is null)
                 {
                     rootNode = node = nextNode;
                 }
@@ -79,7 +79,7 @@ namespace Avalonia.Markup.Parsers
 
         private FindAncestorNode ParseFindAncestor(BindingExpressionGrammar.AncestorNode node)
         {
-            Type ancestorType = null;
+            Type? ancestorType = null;
             var ancestorLevel = node.Level;
 
             if (!(node.Namespace is null) && !(node.TypeName is null))
@@ -97,7 +97,7 @@ namespace Avalonia.Markup.Parsers
 
         private TypeCastNode ParseTypeCastNode(BindingExpressionGrammar.TypeCastNode node)
         {
-            Type castType = null;
+            Type? castType = null;
             if (!(node.Namespace is null) && !(node.TypeName is null))
             {
                 if (_typeResolver == null)
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/ElementNameNode.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/ElementNameNode.cs
index 97198145a8..d6068a15d4 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/ElementNameNode.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/ElementNameNode.cs
@@ -9,7 +9,7 @@ namespace Avalonia.Markup.Parsers.Nodes
     {
         private readonly WeakReference<INameScope> _nameScope;
         private readonly string _name;
-        private IDisposable _subscription;
+        private IDisposable? _subscription;
 
         public ElementNameNode(INameScope nameScope, string name)
         {
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/FindAncestorNode.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/FindAncestorNode.cs
index f304d1e9a2..4124032113 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/FindAncestorNode.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/FindAncestorNode.cs
@@ -7,10 +7,10 @@ namespace Avalonia.Markup.Parsers.Nodes
     public class FindAncestorNode : ExpressionNode
     {
         private readonly int _level;
-        private readonly Type _ancestorType;
-        private IDisposable _subscription;
+        private readonly Type? _ancestorType;
+        private IDisposable? _subscription;
 
-        public FindAncestorNode(Type ancestorType, int level)
+        public FindAncestorNode(Type? ancestorType, int level)
         {
             _level = level;
             _ancestorType = ancestorType;
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs
index f3abd6a5c5..2f1756fc55 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs
@@ -29,15 +29,15 @@ namespace Avalonia.Markup.Parsers.Nodes
             var list = target as IList;
             var dictionary = target as IDictionary;
             var indexerProperty = GetIndexer(typeInfo);
-            var indexerParameters = indexerProperty?.GetIndexParameters();
+            ParameterInfo[] indexerParameters;
 
-            if (indexerProperty != null && indexerParameters.Length == Arguments.Count)
+            if (indexerProperty != null && (indexerParameters = indexerProperty.GetIndexParameters()).Length == Arguments.Count)
             {
                 var convertedObjectArray = new object[indexerParameters.Length];
 
                 for (int i = 0; i < Arguments.Count; i++)
                 {
-                    object temp = null;
+                    object? temp = null;
 
                     if (!TypeUtilities.TryConvert(indexerParameters[i].ParameterType, Arguments[i], CultureInfo.InvariantCulture, out temp))
                     {
@@ -125,7 +125,7 @@ namespace Avalonia.Markup.Parsers.Nodes
 
         public IList<string> Arguments { get; }
 
-        public override Type PropertyType
+        public override Type? PropertyType
         {
             get
             {
@@ -144,15 +144,15 @@ namespace Avalonia.Markup.Parsers.Nodes
             var list = target as IList;
             var dictionary = target as IDictionary;
             var indexerProperty = GetIndexer(typeInfo);
-            var indexerParameters = indexerProperty?.GetIndexParameters();
+            ParameterInfo[] indexerParameters;
 
-            if (indexerProperty != null && indexerParameters.Length == Arguments.Count)
+            if (indexerProperty != null && (indexerParameters = indexerProperty.GetIndexParameters()).Length == Arguments.Count)
             {
                 var convertedObjectArray = new object[indexerParameters.Length];
 
                 for (int i = 0; i < Arguments.Count; i++)
                 {
-                    object temp = null;
+                    object? temp = null;
 
                     if (!TypeUtilities.TryConvert(indexerParameters[i].ParameterType, Arguments[i], CultureInfo.InvariantCulture, out temp))
                     {
@@ -246,7 +246,7 @@ namespace Avalonia.Markup.Parsers.Nodes
             return true;
         }
 
-        private static PropertyInfo GetIndexer(TypeInfo typeInfo)
+        private static PropertyInfo? GetIndexer(TypeInfo? typeInfo)
         {
             PropertyInfo indexer;
 
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/PropertyPathGrammar.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/PropertyPathGrammar.cs
index c5953b514c..8e0a40ccf4 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/PropertyPathGrammar.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/PropertyPathGrammar.cs
@@ -30,7 +30,7 @@ namespace Avalonia.Markup.Parsers
             var parsed = new List<ISyntax>();
             while (state != State.End)
             {
-                ISyntax syntax = null;
+                ISyntax? syntax = null;
                 if (state == State.Start)
                     (state, syntax) = ParseStart(ref r);
                 else if (state == State.Next)
@@ -53,7 +53,7 @@ namespace Avalonia.Markup.Parsers
             return parsed;
         }
         
-        private static (State, ISyntax) ParseNext(ref CharacterReader r)
+        private static (State, ISyntax?) ParseNext(ref CharacterReader r)
         {
             r.SkipWhitespace();
             if (r.End)
@@ -106,7 +106,7 @@ namespace Avalonia.Markup.Parsers
                 });
         }
 
-        static (string ns, string name) ParseXamlIdentifier(ref CharacterReader r)
+        static (string? ns, string name) ParseXamlIdentifier(ref CharacterReader r)
         {
             var ident = r.ParseIdentifier();
             if (ident.IsEmpty)
@@ -147,7 +147,7 @@ namespace Avalonia.Markup.Parsers
             return true;
         }
         
-        private static (State, ISyntax) ParseAfterProperty(ref CharacterReader r)
+        private static (State, ISyntax?) ParseAfterProperty(ref CharacterReader r)
         {
             if (TryParseCasts(ref r, out var rv))
                 return rv;
@@ -184,7 +184,7 @@ namespace Avalonia.Markup.Parsers
 
         public class PropertySyntax : ISyntax
         {
-            public string Name { get; set; }
+            public string Name { get; set; } = string.Empty;
 
             public override bool Equals(object obj)
                 => obj is PropertySyntax other
@@ -193,9 +193,9 @@ namespace Avalonia.Markup.Parsers
         
         public class TypeQualifiedPropertySyntax : ISyntax
         {
-            public string Name { get; set; }
-            public string TypeName { get; set; }
-            public string TypeNamespace { get; set; }
+            public string Name { get; set; } = string.Empty;
+            public string TypeName { get; set; } = string.Empty;
+            public string? TypeNamespace { get; set; }
 
             public override bool Equals(object obj)
                 => obj is TypeQualifiedPropertySyntax other
@@ -212,8 +212,8 @@ namespace Avalonia.Markup.Parsers
         
         public class EnsureTypeSyntax : ISyntax
         {
-            public string TypeName { get; set; }
-            public string TypeNamespace { get; set; }
+            public string TypeName { get; set; } = string.Empty;
+            public string? TypeNamespace { get; set; }
             public override bool Equals(object obj)
                 => obj is EnsureTypeSyntax other
                    && other.TypeName == TypeName
@@ -222,8 +222,8 @@ namespace Avalonia.Markup.Parsers
         
         public class CastTypeSyntax : ISyntax
         {
-            public string TypeName { get; set; }
-            public string TypeNamespace { get; set; }
+            public string TypeName { get; set; } = string.Empty;
+            public string? TypeNamespace { get; set; }
             public override bool Equals(object obj)
                 => obj is CastTypeSyntax other
                    && other.TypeName == TypeName
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs
index b25e9490cd..d821a04d93 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs
@@ -39,7 +39,7 @@ namespace Avalonia.Markup.Parsers
             var selector = new List<ISyntax>();
             while (!r.End && state != State.End)
             {
-                ISyntax syntax = null;
+                ISyntax? syntax = null;
                 switch (state)
                 {
                     case State.Start:
@@ -110,7 +110,7 @@ namespace Avalonia.Markup.Parsers
             return State.TypeName;
         }
 
-        private static (State, ISyntax) ParseMiddle(ref CharacterReader r, char? end)
+        private static (State, ISyntax?) ParseMiddle(ref CharacterReader r, char? end)
         {
             if (r.TakeIf(':'))
             {
@@ -190,7 +190,7 @@ namespace Avalonia.Markup.Parsers
             }
         }
 
-        private static (State, ISyntax) ParseTraversal(ref CharacterReader r)
+        private static (State, ISyntax?) ParseTraversal(ref CharacterReader r)
         {
             r.SkipWhitespace();
             if (r.TakeIf('>'))
@@ -325,7 +325,7 @@ namespace Avalonia.Markup.Parsers
 
         public class OfTypeSyntax : ISyntax, ITypeSyntax
         {
-            public string TypeName { get; set; }
+            public string TypeName { get; set; } = string.Empty;
 
             public string Xmlns { get; set; } = string.Empty;
 
@@ -338,7 +338,7 @@ namespace Avalonia.Markup.Parsers
 
         public class IsSyntax : ISyntax, ITypeSyntax
         {
-            public string TypeName { get; set; }
+            public string TypeName { get; set; } = string.Empty;
 
             public string Xmlns { get; set; } = string.Empty;
 
@@ -351,7 +351,7 @@ namespace Avalonia.Markup.Parsers
 
         public class ClassSyntax : ISyntax
         {
-            public string Class { get; set; }
+            public string Class { get; set; } = string.Empty;
 
             public override bool Equals(object obj)
             {
@@ -361,7 +361,7 @@ namespace Avalonia.Markup.Parsers
 
         public class NameSyntax : ISyntax
         {
-            public string Name { get; set; }
+            public string Name { get; set; } = string.Empty;
 
             public override bool Equals(object obj)
             {
@@ -371,9 +371,9 @@ namespace Avalonia.Markup.Parsers
 
         public class PropertySyntax : ISyntax
         {
-            public string Property { get; set; }
+            public string Property { get; set; } = string.Empty;
 
-            public string Value { get; set; }
+            public string Value { get; set; } = string.Empty;
 
             public override bool Equals(object obj)
             {
@@ -409,7 +409,7 @@ namespace Avalonia.Markup.Parsers
 
         public class NotSyntax : ISyntax
         {
-            public IEnumerable<ISyntax> Argument { get; set; }
+            public IEnumerable<ISyntax> Argument { get; set; } = Enumerable.Empty<ISyntax>();
 
             public override bool Equals(object obj)
             {
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs
index 04519bf2bb..92ba744ee1 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs
@@ -31,13 +31,13 @@ namespace Avalonia.Markup.Parsers
         /// </summary>
         /// <param name="s">The string.</param>
         /// <returns>The parsed selector.</returns>
-        public Selector Parse(string s)
+        public Selector? Parse(string s)
         {
             var syntax = SelectorGrammar.Parse(s);
             return Create(syntax);
         }
 
-        private Selector Create(IEnumerable<SelectorGrammar.ISyntax> syntax)
+        private Selector? Create(IEnumerable<SelectorGrammar.ISyntax> syntax)
         {
             var result = default(Selector);
             var results = default(List<Selector>);
@@ -110,7 +110,7 @@ namespace Avalonia.Markup.Parsers
                             results = new List<Selector>();
                         }
 
-                        results.Add(result);
+                        results.Add(result ?? throw new NotSupportedException("Invalid selector!"));
                         result = null;
                         break;
                     default: