Added nullable type converter proxy

src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj

@@ -11,6 +11,7 @@
         <Compile Include="Converters\AvaloniaUriTypeConverter.cs" />
         <Compile Include="Converters\FontFamilyTypeConverter.cs" />
         <Compile Include="Converters\MemberSelectorTypeConverter.cs" />
+        <Compile Include="Converters\NullableTypeConverter.cs" />
         <Compile Include="Converters\ParseTypeConverter.cs" />
         <Compile Include="Converters\SetterValueTypeConverter.cs" />
         <Compile Include="Converters\TimeSpanTypeConverter.cs" />

src/Markup/Avalonia.Markup.Xaml/AvaloniaTypeConverters.cs

@@ -51,6 +51,14 @@ namespace Avalonia.Markup.Xaml
         /// <returns>The type converter.</returns>
         public static Type GetTypeConverter(Type type)
         {
+            if (type.IsConstructedGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
+            {
+                var inner = GetTypeConverter(type.GetGenericArguments()[0]);
+                if (inner == null)
+                    return null;
+                return typeof(NullableTypeConverter<>).MakeGenericType(inner);
+            }
+            
             if (_converters.TryGetValue(type, out var result))
             {
                 return result;

src/Markup/Avalonia.Markup.Xaml/Converters/NullableTypeConverter.cs

@@ -0,0 +1,89 @@
+using System;
+using System.Collections;
+using System.ComponentModel;
+using System.Globalization;
+
+namespace Avalonia.Markup.Xaml.Converters
+{
+    public class NullableTypeConverter<T> : TypeConverter where T : TypeConverter, new()
+    {
+        private TypeConverter _inner;
+
+        public NullableTypeConverter()
+        {
+            _inner = new T();
+        }
+
+        public NullableTypeConverter(TypeConverter inner)
+        {
+            _inner = inner;
+        }
+
+        
+        public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
+        {
+            if (value == null)
+                return null;
+            return _inner.ConvertTo(context, culture, value, destinationType);
+        }
+
+        public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
+        {
+            if (value == null)
+                return null;
+            if (value as string == "")
+                return null;
+            return _inner.ConvertFrom(context, culture, value);
+        }
+        
+        public override object CreateInstance(ITypeDescriptorContext context, IDictionary propertyValues)
+        {
+            return _inner.CreateInstance(context, propertyValues);
+        }
+        
+        public override bool GetStandardValuesSupported(ITypeDescriptorContext context)
+        {
+            return _inner.GetStandardValuesSupported(context);
+        }
+
+        public override bool GetStandardValuesExclusive(ITypeDescriptorContext context)
+        {
+            return _inner.GetStandardValuesExclusive(context);
+        }
+
+        public override bool GetCreateInstanceSupported(ITypeDescriptorContext context)
+        {
+            return _inner.GetCreateInstanceSupported(context);
+        }
+
+        public override bool GetPropertiesSupported(ITypeDescriptorContext context)
+        {
+            return _inner.GetPropertiesSupported(context);
+        }
+
+        public override StandardValuesCollection GetStandardValues(ITypeDescriptorContext context)
+        {
+            return _inner.GetStandardValues(context);
+        }
+
+        public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext context, object value, Attribute[] attributes)
+        {
+            return _inner.GetProperties(context, value, attributes);
+        }
+
+        public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
+        {
+            return _inner.CanConvertTo(context, destinationType);
+        }
+
+        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
+        {
+            return _inner.CanConvertFrom(context, sourceType);
+        }
+
+        public override bool IsValid(ITypeDescriptorContext context, object value)
+        {
+            return _inner.IsValid(context, value);
+        }
+    }
+}

tests/Avalonia.Markup.Xaml.UnitTests/Converters/NullableConverterTests.cs

@@ -0,0 +1,31 @@
+using Avalonia.Controls;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Markup.Xaml.UnitTests.Converters
+{
+    public class ClassWithNullableProperties
+    {
+        public Thickness? Thickness { get; set; }
+        public Orientation? Orientation { get; set; }
+    }
+    
+    public class NullableConverterTests
+    {
+        [Fact]
+        public void Nullable_Types_Should_Still_Be_Converted_Properly()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformWrapper))
+            {
+                var xaml = @"<ClassWithNullableProperties 
+xmlns='clr-namespace:Avalonia.Markup.Xaml.UnitTests.Converters'
+    Thickness = '5' Orientation='Vertical'
+></ClassWithNullableProperties>";
+                var loader = new AvaloniaXamlLoader();
+                var data = (ClassWithNullableProperties)loader.Load(xaml, typeof(ClassWithNullableProperties).Assembly);
+                Assert.Equal(new Thickness(5), data.Thickness);
+                Assert.Equal(Orientation.Vertical, data.Orientation);
+            }                
+        }
+    }
+}