fix: Don't use nullable reference types in generated code

Author: Youssef1313

src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs

@@ -3875,7 +3875,7 @@ string buildBindBack()
 						{
 							if (!string.IsNullOrWhiteSpace(rawBindBack))
 							{
-								var targetPropertyType = GetXBindPropertyPathType(propertyPaths.properties[0], GetType(dataType));
+								var targetPropertyType = GetXBindPropertyPathType(propertyPaths.properties[0], GetType(dataType)).ToDisplayString(NullableFlowState.None);
 								return $"(___ctx, __value) => {{ if(___ctx is {dataType} ___tctx) {{ ___tctx.{rawBindBack}(({propertyType})__value); }} }}";
 							}
 							else
@@ -3887,8 +3887,8 @@ string buildBindBack()
 						{
 							if (propertyPaths.properties.Length == 1)
 							{
-								var targetPropertyType = GetXBindPropertyPathType(propertyPaths.properties[0], GetType(dataType));
-								return $"(___ctx, __value) => {{ if(___ctx is {dataType} ___tctx) {{ {contextFunction} = ({targetPropertyType})global::Windows.UI.Xaml.Markup.XamlBindingHelper.ConvertValue(typeof({targetPropertyType.ToDisplayString(NullableFlowState.None)}), __value); }} }}";
+								var targetPropertyType = GetXBindPropertyPathType(propertyPaths.properties[0], GetType(dataType)).ToDisplayString(NullableFlowState.None);
+								return $"(___ctx, __value) => {{ if(___ctx is {dataType} ___tctx) {{ {contextFunction} = ({targetPropertyType})global::Windows.UI.Xaml.Markup.XamlBindingHelper.ConvertValue(typeof({targetPropertyType}), __value); }} }}";
 							}
 							else
 							{
@@ -3928,10 +3928,10 @@ string buildBindBack()
 						{
 							if (propertyPaths.properties.Length == 1)
 							{
-								var targetPropertyType = GetXBindPropertyPathType(propertyPaths.properties[0]);
+								var targetPropertyType = GetXBindPropertyPathType(propertyPaths.properties[0]).ToDisplayString(NullableFlowState.None);
 								return $"(___ctx, __value) => {{ " +
 									$"if(___ctx is global::{_className.ns + "." + _className.className} ___tctx) " +
-									$"{rawFunction} = ({targetPropertyType})global::Windows.UI.Xaml.Markup.XamlBindingHelper.ConvertValue(typeof({targetPropertyType.ToDisplayString(NullableFlowState.None)}), __value);" +
+									$"{rawFunction} = ({targetPropertyType})global::Windows.UI.Xaml.Markup.XamlBindingHelper.ConvertValue(typeof({targetPropertyType}), __value);" +
 									$" }}";
 							}
 							else
@@ -4105,7 +4105,7 @@ private string GetCustomMarkupExtensionValue(XamlMemberDefinition member)
 				{
 					// The target property implements IConvertible, therefore
 					// cast ProvideValue() using Convert.ChangeType
-					var targetTypeDisplay = propertyType.ToDisplayString();
+					var targetTypeDisplay = propertyType.ToDisplayString(NullableFlowState.None);
 					var targetType = $"typeof({targetTypeDisplay})";
 
 					// It's important to cast to string before performing the conversion

src/SourceGenerators/XamlGenerationTests/TestNullable_ResourceDictionary.xaml

@@ -0,0 +1,17 @@
+<ResourceDictionary
+    x:Class="UnoNullableContextGeneratedBug.TestTemplate"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:UnoNullableContextGeneratedBug">
+
+    <DataTemplate x:Key="TestDataTemplate" x:DataType="local:TestViewModel">
+        <StackPanel>
+            <!--This works-->
+            <TextBlock Text="{x:Bind ObjectInstance, Mode=OneWay}"/>
+        
+            <!-- This generates code with a nullable error at compile time -->
+            <TextBlock Text="{x:Bind ObjectInstance, Mode=TwoWay}"/>
+        </StackPanel>
+    </DataTemplate>
+
+</ResourceDictionary>

src/SourceGenerators/XamlGenerationTests/TestNullable_ViewModel.cs

@@ -0,0 +1,24 @@
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+using System.Text;
+
+namespace UnoNullableContextGeneratedBug
+{
+    public class TestViewModel
+    {
+        public TestViewModel()
+        {
+            ObjectInstance = new ThrowawayClass();
+        }
+
+        public ThrowawayClass? ObjectInstance { get; set; } = new ThrowawayClass();
+    }
+
+    public class ThrowawayClass
+    {
+    }
+}

src/SourceGenerators/XamlGenerationTests/XamlGenerationTests.csproj

@@ -2,6 +2,9 @@
 	<PropertyGroup>
 		<TargetFrameworks>MonoAndroid11.0;xamarinios10;netstandard2.0</TargetFrameworks>
 		<TargetFrameworksCI>MonoAndroid11.0;MonoAndroid10.0;xamarinios10;netstandard2.0</TargetFrameworksCI>
+
+		<LangVersion>8.0</LangVersion>
+		<WarningsAsErrors>nullable</WarningsAsErrors>
 	</PropertyGroup>
 
 	<PropertyGroup>