Support for XSD nillable and xsi:nil="true" (#60)

* WIP

* Completed typing and getters
WIP: needs runtime support for setters

* Fix "hides parent InnerException property" C# warning

* xsi:nil support for setting scalars
WIP: setting collections not supported yet

* XList support for nil values
WIP: codgen must be modified to take advantage of new option

* Refactor XNil apis and fix a few bugs

* Codegen nillable lists instances
Fix derived XList not supporting XNil.Value properly

* Fix overriden XList.Add nil support

* First test + Fix prevent xsi:type=anyType

* More tests and some fixes

* Added polysharp to XObjectsTests. Consolidated versions across the solution.

* Added global.json file for controlling which version of the .NET SDK to use.

* Revert "Added global.json file for controlling which version of the .NET SDK to use."

This reverts commit 9b1e83f.

* Lang version set to preview.

* Incremented version.

---------

Co-authored-by: Muhammad Miftah <mamift@gmail.com>
Co-authored-by: Muhammad Miftah <mamift@users.noreply.github.com>

GeneratedSchemaLibraries/Multi-namespaces/Multi-namespaces.csproj

@@ -17,12 +17,6 @@
     <None Remove="TestNs2.xsd" />
     <None Remove="TestNs3ToBeIncluded.xsd" />
   </ItemGroup>
-  <ItemGroup>
-    <PackageReference Include="PolySharp" Version="1.13.2">
-      <PrivateAssets>all</PrivateAssets>
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-    </PackageReference>
-  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\XObjectsCore\XObjectsCore.csproj">
       <SetTargetFramework>TargetFramework=netstandard2.0</SetTargetFramework>

LinqToXsd.Schemas/LinqToXsd.Schemas.csproj

@@ -35,7 +35,7 @@
     </ProjectReference>
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="PolySharp" Version="1.13.2">
+    <PackageReference Include="PolySharp" Version="1.14.1">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>

LinqToXsd.Schemas/Tests/Nil/NilTest.xsd

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="utf-8"?>
+<xs:schema
+    targetNamespace="http://linqtoxsd.schemas.org/test/nil-test.xsd"
+    xmlns="http://linqtoxsd.schemas.org/test/nil-test.xsd"
+    xmlns:xs="http://www.w3.org/2001/XMLSchema"
+    elementFormDefault="qualified"
+>
+	<xs:complexType name="ValueHolder">
+		<xs:sequence>
+			<xs:element name="Value" type="xs:string" />
+		</xs:sequence>
+	</xs:complexType>
+
+	<xs:element name="Root">
+		<xs:complexType>
+			<xs:sequence>
+				<xs:element name="RequiredRef" type="xs:string" nillable="true" />
+				<xs:element name="RequiredVal" type="xs:int" nillable="true" />
+				<xs:element name="RequiredEl" type="ValueHolder" nillable="true" />
+				<xs:element name="OptionalRef" type="xs:string" minOccurs="0" nillable="true" />
+				<xs:element name="OptionalVal" type="xs:int" minOccurs="0" nillable="true" />
+				<xs:element name="OptionalEl" type="ValueHolder" minOccurs="0" nillable="true" />
+				<xs:element name="ListRef" type="xs:string" minOccurs="0" maxOccurs="unbounded" nillable="true" />
+				<xs:element name="ListVal" type="xs:int" minOccurs="0" maxOccurs="unbounded" nillable="true" />
+				<xs:element name="ListEl" type="ValueHolder" minOccurs="0" maxOccurs="unbounded" nillable="true" />
+			</xs:sequence>
+		</xs:complexType>
+	</xs:element>
+</xs:schema>

LinqToXsd.Schemas/Tests/Nil/NilTest.xsd.config

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Configuration xmlns="http://www.microsoft.com/xml/schema/linq">
+	<CodeGeneration>
+		<NullableReferences>true</NullableReferences>
+	</CodeGeneration>
+    <Namespaces>
+        <Namespace DefaultVisibility="public" Schema="http://linqtoxsd.schemas.org/test/nil-test.xsd" Clr="LinqToXsd.Schemas.Test.NilTest" />
+    </Namespaces>
+    <Validation>
+        <VerifyRequired>false</VerifyRequired>
+    </Validation>
+    <Transformation>
+        <Deanonymize strict="false" />
+    </Transformation>
+</Configuration>