fix: keep nil values during (de)serialization

backend/src/DataModeling/Converter/Csharp/JsonMetadataToCsharpConverter.cs

@@ -182,7 +182,7 @@ private void ParseFieldProperty(ElementMetadata element, StringBuilder classBuil
             else
             {
                 elementOrder += 1;
-                AddXmlElementAnnotation(element, classBuilder, elementOrder);
+                AddXmlElementAnnotation(element, classBuilder, elementOrder, !isValueType && (element.Nillable ?? false));
 
                 // Temporary fix - as long as we use System.Text.Json for serialization and  Newtonsoft.Json for
                 // deserialization, we need both JsonProperty and JsonPropertyName annotations.
@@ -265,16 +265,21 @@ private void ParseGroupProperty(ElementMetadata element, StringBuilder classBuil
             }
         }
 
-        private void AddXmlElementAnnotation(ElementMetadata element, StringBuilder classBuilder, int elementOrder)
+        private void AddXmlElementAnnotation(ElementMetadata element, StringBuilder classBuilder, int elementOrder, bool addNillableAttribute = false)
         {
-            if (element.OrderOblivious)
+            string additionalAttributeParams = string.Empty;
+            if (!element.OrderOblivious)
             {
-                classBuilder.AppendLine($"""{Indent(2)}[XmlElement("{element.XName}")]""");
+                additionalAttributeParams += $", Order = {elementOrder}";
             }
-            else
+
+            if (addNillableAttribute)
             {
-                classBuilder.AppendLine($"""{Indent(2)}[XmlElement("{element.XName}", Order = {elementOrder})]""");
+                additionalAttributeParams += ", IsNullable = true";
             }
+
+
+            classBuilder.AppendLine($"""{Indent(2)}[XmlElement("{element.XName}"{additionalAttributeParams})]""");
         }
 
         private void AddShouldSerializeForTagContent(ElementMetadata element, StringBuilder classBuilder, ModelMetadata modelMetadata)

backend/tests/DataModeling.Tests/CsharpEnd2EndGenerationTests.cs

@@ -76,7 +76,7 @@ public void JsonSchemaWithStringFieldInUriFormatShouldConvertToCSharp(string jso
                 .Then.CompiledAssembly.Should().NotBeNull();
         }
 
-        private void GeneratedClassesShouldBeEquivalentToExpected(string expectedCsharpClassPath)
+        private void GeneratedClassesShouldBeEquivalentToExpected(string expectedCsharpClassPath, bool overwriteExpected = false)
         {
             string expectedClasses = SharedResourcesHelper.LoadTestDataAsString(expectedCsharpClassPath);
 
@@ -86,7 +86,10 @@ private void GeneratedClassesShouldBeEquivalentToExpected(string expectedCsharpC
             _testOutput.WriteLine(CSharpClasses);
 
             // Save the current generated classes to the expected file so they can be compared with git diff.
-            SharedResourcesHelper.WriteUpdatedTestData(expectedCsharpClassPath, CSharpClasses);
+            if (overwriteExpected)
+            {
+                SharedResourcesHelper.WriteUpdatedTestData(expectedCsharpClassPath, CSharpClasses);
+            }
 
             var expectedAssembly = Compiler.CompileToAssembly(expectedClasses);
 

backend/tests/DataModeling.Tests/Utils/SerializationHelper.cs

@@ -2,6 +2,7 @@
 using System.IO;
 using System.Text;
 using System.Xml.Serialization;
+using SharedResources.Tests;
 
 namespace DataModeling.Tests.Utils
 {
@@ -20,7 +21,7 @@ public static object Deserialize(string xml, Type type)
         public static string SerializeXml(object o)
         {
             var xmlSerializer = new XmlSerializer(o.GetType());
-            using var textWriter = new StringWriter();
+            using var textWriter = new Utf8StringWriter();
             xmlSerializer.Serialize(textWriter, o);
             return textWriter.ToString();
         }

backend/tests/DataModeling.Tests/XmlDeserializeSerializeTests.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Linq;
+using System.Xml.Linq;
+using System.Xml.Serialization;
+using DataModeling.Tests.BaseClasses;
+using DataModeling.Tests.Utils;
+using FluentAssertions;
+using SharedResources.Tests;
+using Xunit;
+
+namespace DataModeling.Tests;
+
+public class XmlDeserializeSerializeTests : CsharpModelConversionTestsBase<XmlDeserializeSerializeTests>
+{
+    [Theory]
+    [InlineData("Model/XmlSchema/XsAll/xsall-example.xsd", "Model/Xml/XsAll/xsall-example-nillable-sample.xml")]
+    public void XmlDeserializeSerializeShouldKeepNillValue(string xsdSchemaPath, string xmlPath)
+    {
+        Given.That.XsdSchemaLoaded(xsdSchemaPath)
+            .When.LoadedXsdSchemaConvertedToJsonSchema()
+            .And.ConvertedJsonSchemaConvertedToModelMetadata()
+            .And.ModelMetadataConvertedToCsharpClass()
+            .And.CSharpClassesCompiledToAssembly()
+            .Then.CompiledAssembly.Should().NotBeNull();
+
+        And.DeserializeAndSerializeShouldProduceSameXml(xmlPath);
+    }
+
+    private void DeserializeAndSerializeShouldProduceSameXml(string xmlPath)
+    {
+        Type csharpType = CompiledAssembly.Types().Single(type => type.CustomAttributes.Any(att => att.AttributeType == typeof(XmlRootAttribute)));
+
+        string loadedXml = SharedResourcesHelper.LoadTestDataAsString(xmlPath);
+
+        // Deserialize xml to new object of type csharpType
+        object deserializedObject = SerializationHelper.Deserialize(loadedXml, csharpType);
+
+        // serialize xml back to string
+        string serializedXml = SerializationHelper.SerializeXml(deserializedObject);
+
+        // Compare the original xml with the serialized xml
+        var expected = XDocument.Parse(loadedXml);
+        var result = XDocument.Parse(serializedXml);
+        Assert.True(XNode.DeepEquals(expected, result));
+    }
+}

testdata/Model/CSharp/Gitea/Kursdomene_APINøkkel_M_2020-05-26_5702_34556_SERES.cs

@@ -63,12 +63,12 @@ public class Applikasjon
     [JsonPropertyName("beskrivelse")]
     public string beskrivelse { get; set; }
 
-    [XmlElement("miljoe", Order = 6)]
+    [XmlElement("miljoe", Order = 6, IsNullable = true)]
     [JsonProperty("miljoe")]
     [JsonPropertyName("miljoe")]
     public string miljoe { get; set; }
 
-    [XmlElement("nettleserApplikasjonWebadresse", Order = 7)]
+    [XmlElement("nettleserApplikasjonWebadresse", Order = 7, IsNullable = true)]
     [JsonProperty("nettleserApplikasjonWebadresse")]
     [JsonPropertyName("nettleserApplikasjonWebadresse")]
     public string nettleserApplikasjonWebadresse { get; set; }
@@ -77,32 +77,32 @@ public class Applikasjon
 
   public class APIRessurs
   {
-    [XmlElement("profile", Order = 1)]
+    [XmlElement("profile", Order = 1, IsNullable = true)]
     [JsonProperty("profile")]
     [JsonPropertyName("profile")]
     public string profile { get; set; }
 
-    [XmlElement("organizationReportee", Order = 2)]
+    [XmlElement("organizationReportee", Order = 2, IsNullable = true)]
     [JsonProperty("organizationReportee")]
     [JsonPropertyName("organizationReportee")]
     public string organizationReportee { get; set; }
 
-    [XmlElement("meldingsboks", Order = 3)]
+    [XmlElement("meldingsboks", Order = 3, IsNullable = true)]
     [JsonProperty("meldingsboks")]
     [JsonPropertyName("meldingsboks")]
     public string meldingsboks { get; set; }
 
-    [XmlElement("lookup", Order = 4)]
+    [XmlElement("lookup", Order = 4, IsNullable = true)]
     [JsonProperty("lookup")]
     [JsonPropertyName("lookup")]
     public string lookup { get; set; }
 
-    [XmlElement("broker", Order = 5)]
+    [XmlElement("broker", Order = 5, IsNullable = true)]
     [JsonProperty("broker")]
     [JsonPropertyName("broker")]
     public string broker { get; set; }
 
-    [XmlElement("autorisasjon", Order = 6)]
+    [XmlElement("autorisasjon", Order = 6, IsNullable = true)]
     [JsonProperty("autorisasjon")]
     [JsonPropertyName("autorisasjon")]
     public string autorisasjon { get; set; }

testdata/Model/CSharp/Gitea/Kursdomene_BliTjenesteeier_M_2020-05-25_5703_34553_SERES.cs

@@ -59,12 +59,12 @@ public class Tjenesteeier
     [JsonPropertyName("sektor")]
     public string sektor { get; set; }
 
-    [XmlElement("navnNynorsk", Order = 4)]
+    [XmlElement("navnNynorsk", Order = 4, IsNullable = true)]
     [JsonProperty("navnNynorsk")]
     [JsonPropertyName("navnNynorsk")]
     public string navnNynorsk { get; set; }
 
-    [XmlElement("navnEngelsk", Order = 5)]
+    [XmlElement("navnEngelsk", Order = 5, IsNullable = true)]
     [JsonProperty("navnEngelsk")]
     [JsonPropertyName("navnEngelsk")]
     public string navnEngelsk { get; set; }

testdata/Model/CSharp/Gitea/dat-skjema.cs

@@ -207,7 +207,7 @@ public class InnholdSkjema
     public bool? BekreftRiktig { get; set; }
 
     [RegularExpression(@"^[0-9]{4}-(0[1-9]|1[0-2])-(0[1-9]|[1-2][0-9]|3[0-1])$")]
-    [XmlElement("AnsatteInnen", Order = 4)]
+    [XmlElement("AnsatteInnen", Order = 4, IsNullable = true)]
     [JsonProperty("AnsatteInnen")]
     [JsonPropertyName("AnsatteInnen")]
     public string AnsatteInnen { get; set; }

testdata/Model/CSharp/Gitea/nbib-melding.cs

@@ -12,32 +12,32 @@ namespace Altinn.App.Models
   [XmlRoot(ElementName="Message")]
   public class MessageV1
   {
-    [XmlElement("ProcessTask", Order = 1)]
+    [XmlElement("ProcessTask", Order = 1, IsNullable = true)]
     [JsonProperty("ProcessTask")]
     [JsonPropertyName("ProcessTask")]
     public string ProcessTask { get; set; }
 
-    [XmlElement("ServiceName", Order = 2)]
+    [XmlElement("ServiceName", Order = 2, IsNullable = true)]
     [JsonProperty("ServiceName")]
     [JsonPropertyName("ServiceName")]
     public string ServiceName { get; set; }
 
-    [XmlElement("Title", Order = 3)]
+    [XmlElement("Title", Order = 3, IsNullable = true)]
     [JsonProperty("Title")]
     [JsonPropertyName("Title")]
     public string Title { get; set; }
 
-    [XmlElement("Body", Order = 4)]
+    [XmlElement("Body", Order = 4, IsNullable = true)]
     [JsonProperty("Body")]
     [JsonPropertyName("Body")]
     public string Body { get; set; }
 
-    [XmlElement("Reference", Order = 5)]
+    [XmlElement("Reference", Order = 5, IsNullable = true)]
     [JsonProperty("Reference")]
     [JsonPropertyName("Reference")]
     public string Reference { get; set; }
 
-    [XmlElement("Sender", Order = 6)]
+    [XmlElement("Sender", Order = 6, IsNullable = true)]
     [JsonProperty("Sender")]
     [JsonPropertyName("Sender")]
     public string Sender { get; set; }

testdata/Model/CSharp/Gitea/skjema.cs

@@ -331,7 +331,7 @@ public class InnholdSkjema
     public string SamfunnskritiskBransje { get; set; }
 
     [RegularExpression(@"^[0-9]{4}-(0[1-9]|1[0-2])-(0[1-9]|[1-2][0-9]|3[0-1])$")]
-    [XmlElement("BenyttesFraDato", Order = 15)]
+    [XmlElement("BenyttesFraDato", Order = 15, IsNullable = true)]
     [JsonProperty("BenyttesFraDato")]
     [JsonPropertyName("BenyttesFraDato")]
     public string BenyttesFraDato { get; set; }