Fix XML Encoding of Null Variant to conform to spec (#3062)

* fix XML Encoding of Null Variant to conform to spec

* fix XmlEncodedValue for null variants values to be the null (same behaviour as ExtensionObject)

* Update Encoder to Produce <uax:Value xsi:nil=\"true\" /> output

Author: romanett

Stack/Opc.Ua.Core/Types/BuiltIn/Variant.cs

@@ -778,6 +778,12 @@ private XmlElement XmlEncodedValue
         {
             get
             {
+                // check for null.
+                if (m_value == null)
+                {
+                    return null;
+                }
+
                 // create encoder.
                 using (XmlEncoder encoder = new XmlEncoder(MessageContextExtension.CurrentContext))
                 {

Stack/Opc.Ua.Core/Types/Encoders/XmlEncoder.cs

@@ -781,9 +781,9 @@ public void WriteVariant(string fieldName, Variant value)
                 {
                     PushNamespace(Namespaces.OpcUaXsd);
 
-                    m_writer.WriteStartElement("Value", Namespaces.OpcUaXsd);
-                    WriteVariantContents(value.Value, value.TypeInfo);
-                    m_writer.WriteEndElement();
+                        m_writer.WriteStartElement("Value", Namespaces.OpcUaXsd);
+                        WriteVariantContents(value.Value, value.TypeInfo);
+                        m_writer.WriteEndElement();
 
                     PopNamespace();
 
@@ -1800,8 +1800,7 @@ public void WriteVariantContents(object value, TypeInfo typeInfo)
             // check for null.
             if (value == null)
             {
-                m_writer.WriteStartElement("Null", Namespaces.OpcUaXsd);
-                m_writer.WriteEndElement();
+                m_writer.WriteAttributeString("nil", Namespaces.XmlSchemaInstance, "true");
                 return;
             }
 

Tests/Opc.Ua.Core.Tests/Types/Encoders/XmlEncoderTests.cs

@@ -176,6 +176,45 @@ public void EncodeDecodeVariantMatrix()
             // Check decode result against input value
             Assert.AreEqual(actualVariant, variant);
         }
+
+        // <summary>
+        /// Validate the encoding and decoding of the a variant that contains a null value
+        /// </summary>
+        [Test]
+        public void EncodeDecodeVariantNul()
+        {
+            
+            var variant = Variant.Null;
+
+            string expected = "<?xml version=\"1.0\" encoding=\"utf-16\"?>\r\n<uax:VariantTest xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:uax=\"http://opcfoundation.org/UA/2008/02/Types.xsd\">\r\n  <uax:Test>\r\n    <uax:Value xsi:nil=\"true\" />\r\n  </uax:Test>\r\n</uax:VariantTest>";
+
+            // Encode
+            var context = new ServiceMessageContext();
+            string actualXmlValue;
+            using (IEncoder xmlEncoder = new XmlEncoder(new XmlQualifiedName("VariantTest", Namespaces.OpcUaXsd), null, context))
+            {
+                xmlEncoder.PushNamespace(Namespaces.OpcUaXsd);
+                xmlEncoder.WriteVariant("Test", variant);
+                xmlEncoder.PopNamespace();
+                actualXmlValue = xmlEncoder.CloseAndReturnText();
+            }
+
+            // Check encode result against expected XML value
+            Assert.AreEqual(expected.Replace("\r", "").Replace("\n", ""), actualXmlValue.Replace("\r", "").Replace("\n", ""));
+
+            // Decode
+            Variant actualVariant;
+            using (XmlReader reader = XmlReader.Create(new StringReader(actualXmlValue)))
+            {
+                using (IDecoder xmlDecoder = new XmlDecoder(null, reader, context))
+                {
+                    actualVariant = xmlDecoder.ReadVariant("Test");
+                }
+            }
+
+            // Check decode result against input value
+            Assert.AreEqual(actualVariant, variant);
+        }
         #endregion
     }
 }