Fix NullReferenceException during ValueSet serialization in R4/R4B

src/Hl7.Fhir.Shims.Base/Specification/Terminology/ValueSetExpander.cs

@@ -383,7 +383,7 @@ internal static ValueSet.ContainsComponent ToContainsComponent(this CodeSystem.C
             if (inactiveProperty?.Value is FhirBoolean isInactive)
                 newContains.Inactive = isInactive.Value;
 
-#if !STU3
+#if !(STU3 || R4)
             if (source.Property.Any())
             {
                 newContains.Property = source.Property.Select(p => new ValueSet.ConceptPropertyComponent { Code = p.Code, Value = p.Value }).ToList();

src/Hl7.Fhir.Specification.Shared.Tests/Source/TerminologyTests.cs

@@ -1,6 +1,7 @@
 using FluentAssertions;
 using Hl7.Fhir.Model;
 using Hl7.Fhir.Rest;
+using Hl7.Fhir.Serialization;
 using Hl7.Fhir.Specification.Source;
 using Hl7.Fhir.Specification.Terminology;
 using Hl7.Fhir.Validation;
@@ -762,6 +763,54 @@ void expandAction(string url)
             }
         }
 
+        /// <summary>
+        /// Test for issue with ValueSet expansion causing NullReferenceException during serialization 
+        /// when the expansion contains property field that doesn't exist in R4/R4B.
+        /// The Property field in ValueSet.expansion.contains was introduced in R5.
+        /// Uses item-type ValueSet which has properties in R5.
+        /// </summary>
+        [Fact]
+        public async Tasks.Task ExpandedValueSetShouldSerializeSuccessfully()
+        {
+            var server = new LocalTerminologyService(_resolver);
+            var parameters = new Parameters()
+            {
+                Parameter = new List<Parameters.ParameterComponent>
+                {
+                    new Parameters.ParameterComponent()
+                    {
+                        Name = "url",
+                        Value = new FhirUri("http://hl7.org/fhir/ValueSet/item-type"),
+                    },
+                },
+            };
+
+            var resource = await server.Expand(parameters);
+
+            // This should not throw a NullReferenceException
+            var json = resource.ToJson();
+
+            // Verify the expansion was successful
+            Assert.NotNull(json);
+            Assert.NotEmpty(json);
+            var valueSet = resource as ValueSet;
+            Assert.NotNull(valueSet);
+            Assert.True(valueSet.HasExpansion);
+            Assert.True(valueSet.Expansion.Contains.Any());
+
+#if R5 || R6
+            // In R5 and R6, verify that the Property element is correctly set in the expansion
+            // when the CodeSystem has properties defined. The item-type CodeSystem has the
+            // "notSelectable" property defined for some concepts (e.g., "question").
+            var containsWithProperties = valueSet.Expansion.Contains
+                .Where(c => c.Property != null && c.Property.Any())
+                .ToList();
+
+            // The item-type ValueSet should have at least one concept with properties in R5+
+            Assert.NotEmpty(containsWithProperties);
+#endif
+        }
+
         [Fact]
         public void TestValidateCodeParametersCode()
         {