keep snapshot when differential is missing

src/Hl7.Fhir.Conformance/Specification/Snapshot/SnapshotGenerator.cs

@@ -143,14 +143,15 @@ private static IAsyncResourceResolver verifySource(IAsyncResourceResolver resolv
         /// <param name="structure">A <see cref="StructureDefinition"/> instance.</param>
         public async T.Task UpdateAsync(StructureDefinition structure)
         {
-            structure.Snapshot = new StructureDefinition.SnapshotComponent()
-            {
-                Element = await GenerateAsync(structure).ConfigureAwait(false)
-            };
-            structure.Snapshot.SetCreatedBySnapshotGenerator();
+                var result = await GenerateAsync(structure).ConfigureAwait(false);
+
+                if (result == null && structure.Snapshot?.Element != null) return;
+
+                structure.Snapshot = new StructureDefinition.SnapshotComponent { Element = result };
+                structure.Snapshot.SetCreatedBySnapshotGenerator();
 
-            // [WMR 20170209] TODO: also merge global StructureDefinition.Mapping components
-            // structure.Mappings = ElementDefnMerger.Merge(...)
+                // [WMR 20170209] TODO: also merge global StructureDefinition.Mapping components
+                // structure.Mappings = ElementDefnMerger.Merge(...)
         }
 
         /// <inheritdoc cref="UpdateAsync(StructureDefinition)"/>

src/Hl7.Fhir.Specification.Shared.Tests/Snapshot/SnapshotGeneratorTest.cs

@@ -3193,6 +3193,42 @@ public async T.Task FindComplexTestExtensions()
             }
         };
 
+        private static StructureDefinition LogicalModelWithoutDiff => new()
+        {
+            Name = "MyLogicalModel",
+            Url = "http://example.org/fhir/StructureDefinition/MyLogicalModel",
+            Type = "http://foo.org/bar",
+            Derivation = StructureDefinition.TypeDerivationRule.Constraint,
+            BaseDefinition = "http://hl7.org/fhir/StructureDefinition/Base",
+            Kind = StructureDefinition.StructureDefinitionKind.Logical,
+            Snapshot = new()
+            {
+                Element = new()
+               {
+                   new()
+                   {
+                       ElementId = "logicalmodel",
+                       Path = "logicalmodel",
+                       Short = "root"
+
+                   },
+                    new()
+                   {
+                       ElementId = "logicalmodel.Identifier",
+                       Path = "logicalmodel.Identifier",
+                       Short = "Identifier",
+                       Min = 1,
+                       Max = "*",
+                       Type = new()
+                       {
+                           new() {Code = "Identifier"}
+                       }
+                   },
+               }
+            }
+
+        };
+
         [Conditional("DEBUG")]
         private void dumpElements(IEnumerable<ElementDefinition> elements, string header = null) => dumpElements(elements.ToList(), header);
 
@@ -3295,6 +3331,23 @@ public async T.Task TestMissingDifferential()
             expanded.Snapshot.Element.Dump();
         }
 
+        [TestMethod]
+        public async T.Task TestMissingDifferentialLogicalModel()
+        {
+            // Create a profile without a differential
+            var profile = LogicalModelWithoutDiff;
+
+            var resolver = new InMemoryResourceResolver(profile);
+            var multiResolver = new MultiResolver(_testResolver, resolver);
+            _generator = new SnapshotGenerator(multiResolver, _settings);
+
+            var (_, expanded) = await generateSnapshotAndCompare(profile);
+            Assert.IsNotNull(expanded);
+            Assert.IsTrue(expanded.HasSnapshot);
+
+            expanded.Snapshot.Element.Dump();
+        }
+
         [TestMethod]
         public async T.Task TestUnresolvedBaseProfile()
         {