Fix edit in one project not being reflected in consuming project

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.cs

@@ -173,16 +173,37 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                             return true;
                         }
 
-                        if (oldSymbol is IAssemblySymbol oldAssembly && newSymbol is IAssemblySymbol newAssembly)
+                        if (oldSymbol is not IAssemblySymbol oldAssembly || newSymbol is not IAssemblySymbol newAssembly)
                         {
-                            var oldModuleMVIDs = oldAssembly.Modules.Select(GetMVID);
-                            var newModuleMVIDs = newAssembly.Modules.Select(GetMVID);
-                            return oldModuleMVIDs.SequenceEqual(newModuleMVIDs);
+                            return false;
+                        }
+
+                        // Compare the MVIDs of the modules in each assembly. If they aren't present or don't match we don't consider them equal
+                        var oldModules = oldAssembly.Modules.ToArray();
+                        var newModules = newAssembly.Modules.ToArray();
+                        if (oldModules.Length != newModules.Length)
+                        {
+                            return false;
+                        }
 
-                            static Guid GetMVID(IModuleSymbol m) => m.GetMetadata()?.GetModuleVersionId() ?? Guid.Empty;
+                        for (int i = 0; i < oldModules.Length; i++)
+                        {
+                            var oldMetadata = oldModules[i].GetMetadata();
+                            var newMetadata = newModules[i].GetMetadata();
+
+                            if (oldMetadata is null || newMetadata is null)
+                            {
+                                return false;
+                            }
+
+                            if (oldMetadata.GetModuleVersionId() != newMetadata.GetModuleVersionId())
+                            {
+                                return false;
+                            }
                         }
 
-                        return false;
+                        // All module MVIDs matched.
+                        return true;
                     })))
                     {
                         return false;

src/Compiler/test/Microsoft.NET.Sdk.Razor.SourceGenerators.Tests/RazorSourceGeneratorTests.cs

@@ -3605,5 +3605,85 @@ public async Task IncrementalCompilation_RerunsGenerator_When_AdditionalFileRena
             var newCouNterSource = result.GeneratedSources.FirstOrDefault(s => s.HintName.Contains("NewCouNter"));
             Assert.Contains("public partial class NewCouNter", newCouNterSource.SourceText.ToString());
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/razor/issues/12316")]
+        public async Task RazorClassLibrary_Change_Updates_DependentProject_WhenReferencedAsCompilation()
+        {
+            var rclProject = CreateTestProject(new()
+            {
+                ["LibComponent.razor"] = "<p>Library component</p>",
+            });
+            rclProject = rclProject.WithAssemblyName("RazorClassLibrary");
+
+            var rclCompilation = await rclProject.GetCompilationAsync();
+            var rclDriver = await GetDriverAsync(rclProject);
+            var rclRun = RunGenerator(rclCompilation!, ref rclDriver, out var rclOutputCompilation);
+            Assert.Empty(rclRun.Diagnostics);
+            Assert.Single(rclRun.GeneratedSources); // LibComponent
+
+            // Explicitly use a CompilationReference
+            var rclReference = rclOutputCompilation.ToMetadataReference();
+
+            // Create the main project that references the RCL and uses its component.
+            var mainProject = CreateTestProject(new()
+            {
+                ["Pages/Index.razor"] = "<LibComponent />",
+            });
+            mainProject = mainProject.AddMetadataReference(rclReference);
+
+            var mainCompilation = await mainProject.GetCompilationAsync();
+            var (mainDriver, mainAdditionalTexts) = await GetDriverWithAdditionalTextAsync(mainProject);
+            var mainRun = RunGenerator(mainCompilation!, ref mainDriver);
+            Assert.Empty(mainRun.Diagnostics);
+            Assert.Single(mainRun.GeneratedSources);
+
+            // Rename the component in the RCL: LibComponent -> RenamedComponent
+            rclProject = CreateTestProject(new()
+            {
+                ["RenamedComponent.razor"] = "<p>Library component</p>",
+            }).WithAssemblyName("RazorClassLibrary");
+
+            rclCompilation = await rclProject.GetCompilationAsync()!;
+            rclDriver = await GetDriverAsync(rclProject);
+            rclRun = RunGenerator(rclCompilation!, ref rclDriver, out rclOutputCompilation);
+            Assert.Empty(rclRun.Diagnostics);
+            Assert.Single(rclRun.GeneratedSources); // RenamedComponent
+
+            var rclReference2 = rclOutputCompilation.ToMetadataReference();
+
+            // Update main project to point to the new reference (with renamed component).
+            mainProject = mainProject.RemoveMetadataReference(rclReference)
+                                     .AddMetadataReference(rclReference2);
+            mainCompilation = await mainProject.GetCompilationAsync();
+
+            // Re-run generator: expect missing component diagnostic (RZ10012).
+            mainRun = RunGenerator(mainCompilation!, ref mainDriver);
+            var missing = Assert.Single(mainRun.Diagnostics);
+            Assert.Equal("RZ10012", missing.Id);
+
+            // Update main project's Index.razor to use the renamed component.
+            var updatedIndex = new TestAdditionalText("Pages/Index.razor", SourceText.From("<RenamedComponent />", Encoding.UTF8));
+            mainDriver = mainDriver.ReplaceAdditionalText(
+                mainAdditionalTexts.First(t => t.Path.EndsWith("Index.razor", StringComparison.OrdinalIgnoreCase)),
+                updatedIndex);
+
+            // Re-run generator: should compile cleanly again.
+            mainRun = RunGenerator(mainCompilation!, ref mainDriver);
+            Assert.Empty(mainRun.Diagnostics);
+            Assert.Single(mainRun.GeneratedSources);
+
+            // Update the compilation, which will cause us to re-run. 
+            RazorEventListener eventListener = new RazorEventListener();
+
+            mainCompilation = mainCompilation!.WithOptions(mainCompilation.Options.WithModuleName("newMain"));
+            mainRun = RunGenerator(mainCompilation!, ref mainDriver);
+            Assert.Empty(mainRun.Diagnostics);
+            Assert.Single(mainRun.GeneratedSources);
+
+            // Confirm that the tag helpers from metadata refs _didn't_ re-run
+            Assert.Collection(eventListener.Events,
+                     e => Assert.Equal("DiscoverTagHelpersFromCompilationStart", e.EventName),
+                     e => Assert.Equal("DiscoverTagHelpersFromCompilationStop", e.EventName));
+        }
     }
 }