perf(csharp): pre-write internalize provider reference map

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: live1206

.chronus/changes/mtg-hybrid-reference-map-prewrite-internalization-2026-06-29-0759.md

@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/http-client-csharp"
+---
+
+Improve C# generator post-processing performance by using provider-based reference-map construction with pre-write internalization.

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/CSharpGen.cs

@@ -37,6 +37,7 @@ public async Task ExecuteAsync()
         {
             CodeModelGenerator.Instance.Emitter.Info("Starting code generation");
             CodeModelGenerator.Instance.Stopwatch.Start();
+            ProviderReferenceMapAnalyzer.ResetPreWriteAccessibility();
 
             var outputPath = CodeModelGenerator.Instance.Configuration.OutputDirectory;
             var generatedSourceOutputPath = CodeModelGenerator.Instance.Configuration.ProjectGeneratedDirectory;
@@ -100,7 +101,12 @@ await GeneratedCodeWorkspace.LoadBaselineContract(),
             {
                 // Ensure back-compatibility processing is done after all visitors have run
                 outputType.ProcessTypeForBackCompatibility();
+            }
+
+            generatedCodeWorkspace.ApplyPreWriteAccessibility(output.TypeProviders);
 
+            foreach (var outputType in output.TypeProviders)
+            {
                 var writer = CodeModelGenerator.Instance.GetWriter(outputType);
                 generateFilesTasks.Add(generatedCodeWorkspace.AddGeneratedFile(writer.Write()));
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/PostProcessing/GeneratedCodeWorkspace.cs

@@ -89,6 +89,11 @@ internal void AnalyzeProviderReferenceMap(IReadOnlyList<TypeProvider> providers)
             ProviderReferenceMapAnalyzer.Analyze(providers, _project);
         }
 
+        internal void ApplyPreWriteAccessibility(IReadOnlyList<TypeProvider> providers)
+        {
+            ProviderReferenceMapAnalyzer.ApplyPreWriteAccessibility(providers, _project);
+        }
+
         private async Task UpdateProject(Document document)
         {
             var root = await document.GetSyntaxRootAsync();
@@ -284,10 +289,16 @@ public async Task PostProcessAsync()
                 case Configuration.UnreferencedTypesHandlingOption.KeepAll:
                     break;
                 case Configuration.UnreferencedTypesHandlingOption.Internalize:
-                    _project = await postProcessor.InternalizeAsync(_project);
+                    if (!ProviderReferenceMapAnalyzer.PreWriteAccessibilityApplied)
+                    {
+                        _project = await postProcessor.InternalizeAsync(_project);
+                    }
                     break;
                 case Configuration.UnreferencedTypesHandlingOption.RemoveOrInternalize:
-                    _project = await postProcessor.InternalizeAsync(_project);
+                    if (!ProviderReferenceMapAnalyzer.PreWriteAccessibilityApplied)
+                    {
+                        _project = await postProcessor.InternalizeAsync(_project);
+                    }
                     _project = await postProcessor.RemoveAsync(_project);
                     break;
             }