always add default property (Azure#49447)

Author: m-nash

sdk/core/System.ClientModel/gen/ModelReaderWriterContextGenerator.Emitter.cs

@@ -83,36 +83,33 @@ private void EmitContextClass(ModelReaderWriterContextGenerationSpec contextGene
                 builder.AppendLine();
             }
 
-            if (contextGenerationSpec.TypeBuilders.Count > 0)
-            {
-                builder.AppendLine(indent, $"private static {contextName} _{contextName.ToCamelCase()};");
-                builder.AppendLine(indent, "/// <summary> Gets the default instance </summary>");
-                builder.AppendLine(indent, $"public static {contextName} Default => _{contextName.ToCamelCase()} ??= new();");
-                builder.AppendLine();
+            builder.AppendLine(indent, $"private static {contextName} _{contextName.ToCamelCase()};");
+            builder.AppendLine(indent, "/// <summary> Gets the default instance </summary>");
+            builder.AppendLine(indent, $"public static {contextName} Default => _{contextName.ToCamelCase()} ??= new();");
+            builder.AppendLine();
 
-                builder.AppendLine(indent, $"private {contextName}()");
-                builder.AppendLine(indent, "{");
-                indent++;
-                foreach (var modelInfo in contextGenerationSpec.TypeBuilders)
+            builder.AppendLine(indent, $"private {contextName}()");
+            builder.AppendLine(indent, "{");
+            indent++;
+            foreach (var modelInfo in contextGenerationSpec.TypeBuilders)
+            {
+                builder.Append(indent, $"_typeBuilderFactories.Add(typeof({modelInfo.Type.FullyQualifiedName}), () => ");
+                if (ShouldGenerateAsLocal(contextGenerationSpec, modelInfo))
                 {
-                    builder.Append(indent, $"_typeBuilderFactories.Add(typeof({modelInfo.Type.FullyQualifiedName}), () => ");
-                    if (ShouldGenerateAsLocal(contextGenerationSpec, modelInfo))
-                    {
-                        builder.AppendLine($" new global::{modelInfo.Type.GetInnerItemType().Namespace}.{modelInfo.Type.TypeCaseName}Builder());");
-                    }
-                    else
-                    {
-                        builder.AppendLine($" s_referenceContexts[typeof({modelInfo.ContextType.FullyQualifiedName})].GetTypeBuilder(typeof({modelInfo.Type.FullyQualifiedName})));");
-                    }
+                    builder.AppendLine($" new global::{modelInfo.Type.GetInnerItemType().Namespace}.{modelInfo.Type.TypeCaseName}Builder());");
                 }
-                builder.AppendLine();
+                else
+                {
+                    builder.AppendLine($" s_referenceContexts[typeof({modelInfo.ContextType.FullyQualifiedName})].GetTypeBuilder(typeof({modelInfo.Type.FullyQualifiedName})));");
+                }
+            }
+            builder.AppendLine();
 
-                builder.AppendLine(indent, "AddAdditionalFactories(_typeBuilderFactories);");
+            builder.AppendLine(indent, "AddAdditionalFactories(_typeBuilderFactories);");
 
-                indent--;
-                builder.AppendLine(indent, "}");
-                builder.AppendLine();
-            }
+            indent--;
+            builder.AppendLine(indent, "}");
+            builder.AppendLine();
 
             builder.AppendLine(indent, "/// <inheritdoc/>");
             builder.Append(indent, "protected override bool TryGetTypeBuilderCore(");

sdk/core/System.ClientModel/tests/gen.unit/CompilationHelper.cs

@@ -95,6 +95,9 @@ public static Compilation CreateCompilation(
         }
 
         public static GeneratorResult RunSourceGenerator(Compilation compilation, bool disableDiagnosticValidation = false)
+            => RunSourceGenerator(compilation, out _, disableDiagnosticValidation);
+
+        public static GeneratorResult RunSourceGenerator(Compilation compilation, out Compilation newCompilation, bool disableDiagnosticValidation = false)
         {
             ModelReaderWriterContextGenerationSpec? generatedSpecs = null;
             var generator = new ModelReaderWriterContextGenerator
@@ -103,7 +106,7 @@ public static GeneratorResult RunSourceGenerator(Compilation compilation, bool d
             };
 
             CSharpGeneratorDriver driver = CreateJsonSourceGeneratorDriver(compilation, generator);
-            driver.RunGeneratorsAndUpdateCompilation(compilation, out Compilation outCompilation, out ImmutableArray<Diagnostic> diagnostics);
+            driver.RunGeneratorsAndUpdateCompilation(compilation, out newCompilation, out ImmutableArray<Diagnostic> diagnostics);
 
             return new()
             {

sdk/core/System.ClientModel/tests/gen.unit/ContextGeneratorTests.cs

@@ -653,5 +653,46 @@ public void Write(Utf8JsonWriter writer, ModelReaderWriterOptions options)
             Assert.AreEqual(1, result.Diagnostics.Length);
             Assert.AreEqual(ModelReaderWriterContextGenerator.DiagnosticDescriptors.BuildableAttributeRequiresContext.Id, result.Diagnostics[0].Id);
         }
+
+        [Test]
+        public void ValidateDefaultExistsWithNoBuilders()
+        {
+            string source =
+$$"""
+using System;
+using System.ClientModel.Primitives;
+using System.Collections.Generic;
+using System.Text.Json;
+
+namespace TestProject
+{
+    public partial class LocalContext : ModelReaderWriterContext { }
+}
+""";
+
+            Compilation compilation = CompilationHelper.CreateCompilation(source);
+            var result = CompilationHelper.RunSourceGenerator(compilation, out var newCompilation);
+
+            Assert.IsNotNull(result.GenerationSpec);
+            Assert.AreEqual("LocalContext", result.GenerationSpec!.Type.Name);
+            Assert.AreEqual("TestProject", result.GenerationSpec.Type.Namespace);
+            Assert.AreEqual(0, result.GenerationSpec.ReferencedContexts.Count);
+            Assert.AreEqual(0, result.GenerationSpec.TypeBuilders.Count);
+            Assert.AreEqual(0, result.Diagnostics.Length);
+
+            var localContextSymbol = newCompilation.GetTypeByMetadataName("TestProject.LocalContext");
+            Assert.IsNotNull(localContextSymbol);
+            var defaultProp = localContextSymbol!.GetMembers("Default")
+                .OfType<IPropertySymbol>()
+                .FirstOrDefault(p => p.IsStatic);
+
+            Assert.IsNotNull(defaultProp, "Default property should not be null.");
+
+            Assert.IsTrue(SymbolEqualityComparer.Default.Equals(defaultProp!.Type, localContextSymbol));
+
+            Assert.AreEqual(1, localContextSymbol.Constructors.Length);
+            var ctor = localContextSymbol.Constructors[0];
+            Assert.AreEqual(Accessibility.Private, ctor.DeclaredAccessibility);
+        }
     }
 }