Support back-compat model constructors in the C# generator (#11503)

When a previously-required model property is relaxed to optional, the
generator drops the corresponding parameter from the model's
initialization constructor — a source-breaking change for callers that
construct the model positionally (issue #11460). This restores the
previously-published public constructor via the `LastContractView`
back-compat mechanism.

### Changes

- **`ModelProvider.BuildConstructorsForBackCompatibility` (new
override)** — For each public constructor in `LastContractView` that has
no current equivalent, reconstructs it as a public overload that chains
to the closest current public constructor (`: this(...)`) and assigns
the extra properties in its body.
- Chain target is the current public constructor whose parameters form
an in-order subsequence of the previous ones (closest match preferred).
- Each dropped parameter must map to a settable property with an
unchanged type — matched by property name, or by `OriginalName` for
codegen renames. If any dropped parameter cannot be mapped, restoration
is skipped.
- Kept parameters forward to the chained constructor (validation
stripped, since the target validates); non-nullable reference-type
extras reinstate their `AssertNotNull` check.
- **`BackCompatibilityChangeCategory`** — Adds
`ConstructorAddedFromLastContract` and
`ConstructorAddedFromLastContractSkipped` for emitter diagnostics.

### Example

Given a model where `resources` was required and is now optional, the
restored overload:

```csharp
public MockInputModel(string name, string resources) : this(name)
{
    Argument.AssertNotNull(resources, nameof(resources));
    Resources = resources;
}
```

### Tests

Added unit tests covering the restore case (required→optional), and the
negative cases where the property was removed entirely or no last
contract exists.

<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #11460

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jorgerangel-msft <102122018+jorgerangel-msft@users.noreply.github.com>
Co-authored-by: Jorge Rangel <jorgerangel@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: e815ae8d-8fc2-4c43-982e-1e98f84f4f54

Author: 3 people

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs

@@ -125,6 +125,9 @@ public MrwSerializationTypeDefinition(InputModelType inputModel, ModelProvider m
         protected override IReadOnlyList<MethodProvider> BuildMethodsForBackCompatibility(IEnumerable<MethodProvider> originalMethods)
             => [.. originalMethods];
 
+        protected override IReadOnlyList<ConstructorProvider> BuildConstructorsForBackCompatibility(IEnumerable<ConstructorProvider> originalConstructors)
+            => [.. originalConstructors];
+
         private ConstructorProvider SerializationConstructor => _serializationConstructor ??= _model.FullConstructor;
         private PropertyProvider[] AdditionalProperties => _additionalProperties.Value;
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/ScmModelProviderTests.cs

@@ -205,6 +205,75 @@ await MockHelpers.LoadMockGeneratorAsync(
             Assert.AreEqual(Helpers.GetExpectedFromFile(), file.Content);
         }
 
+        [Test]
+        public async Task BackCompat_ParameterlessConstructorRestoredRemovesMockingConstructor()
+        {
+            // The last contract published a parameterless `protected BaseModel()`. The current generation
+            // makes the discriminator required, so the abstract base's initialization constructor now takes a
+            // parameter and the parameterless constructor is dropped. It is restored, and the generated
+            // parameterless mocking constructor on the serialization partial is removed to avoid a duplicate.
+            var derivedInputModel = InputFactory.Model(
+                "derivedModel",
+                discriminatedKind: "one",
+                properties:
+                [
+                    InputFactory.Property("kind", InputPrimitiveType.String, isRequired: true, isDiscriminator: true)
+                ]);
+            var inputModel = InputFactory.Model(
+                "baseModel",
+                properties:
+                [
+                    InputFactory.Property("kind", InputPrimitiveType.String, isRequired: true, isDiscriminator: true)
+                ],
+                discriminatedModels: new Dictionary<string, InputModelType>() { { "one", derivedInputModel } });
+
+            await MockHelpers.LoadMockGeneratorAsync(
+                lastContractCompilation: async () => await Helpers.GetCompilationFromDirectoryAsync(),
+                inputModels: () => [inputModel]);
+
+            var model = ScmCodeModelGenerator.Instance.OutputLibrary.TypeProviders
+                .OfType<ScmModel>().Single(t => t.Name == "BaseModel");
+
+            model.ProcessTypeForBackCompatibility();
+
+            // The model gains the restored standalone parameterless constructor.
+            var modelContent = new TypeProviderWriter(model).Write().Content;
+            Assert.AreEqual(Helpers.GetExpectedFromFile("Model"), modelContent);
+
+            // The serialization partial no longer carries the parameterless mocking constructor (avoids CS0111).
+            var serializationContent = new TypeProviderWriter(model.SerializationProviders.Single()).Write().Content;
+            Assert.AreEqual(Helpers.GetExpectedFromFile("Serialization"), serializationContent);
+        }
+
+        [Test]
+        public async Task BackCompat_StructParameterlessConstructorNotMovedFromSerialization()
+        {
+            // A struct always exposes a public parameterless constructor via its serialization (mocking)
+            // constructor, so the last contract's parameterless constructor is already present. It must not
+            // be moved onto the model partial, which would be pointless churn with no public API change.
+            var inputModel = InputFactory.Model(
+                "structModel",
+                modelAsStruct: true,
+                properties:
+                [
+                    InputFactory.Property("prop", InputPrimitiveType.String, isRequired: true)
+                ]);
+
+            await MockHelpers.LoadMockGeneratorAsync(
+                lastContractCompilation: async () => await Helpers.GetCompilationFromDirectoryAsync(),
+                inputModels: () => [inputModel]);
+
+            var model = ScmCodeModelGenerator.Instance.OutputLibrary.TypeProviders
+                .OfType<ScmModel>().Single(t => t.Name == "StructModel");
+
+            model.ProcessTypeForBackCompatibility();
+
+            Assert.IsFalse(model.Constructors.Any(c => c.Signature.Parameters.Count == 0),
+                "Struct model must not gain a parameterless constructor on the model partial.");
+            Assert.IsTrue(model.SerializationProviders.Single().Constructors.Any(c => c.Signature.Parameters.Count == 0),
+                "Struct serialization partial must retain its parameterless constructor.");
+        }
+
         [Test]
         public void TestDynamicModelWithUnionAdditionalProps()
         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BackCompat_ParameterlessConstructorRestoredRemovesMockingConstructor(Model).cs

@@ -0,0 +1,31 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System;
+using System.Collections.Generic;
+
+namespace Sample.Models
+{
+    public abstract partial class BaseModel
+    {
+        private protected readonly global::System.Collections.Generic.IDictionary<string, global::System.BinaryData> _additionalBinaryDataProperties;
+
+        private protected BaseModel(string kind)
+        {
+            Kind = kind;
+        }
+
+        internal BaseModel(string kind, global::System.Collections.Generic.IDictionary<string, global::System.BinaryData> additionalBinaryDataProperties)
+        {
+            Kind = kind;
+            _additionalBinaryDataProperties = additionalBinaryDataProperties;
+        }
+
+        protected BaseModel() : this(default)
+        {
+        }
+
+        internal string Kind { get; set; }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BackCompat_ParameterlessConstructorRestoredRemovesMockingConstructor(Serialization).cs

@@ -0,0 +1,111 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System;
+using System.ClientModel.Primitives;
+using System.Text.Json;
+using Sample;
+
+namespace Sample.Models
+{
+    [global::System.ClientModel.Primitives.PersistableModelProxyAttribute(typeof(global::Sample.Models.UnknownBaseModel))]
+    public abstract partial class BaseModel : global::System.ClientModel.Primitives.IJsonModel<global::Sample.Models.BaseModel>
+    {
+        protected virtual global::Sample.Models.BaseModel PersistableModelCreateCore(global::System.BinaryData data, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            string format = (options.Format == "W") ? ((global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.BaseModel>)this).GetFormatFromOptions(options) : options.Format;
+            switch (format)
+            {
+                case "J":
+                    using (global::System.Text.Json.JsonDocument document = global::System.Text.Json.JsonDocument.Parse(data, global::Sample.ModelSerializationExtensions.JsonDocumentOptions))
+                    {
+                        return global::Sample.Models.BaseModel.DeserializeBaseModel(document.RootElement, options);
+                    }
+                default:
+                    throw new global::System.FormatException($"The model {nameof(global::Sample.Models.BaseModel)} does not support reading '{options.Format}' format.");
+            }
+        }
+
+        protected virtual global::System.BinaryData PersistableModelWriteCore(global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            string format = (options.Format == "W") ? ((global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.BaseModel>)this).GetFormatFromOptions(options) : options.Format;
+            switch (format)
+            {
+                case "J":
+                    return global::System.ClientModel.Primitives.ModelReaderWriter.Write(this, options, global::Sample.SampleContext.Default);
+                default:
+                    throw new global::System.FormatException($"The model {nameof(global::Sample.Models.BaseModel)} does not support writing '{options.Format}' format.");
+            }
+        }
+
+        global::System.BinaryData global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.BaseModel>.Write(global::System.ClientModel.Primitives.ModelReaderWriterOptions options) => this.PersistableModelWriteCore(options);
+
+        global::Sample.Models.BaseModel global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.BaseModel>.Create(global::System.BinaryData data, global::System.ClientModel.Primitives.ModelReaderWriterOptions options) => this.PersistableModelCreateCore(data, options);
+
+        string global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.BaseModel>.GetFormatFromOptions(global::System.ClientModel.Primitives.ModelReaderWriterOptions options) => "J";
+
+        void global::System.ClientModel.Primitives.IJsonModel<global::Sample.Models.BaseModel>.Write(global::System.Text.Json.Utf8JsonWriter writer, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            writer.WriteStartObject();
+            this.JsonModelWriteCore(writer, options);
+            writer.WriteEndObject();
+        }
+
+        protected virtual void JsonModelWriteCore(global::System.Text.Json.Utf8JsonWriter writer, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            string format = (options.Format == "W") ? ((global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.BaseModel>)this).GetFormatFromOptions(options) : options.Format;
+            if ((format != "J"))
+            {
+                throw new global::System.FormatException($"The model {nameof(global::Sample.Models.BaseModel)} does not support writing '{format}' format.");
+            }
+            writer.WritePropertyName("kind"u8);
+            writer.WriteStringValue(Kind);
+            if (((options.Format != "W") && (_additionalBinaryDataProperties != null)))
+            {
+                foreach (var item in _additionalBinaryDataProperties)
+                {
+                    writer.WritePropertyName(item.Key);
+#if NET6_0_OR_GREATER
+                    writer.WriteRawValue(item.Value);
+#else
+                    using (global::System.Text.Json.JsonDocument document = global::System.Text.Json.JsonDocument.Parse(item.Value))
+                    {
+                        global::System.Text.Json.JsonSerializer.Serialize(writer, document.RootElement);
+                    }
+#endif
+                }
+            }
+        }
+
+        global::Sample.Models.BaseModel global::System.ClientModel.Primitives.IJsonModel<global::Sample.Models.BaseModel>.Create(ref global::System.Text.Json.Utf8JsonReader reader, global::System.ClientModel.Primitives.ModelReaderWriterOptions options) => this.JsonModelCreateCore(ref reader, options);
+
+        protected virtual global::Sample.Models.BaseModel JsonModelCreateCore(ref global::System.Text.Json.Utf8JsonReader reader, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            string format = (options.Format == "W") ? ((global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.BaseModel>)this).GetFormatFromOptions(options) : options.Format;
+            if ((format != "J"))
+            {
+                throw new global::System.FormatException($"The model {nameof(global::Sample.Models.BaseModel)} does not support reading '{format}' format.");
+            }
+            using global::System.Text.Json.JsonDocument document = global::System.Text.Json.JsonDocument.ParseValue(ref reader);
+            return global::Sample.Models.BaseModel.DeserializeBaseModel(document.RootElement, options);
+        }
+
+        internal static global::Sample.Models.BaseModel DeserializeBaseModel(global::System.Text.Json.JsonElement element, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            if ((element.ValueKind == global::System.Text.Json.JsonValueKind.Null))
+            {
+                return null;
+            }
+            if (element.TryGetProperty("kind"u8, out global::System.Text.Json.JsonElement discriminator))
+            {
+                switch (discriminator.GetString())
+                {
+                    case "one":
+                        return global::Sample.Models.DerivedModel.DeserializeDerivedModel(element, options);
+                }
+            }
+            return global::Sample.Models.UnknownBaseModel.DeserializeUnknownBaseModel(element, options);
+        }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BackCompat_ParameterlessConstructorRestoredRemovesMockingConstructor/BaseModel.cs

@@ -0,0 +1,10 @@
+namespace Sample.Models
+{
+    public abstract partial class BaseModel
+    {
+        /// <summary> Initializes a new instance of BaseModel. </summary>
+        protected BaseModel()
+        {
+        }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BackCompat_StructParameterlessConstructorNotMovedFromSerialization/StructModel.cs

@@ -0,0 +1,10 @@
+namespace Sample.Models
+{
+    public partial struct StructModel
+    {
+        /// <summary> Initializes a new instance of StructModel. </summary>
+        public StructModel()
+        {
+        }
+    }
+}

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

@@ -54,5 +54,11 @@ public enum BackCompatibilityChangeCategory
 
         /// <summary>A fixed enum member was re-added to preserve a member that existed in the last contract but is no longer produced by the current spec.</summary>
         EnumMemberAddedFromLastContract,
+
+        /// <summary>A back-compat model constructor was re-added to preserve a public constructor that existed in the last contract but is no longer produced by the current spec.</summary>
+        ConstructorAddedFromLastContract,
+
+        /// <summary>A back-compat model constructor could not be reconstructed from the last contract and was skipped.</summary>
+        ConstructorAddedFromLastContractSkipped,
     }
 }