Add External property to InputType to preserve original type information

packages/http-client-csharp/emitter/src/lib/type-converter.ts

@@ -29,7 +29,7 @@ import {
   InputDurationType,
   InputEnumType,
   InputEnumValueType,
-  InputExternalType,
+  InputExternalTypeMetadata,
   InputLiteralType,
   InputModelProperty,
   InputModelType,
@@ -81,19 +81,13 @@ export function fromSdkType<T extends SdkType>(
     return retVar as any;
   }
 
-  // Check if this type references an external type
-  if ((sdkType as any).external) {
-    retVar = fromSdkExternalType(sdkContext, sdkType);
-    sdkContext.__typeCache.updateSdkTypeReferences(sdkType, retVar);
-    return retVar as any;
-  }
-
   switch (sdkType.kind) {
     case "nullable":
       const nullableType: InputNullableType = {
         kind: "nullable",
         type: fromSdkType(sdkContext, sdkType.type, sdkProperty, namespace),
         namespace: sdkType.namespace,
+        external: fromSdkExternalTypeInfo(sdkType),
       };
       retVar = nullableType;
       break;
@@ -146,6 +140,7 @@ export function fromSdkType<T extends SdkType>(
         name: "tuple",
         crossLanguageDefinitionId: "",
         decorators: sdkType.decorators,
+        external: fromSdkExternalTypeInfo(sdkType),
       };
       retVar = tupleType;
       break;
@@ -165,6 +160,7 @@ export function fromSdkType<T extends SdkType>(
         name: "credential",
         crossLanguageDefinitionId: "",
         decorators: sdkType.decorators,
+        external: fromSdkExternalTypeInfo(sdkType),
       };
       retVar = credentialType;
       break;
@@ -200,6 +196,7 @@ function fromSdkModelType(
     summary: modelType.summary,
     discriminatorValue: modelType.discriminatorValue,
     decorators: decorators,
+    external: fromSdkExternalTypeInfo(modelType),
   } as InputModelType;
 
   sdkContext.__typeCache.updateSdkTypeReferences(modelType, inputModelType);
@@ -313,6 +310,7 @@ function createEnumType(
     // constantType.usage, TODO - constant type now does not have usage. TCGC will add it later
     usage: sdkType.kind === "enum" ? sdkType.usage : UsageFlags.None,
     decorators: sdkType.decorators,
+    external: fromSdkExternalTypeInfo(sdkType),
   };
 
   sdkContext.__typeCache.updateSdkTypeReferences(sdkType, inputEnumType);
@@ -340,6 +338,7 @@ function fromSdkDateTimeType(
     crossLanguageDefinitionId: dateTimeType.crossLanguageDefinitionId,
     baseType: dateTimeType.baseType ? fromSdkType(sdkContext, dateTimeType.baseType) : undefined,
     decorators: dateTimeType.decorators,
+    external: fromSdkExternalTypeInfo(dateTimeType),
   };
 }
 
@@ -355,6 +354,7 @@ function fromSdkDurationType(
     crossLanguageDefinitionId: durationType.crossLanguageDefinitionId,
     baseType: durationType.baseType ? fromSdkType(sdkContext, durationType.baseType) : undefined,
     decorators: durationType.decorators,
+    external: fromSdkExternalTypeInfo(durationType),
   };
 }
 
@@ -369,6 +369,7 @@ function fromSdkBuiltInType(
     crossLanguageDefinitionId: builtInType.crossLanguageDefinitionId,
     baseType: builtInType.baseType ? fromSdkType(sdkContext, builtInType.baseType) : undefined,
     decorators: builtInType.decorators,
+    external: fromSdkExternalTypeInfo(builtInType),
   };
 }
 
@@ -385,6 +386,7 @@ function fromUnionType(sdkContext: CSharpEmitterContext, union: SdkUnionType): I
     variantTypes: variantTypes,
     namespace: union.namespace,
     decorators: union.decorators,
+    external: fromSdkExternalTypeInfo(union),
   };
 }
 
@@ -447,6 +449,7 @@ function fromSdkDictionaryType(
     keyType: fromSdkType(sdkContext, dictionaryType.keyType),
     valueType: fromSdkType(sdkContext, dictionaryType.valueType),
     decorators: dictionaryType.decorators,
+    external: fromSdkExternalTypeInfo(dictionaryType),
   };
 }
 
@@ -460,6 +463,7 @@ function fromSdkArrayType(
     valueType: fromSdkType(sdkContext, arrayType.valueType),
     crossLanguageDefinitionId: arrayType.crossLanguageDefinitionId,
     decorators: arrayType.decorators,
+    external: fromSdkExternalTypeInfo(arrayType),
   };
 }
 
@@ -471,20 +475,6 @@ function fromSdkEndpointType(): InputPrimitiveType {
   };
 }
 
-function fromSdkExternalType(
-  sdkContext: CSharpEmitterContext,
-  sdkType: SdkType,
-): InputExternalType {
-  const external = (sdkType as any).external;
-  return {
-    kind: "external",
-    identity: external.identity,
-    package: external.package,
-    minVersion: external.minVersion,
-    decorators: sdkType.decorators,
-  };
-}
-
 /**
  * @beta
  */
@@ -510,3 +500,21 @@ export function getAllModelDecorators(
 
   return Array.from(decoratorMap.values());
 }
+
+/**
+ * Converts TCGC external type information to InputExternalTypeMetadata
+ * @param sdkType - The SDK type that may have external type information
+ * @returns InputExternalTypeMetadata if the type has external info, undefined otherwise
+ */
+function fromSdkExternalTypeInfo(sdkType: SdkType): InputExternalTypeMetadata | undefined {
+  const external = (sdkType as any).external;
+  if (!external) {
+    return undefined;
+  }
+
+  return {
+    identity: external.identity,
+    package: external.package,
+    minVersion: external.minVersion,
+  };
+}

packages/http-client-csharp/emitter/src/type/input-type.ts

@@ -16,6 +16,16 @@ import { InputParameterScope } from "./input-parameter-scope.js";
 import { InputServiceMethod } from "./input-service-method.js";
 import { RequestLocation } from "./request-location.js";
 
+/**
+ * External type information for types that map to external library types.
+ * @beta
+ */
+export interface InputExternalTypeMetadata {
+  identity: string;
+  package?: string;
+  minVersion?: string;
+}
+
 /**
  * The input client type for the CSharp emitter.
  * @beta
@@ -54,6 +64,7 @@ interface InputTypeBase extends DecoratedType {
   summary?: string;
   doc?: string;
   deprecation?: string;
+  external?: InputExternalTypeMetadata;
 }
 
 export type InputType =
@@ -67,8 +78,7 @@ export type InputType =
   | InputEnumValueType
   | InputArrayType
   | InputDictionaryType
-  | InputNullableType
-  | InputExternalType;
+  | InputNullableType;
 
 export interface InputPrimitiveType extends InputTypeBase {
   kind: SdkBuiltInKinds;
@@ -274,10 +284,3 @@ export interface InputDictionaryType extends InputTypeBase {
   keyType: InputType;
   valueType: InputType;
 }
-
-export interface InputExternalType extends InputTypeBase {
-  kind: "external";
-  identity: string;
-  package?: string;
-  minVersion?: string;
-}

packages/http-client-csharp/emitter/test/Unit/type-converter.test.ts

@@ -127,11 +127,18 @@ describe("External types", () => {
     const prop = testModel.properties.find((p) => p.name === "prop");
     ok(prop, "prop should exist");
 
-    // The type should be an external type
-    strictEqual(prop.type.kind, "external");
-    strictEqual((prop.type as any).identity, "Azure.Core.Expressions.DataFactoryExpression");
-    strictEqual((prop.type as any).package, "Azure.Core.Expressions");
-    strictEqual((prop.type as any).minVersion, "1.0.0");
+    // The type should remain a union but with external info
+    strictEqual(prop.type.kind, "union");
+    ok((prop.type as any).external, "Type should have external info");
+    strictEqual(
+      (prop.type as any).external.identity,
+      "Azure.Core.Expressions.DataFactoryExpression",
+    );
+    strictEqual((prop.type as any).external.package, "Azure.Core.Expressions");
+    strictEqual((prop.type as any).external.minVersion, "1.0.0");
+    // Verify union variants are preserved
+    ok((prop.type as any).variantTypes, "Union should have variant types");
+    strictEqual((prop.type as any).variantTypes.length, 2, "Union should have 2 variant types");
   });
 
   it("should convert external type on model", async () => {
@@ -165,10 +172,11 @@ describe("External types", () => {
     const jsonElementProp = testModel.properties.find((p) => p.name === "jsonElement");
     ok(jsonElementProp, "jsonElement property should exist");
 
-    // The type should be an external type
-    strictEqual(jsonElementProp.type.kind, "external");
-    strictEqual((jsonElementProp.type as any).identity, "System.Text.Json.JsonElement");
-    strictEqual((jsonElementProp.type as any).package, "System.Text.Json");
-    strictEqual((jsonElementProp.type as any).minVersion, "8.0.0");
+    // The type should remain a model but with external info
+    strictEqual(jsonElementProp.type.kind, "model");
+    ok((jsonElementProp.type as any).external, "Type should have external info");
+    strictEqual((jsonElementProp.type as any).external.identity, "System.Text.Json.JsonElement");
+    strictEqual((jsonElementProp.type as any).external.package, "System.Text.Json");
+    strictEqual((jsonElementProp.type as any).external.minVersion, "8.0.0");
   });
 });

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/InputExternalType.cs → packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/InputExternalTypeMetadata.cs

@@ -4,17 +4,17 @@
 namespace Microsoft.TypeSpec.Generator.Input
 {
     /// <summary>
-    /// Represents an external type reference in the input model.
+    /// External type information for types that map to external library types.
     /// </summary>
-    public sealed class InputExternalType : InputType
+    public sealed class InputExternalTypeMetadata
     {
         /// <summary>
-        /// Construct a new <see cref="InputExternalType"/> instance
+        /// Construct a new <see cref="InputExternalTypeMetadata"/> instance
         /// </summary>
         /// <param name="identity">The fully qualified name of the external type.</param>
         /// <param name="package">The package that exports the external type.</param>
         /// <param name="minVersion">The minimum version of the package.</param>
-        public InputExternalType(string identity, string? package, string? minVersion) : base("external")
+        public InputExternalTypeMetadata(string identity, string? package, string? minVersion)
         {
             Identity = identity;
             Package = package;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/InputType.cs

@@ -21,6 +21,7 @@ protected InputType(string name)
 
         public string Name { get; internal set; }
         public IReadOnlyList<InputDecoratorInfo> Decorators { get; internal set; } = new List<InputDecoratorInfo>();
+        public InputExternalTypeMetadata? External { get; internal set; }
 
         internal InputType GetCollectionEquivalent(InputType inputType)
         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/Serialization/InputArrayTypeConverter.cs

@@ -28,13 +28,15 @@ public static InputArrayType CreateListType(ref Utf8JsonReader reader, string? i
             string? crossLanguageDefinitionId = null;
             InputType? valueType = null;
             IReadOnlyList<InputDecoratorInfo>? decorators = null;
+            InputExternalTypeMetadata? external = null;
             while (reader.TokenType != JsonTokenType.EndObject)
             {
                 var isKnownProperty = reader.TryReadReferenceId(ref id)
                     || reader.TryReadString("name", ref name)
                     || reader.TryReadString("crossLanguageDefinitionId", ref crossLanguageDefinitionId)
                     || reader.TryReadComplexType("valueType", options, ref valueType)
-                    || reader.TryReadComplexType("decorators", options, ref decorators);
+                    || reader.TryReadComplexType("decorators", options, ref decorators)
+                    || reader.TryReadComplexType("external", options, ref external);
 
                 if (!isKnownProperty)
                 {
@@ -45,7 +47,8 @@ public static InputArrayType CreateListType(ref Utf8JsonReader reader, string? i
             valueType = valueType ?? throw new JsonException("List must have element type");
             var listType = new InputArrayType(name ?? "Array", crossLanguageDefinitionId ?? string.Empty, valueType)
             {
-                Decorators = decorators ?? []
+                Decorators = decorators ?? [],
+                External = external
             };
             if (id != null)
             {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/Serialization/InputDateTimeTypeConverter.cs

@@ -29,6 +29,7 @@ public static InputDateTimeType CreateDateTimeType(ref Utf8JsonReader reader, st
             InputPrimitiveType? wireType = null;
             IReadOnlyList<InputDecoratorInfo>? decorators = null;
             InputDateTimeType? baseType = null;
+            InputExternalTypeMetadata? external = null;
 
             while (reader.TokenType != JsonTokenType.EndObject)
             {
@@ -38,7 +39,8 @@ public static InputDateTimeType CreateDateTimeType(ref Utf8JsonReader reader, st
                     || reader.TryReadString("encode", ref encode)
                     || reader.TryReadComplexType("wireType", options, ref wireType)
                     || reader.TryReadComplexType("baseType", options, ref baseType)
-                    || reader.TryReadComplexType("decorators", options, ref decorators);
+                    || reader.TryReadComplexType("decorators", options, ref decorators)
+                    || reader.TryReadComplexType("external", options, ref external);
 
                 if (!isKnownProperty)
                 {
@@ -52,7 +54,7 @@ public static InputDateTimeType CreateDateTimeType(ref Utf8JsonReader reader, st
             wireType = wireType ?? throw new JsonException("DateTime type must have wireType");
 
             var dateTimeType = Enum.TryParse<DateTimeKnownEncoding>(encode, ignoreCase: true, out var encodeKind)
-                ? new InputDateTimeType(encodeKind, name, crossLanguageDefinitionId, wireType, baseType) { Decorators = decorators ?? [] }
+                ? new InputDateTimeType(encodeKind, name, crossLanguageDefinitionId, wireType, baseType) { Decorators = decorators ?? [], External = external }
                 : throw new JsonException($"Encoding of DateTime type {encode} is unknown.");
 
             if (id != null)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/Serialization/InputDictionaryTypeConverter.cs

@@ -28,12 +28,14 @@ public static InputDictionaryType CreateDictionaryType(ref Utf8JsonReader reader
             InputType? keyType = null;
             InputType? valueType = null;
             IReadOnlyList<InputDecoratorInfo>? decorators = null;
+            InputExternalTypeMetadata? external = null;
             while (reader.TokenType != JsonTokenType.EndObject)
             {
                 var isKnownProperty = reader.TryReadReferenceId(ref id)
                     || reader.TryReadComplexType("keyType", options, ref keyType)
                     || reader.TryReadComplexType("valueType", options, ref valueType)
-                    || reader.TryReadComplexType("decorators", options, ref decorators);
+                    || reader.TryReadComplexType("decorators", options, ref decorators)
+                    || reader.TryReadComplexType("external", options, ref external);
 
                 if (!isKnownProperty)
                 {
@@ -47,6 +49,7 @@ public static InputDictionaryType CreateDictionaryType(ref Utf8JsonReader reader
             var dictType = new InputDictionaryType("Dictionary", keyType, valueType)
             {
                 Decorators = decorators ?? [],
+                External = external
             };
             if (id != null)
             {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/Serialization/InputDurationTypeConverter.cs

@@ -30,6 +30,7 @@ public static InputDurationType CreateDurationType(ref Utf8JsonReader reader, st
             InputPrimitiveType? wireType = null;
             IReadOnlyList<InputDecoratorInfo>? decorators = null;
             InputDurationType? baseType = null;
+            InputExternalTypeMetadata? external = null;
 
             while (reader.TokenType != JsonTokenType.EndObject)
             {
@@ -39,7 +40,8 @@ public static InputDurationType CreateDurationType(ref Utf8JsonReader reader, st
                     || reader.TryReadString("encode", ref encode)
                     || reader.TryReadComplexType("wireType", options, ref wireType)
                     || reader.TryReadComplexType("baseType", options, ref baseType)
-                    || reader.TryReadComplexType("decorators", options, ref decorators);
+                    || reader.TryReadComplexType("decorators", options, ref decorators)
+                    || reader.TryReadComplexType("external", options, ref external);
 
                 if (!isKnownProperty)
                 {
@@ -53,7 +55,7 @@ public static InputDurationType CreateDurationType(ref Utf8JsonReader reader, st
             wireType = wireType ?? throw new JsonException("Duration type must have wireType");
 
             var dateTimeType = DurationKnownEncodingExtensions.TryParse(encode, out var encodeKind)
-                ? new InputDurationType(encodeKind.Value, name, crossLanguageDefinitionId, wireType, baseType) { Decorators = decorators ?? [] }
+                ? new InputDurationType(encodeKind.Value, name, crossLanguageDefinitionId, wireType, baseType) { Decorators = decorators ?? [], External = external }
                 : throw new JsonException($"Encoding of Duration type {encode} is unknown.");
 
             if (id != null)