[Mgmt][CodeGen] AzureTypeFactory drops properties whose model type uses identity-aliased @@alternateType (InputModelType.External.Identity)

[mzhongl524]

Summary

AzureTypeFactory.CreateCSharpTypeCore only handles identity-aliased external types when the input is an InputUnionType (and only for Azure.Core.Expressions.DataFactoryElement). When the same @@alternateType(Model, { identity: "..." }, "csharp") pattern is applied to an InputModelType, the generator has no code path to resolve the alternate identity. Consequently every property whose type is one of those externally-aliased models is silently dropped from the generated C# model (no diagnostic, no warning).

Repro

In azure-rest-api-specs specification/datafactory/resource-manager/Microsoft.DataFactory/DataFactory/client.tsp (lines 661-681), Azure.ResourceManager.DataFactory aliases three TSP models to external Azure.Core.Expressions.DataFactory types:

// Map SecretBase and LinkedServiceReference to the external
// Azure.Core.Expressions.DataFactory types so baseline API surface matches
// (avoids exposing local DataFactorySecret/Reference classes).
@@alternateType(
  SecretBase,
  { identity: "Azure.Core.Expressions.DataFactory.DataFactorySecret" },
  "csharp"
);
@@alternateType(
  LinkedServiceReference,
  { identity: "Azure.Core.Expressions.DataFactory.DataFactoryLinkedServiceReference" },
  "csharp"
);
@@alternateType(
  SecureString,
  { identity: "Azure.Core.Expressions.DataFactory.DataFactorySecretString" },
  "csharp"
);

Steps:

cd sdk/datafactory/Azure.ResourceManager.DataFactory
dotnet build /t:GenerateCode /p:LocalSpecRepo="<path>/specification/datafactory/resource-manager/Microsoft.DataFactory/DataFactory" /p:SaveInputs=true

Concrete cases

A. ZohoLinkedService.AccessToken is dropped

TypeSpec (models.tsp:17623-17660):

model ZohoLinkedServiceTypeProperties {
  connectionProperties?: unknown;
  endpoint?: Dfe<string>;
  accessToken?: SecretBase;             // <-- aliased to DataFactorySecret
  useEncryptedEndpoints?: Dfe<boolean>;
  ...
}

Swagger (stable/2018-06-01/openapi.json:35189-35216) confirms accessToken: { $ref: '#/definitions/SecretBase' }.

Main branch (AutoRest baseline) Azure.ResourceManager.DataFactory.net8.0.cs:

public partial class ZohoLinkedService : DataFactoryLinkedServiceProperties
{
    public ZohoLinkedService() { }
    public DataFactorySecret AccessToken { get; set; }                   // ✅ present
    ...
}

Local MPG-generated Azure.ResourceManager.DataFactory.net8.0.cs (this branch):

public partial class ZohoLinkedService : DataFactoryLinkedServiceProperties
{
    public ZohoLinkedService() { }
    // ❌ no AccessToken!
    public BinaryData ConnectionProperties { get; set; }
    public string EncryptedCredential { get; set; }
    public DataFactoryElement<string> Endpoint { get; set; }
    ...
}

Inspection of the internal flattened-source type (src/Generated/Models/ZohoLinkedServiceTypeProperties.cs lines 34-43) confirms its constructor does not include an accessToken parameter, and the property is missing entirely. The accessToken JSON field instead falls into _additionalBinaryDataProperties during deserialization.

B. ZohoObjectDataset constructor loses its required parameter

TypeSpec (models.tsp:4573-4618):

model Dataset {
  ...Record<unknown>;
  type: string;
  ...
  linkedServiceName: LinkedServiceReference;  // <-- required, aliased
  ...
}

Main branch:

protected DataFactoryDatasetProperties(DataFactoryLinkedServiceReference linkedServiceName) { }
public DataFactoryLinkedServiceReference LinkedServiceName { get; set; }

public partial class ZohoObjectDataset : DataFactoryDatasetProperties
{
    public ZohoObjectDataset(DataFactoryLinkedServiceReference linkedServiceName)
        : base(default(DataFactoryLinkedServiceReference)) { }
    ...
}

Local MPG-generated:

// DataFactoryDatasetProperties no longer has LinkedServiceName at all
private protected DataFactoryDatasetProperties(string datasetType) { ... }

public partial class ZohoObjectDataset : DataFactoryDatasetProperties
{
    public ZohoObjectDataset() : base("ZohoObject") { }   // ❌ parameterless only
    ...
}

Root cause

eng/packages/http-client-csharp/generator/Azure.Generator/src/AzureTypeFactory.cs:

protected override CSharpType? CreateCSharpTypeCore(InputType inputType)
{
    if (inputType is InputPrimitiveType inputPrimitiveType) { ... }
    else if (inputType is InputModelType inputModelType)
    {
        if (KnownAzureTypes.TryGetKnownType(inputModelType.CrossLanguageDefinitionId, out var knownType))
            return knownType;
        // ❌ Never consults inputModelType.External?.Identity
    }
    else if (inputType is InputArrayType inputArrayType) { ... }
    else if (inputType is InputUnionType inputUnionType)
    {
        var dataFactoryElementType = TryCreateDataFactoryElementTypeFromUnion(inputUnionType);
        if (dataFactoryElementType != null) return dataFactoryElementType;
    }
    return base.CreateCSharpTypeCore(inputType);
}

private CSharpType? TryCreateDataFactoryElementTypeFromUnion(InputUnionType inputUnionType)
{
    if (inputUnionType.External?.Identity != AzureClientGenerator.DataFactoryElementIdentity) // ✅ only this one
        return null;
    ...
}

The InputUnionType path correctly recognises External.Identity for Azure.Core.Expressions.DataFactoryElement. The InputModelType path never reads inputModelType.External?.Identity, so identity-aliased models fall through to base.CreateCSharpTypeCore, which returns null (because TCGC excluded the model from generation due to the alternateType), and the consumer property is silently omitted.

CreateFrameworkType (AzureTypeFactory.cs:148-158) only hard-codes a 4-entry switch for ResourceIdentifier/AzureLocation/ResponseError/ETag, with no extensibility point for additional packages.

Impact

Scope across sdk/datafactory/Azure.ResourceManager.DataFactory (a git diff of net8.0 API between local generation and main, restricted to lines mentioning the three external types):

• 167 model classes affected
• 337 public API members (properties + constructors) missing
• Includes every LinkedService secret (password, clientSecret, accountKey, accessToken, privateKey, ...), every Dataset's required LinkedServiceName, and the LinkedServiceReference argument on ~133 backward-compatible constructors

Any future GA service that wants to alternateType a model to an external NuGet type (rather than the built-in DataFactoryElement<T> union case) will hit the same bug.

Proposed fix

In AzureTypeFactory.CreateCSharpTypeCore, also consult inputModelType.External?.Identity and resolve via:

1. A new dispatch (analogous to TryCreateDataFactoryElementTypeFromUnion) that handles known external identities for models, OR
2. An expanded KnownAzureTypes registry / CreateFrameworkType switch covering at least:
   • Azure.Core.Expressions.DataFactory.DataFactorySecret
   • Azure.Core.Expressions.DataFactory.DataFactoryLinkedServiceReference
   • Azure.Core.Expressions.DataFactory.DataFactorySecretString

A diagnostic for the dropped-property case would also have prevented this from being a silent failure.

Workaround in place

Until the generator is fixed, sdk/datafactory/Azure.ResourceManager.DataFactory/src/Custom/Models/ re-exposes the missing properties / constructors via partial classes that back-store through the existing _additionalBinaryDataProperties dictionary (which already preserves the raw JSON across roundtrip). Each customization file links back to this issue.