C# oneOf deserialization silently loses data when no discriminator is defined

[davidwinslowtech]

Description

When an OpenAPI spec defines a oneOf composed type without a discriminator, Kiota generates an IComposedTypeWrapper class with properties for each concrete type. At runtime, deserialization silently produces null values for all typed properties -- the JSON data is effectively lost.

Steps to reproduce

OpenAPI spec:

Effect:
  allOf:
    - \$ref: '#/components/schemas/EffectEntity'
    - type: object
      properties:
        props:
          oneOf:
            - \$ref: '#/components/schemas/AddLoyaltyPointsEffectProps'
            - \$ref: '#/components/schemas/DeductLoyaltyPointsEffectProps'
            - \$ref: '#/components/schemas/TriggerWebhookEffectProps'
            # ... ~30 more types

The JSON response from the API:

{
  "effectType": "addLoyaltyPoints",
  "props": {
    "name": "Product registered",
    "value": 75,
    "programId": 37,
    "transactionUUID": "abc-123"
  }
}

After Kiota deserialization:

effect.Props                              // not null (Effect_props wrapper)
effect.Props.AddLoyaltyPointsEffectProps  // null
effect.Props.DeductLoyaltyPointsEffectProps // null
// All typed properties are null -- data is lost
// AdditionalData on Effect_props is not available (IComposedTypeWrapper doesn't implement IAdditionalDataHolder)

Expected behavior

At minimum, one of:

1. The data should be preserved in AdditionalData on the composed type wrapper so consumers can access it
2. Kiota should attempt to populate all concrete types with matching fields (best-effort deserialization)
3. A warning should be emitted at generation time that oneOf without discriminator will not deserialize correctly in C#

Actual behavior

All typed properties on the IComposedTypeWrapper are null. The JSON data is silently discarded with no error, warning, or fallback. There is no way to access the original data.

Workaround

We replaced the oneOf with a single flat schema containing all fields as optional properties, then regenerated the client. This works but loses the type safety that oneOf is meant to provide.

Environment

• Kiota version: 1.29.0
• Language: C#
• .NET version: 9.0

[davidwinslowtech]

Workaround

We replaced the oneOf in our local copy of the OpenAPI spec with a single flat schema:

EffectProps:
  type: object
  additionalProperties: true
  properties:
    # Loyalty points (addLoyaltyPoints / deductLoyaltyPoints)
    name:
      type: string
    value:
      type: number
    programId:
      type: integer
    subLedgerId:
      type: string
    transactionUUID:
      type: string
    recipientIntegrationId:
      type: string
    cartItemPosition:
      type: number
    cartItemSubPosition:
      type: number
    expiryDate:
      type: string
      format: date-time
    # Tier change (changeLoyaltyTierLevel)
    previousTierName:
      type: string
    newTierName:
      type: string
    # Webhook (callApi)
    webhookId:
      type: integer
    webhookName:
      type: string

Then changed the Effect schema from:

props:
  oneOf:
    - \$ref: '#/components/schemas/AddLoyaltyPointsEffectProps'
    - \$ref: '#/components/schemas/DeductLoyaltyPointsEffectProps'
    # ... ~30 more

To:

props:
  \$ref: '#/components/schemas/EffectProps'

This generates a single EffectProps class that deserializes correctly. Fields not relevant to a given effect type are simply null.

[JonathanWillis]

I feel like I've hit this issue. I have an API that returns an object, which contains 2 properties. One of the properties can be null, because its not yet been saved by the user. The generated client chains up as - _budgetClient.Api.V1.Users["12345"].Budget.GetAsync().Result.LatestRecommendation.BudgetRecommendationResponse.Amount but the data is missing due to the BudgetRecommendationResponse in the middle. The json fails to map onto

The API spec is available as 3.1 or 3.0, made automatically from defined C# ApiEndpoints

<PackageVersion Include="Microsoft.AspNetCore.OpenApi" Version="10.0.3" />
<!-- Microsoft.OpenApi locked to 2.0.0 due to Microsoft.AspNetCore.OpenApi needing it -->
<PackageVersion Include="Microsoft.OpenApi" Version="[2.0.0]" />

OpenAPI 3.1

{
  "BudgetResponse": {
    "required": [
      "latestRecommendation",
      "currentBudget"
    ],
    "type": "object",
    "properties": {
      "latestRecommendation": {
        "oneOf": [
          {
            "type": "null"
          },
          {
            "$ref": "#/components/schemas/BudgetRecommendationResponse"
          }
        ]
      },
      "currentBudget": {
        "oneOf": [
          {
            "type": "null"
          },
          {
            "$ref": "#/components/schemas/BudgetInfoResponse"
          }
        ]
      }
    }
  }
}

OpenAPI 3.0

{
  "BudgetResponse": {
    "required": [
      "latestRecommendation",
      "currentBudget"
    ],
    "type": "object",
    "properties": {
      "latestRecommendation": {
        "oneOf": [
          {
            "nullable": true
          },
          {
            "$ref": "#/components/schemas/BudgetRecommendationResponse"
          }
        ]
      },
      "currentBudget": {
        "oneOf": [
          {
            "nullable": true
          },
          {
            "$ref": "#/components/schemas/BudgetInfoResponse"
          }
        ]
      }
    }
  }
}