Fix duplicate discriminator property in derived types with matching serialized names

[Copilot]

• Investigate the issue and understand the root cause
• Write failing tests that reproduce the bug
• Fix the issue by comparing serialized names instead of C# property names for discriminator de-duplication
• Verify all related tests pass (1233 generator tests)
• Run code review and address feedback
• Add null checks for SerializedName as suggested in review

Root Cause Analysis

The bug occurs in ModelProvider.BuildProperties() where the check for duplicate discriminator properties only compared by C# property name (property.Name) instead of also checking the serialized name (property.SerializedName).

When the base model has a discriminator property with a different C# name than the derived model's discriminator property (but the same wire name like @odata.type), the check failed and the discriminator property was incorrectly added to the derived class.

Fix

Added a HashSet of base discriminator serialized names and extended the check to skip discriminator properties that match by either:

1. C# property name (existing check)
2. Serialized name (new check)

Added null checks for SerializedName as per code review feedback to ensure defensive coding practices.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: Duplicate Discriminator Property in Derived Types</issue_title>
<issue_description>### Describe the bug

Description

The TypeSpec C# generator creates a duplicate OdataType property in derived classes when the base class already defines it as the discriminator. This causes CS0108 "hides inherited member" compiler warnings/errors.

Reproduction

In ContentUnderstandingSkill.cs (derived from SearchIndexerSkill):

Base class (SearchIndexerSkill.cs):

internal string OdataType { get; set; }

Derived class (ContentUnderstandingSkill.cs) incorrectly re-declares:

internal string OdataType { get; set; } = "#Microsoft.Skills.Util.ContentUnderstandingSkill";

The internal constructor also has a redundant parameter:

internal ContentUnderstandingSkill(
    string odataType,           // ? passed to base class
    ...,
    string odataType0)          // ? duplicate, assigned to local OdataType
    : base(odataType, ...)
{
    // ...
    OdataType = odataType0;     // ? This shouldn't exist
}

Expected Behavior

Derived types should NOT re-declare the discriminator property. The value should be set via the base class constructor only, which is already being done correctly in the public constructor:

public ContentUnderstandingSkill(...) 
    : base("#Microsoft.Skills.Util.ContentUnderstandingSkill", inputs, outputs)

Compiler Error

CS0108: 'ContentUnderstandingSkill.OdataType' hides inherited member 'SearchIndexerSkill.OdataType'. 
Use the new keyword if hiding was intended.

Root Cause

The TypeSpec definition likely has the @odata.type discriminator property appearing in both the base type and derived type definitions, or there's a conflict between the discriminator and a regular property with the same JSON name.

Affected Types

• ContentUnderstandingSkill
• ChatCompletionSkill
• Potentially all derived skill types and any other polymorphic hierarchy with a discriminator property

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Reproduction

Existing File with error : https://github.com/Azure/azure-sdk-for-net/blob/3df80e71cc22102f60910e9d188ca864ea18849d/sdk/search/Azure.Search.Documents/src/Generated/Models/ChatCompletionSkill.cs#L138C1-L139C1

Previous file: https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/search/Azure.Search.Documents/src/Generated/Models/ChatCompletionSkill.cs

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.</issue_description>

<agent_instructions>This fix applies to packages\http-client-csharp</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: Duplicate Discriminator Property in Derived Types #9437

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