Generated Back Compat Service Methods Do Not Compile If Params Have Reserved Names

[jorgerangel-msft]

As part of 72a95c2, back compat support was added for service operations. There two bugs that were found as part of Azure/azure-sdk-for-net#59156

• If the previous version of the method had reserved param names select, top, skip, etc we need to correctly generate the named argument.
  In example:

/// <summary> Lists all indexes available for a search service. </summary>
/// <param name="select"> Selects which top-level properties to retrieve. Specified as a comma-separated list of JSON property names, or '*' for all properties. The default is all properties. </param>
/// <param name="cancellationToken"> The cancellation token that can be used to cancel the operation. </param>
[EditorBrowsable(EditorBrowsableState.Never)]
public virtual AsyncPageable<SearchIndexResponse> GetIndexesWithSelectedPropertiesAsync(IEnumerable<string> @select, CancellationToken cancellationToken)
{
    return this.GetIndexesWithSelectedPropertiesAsync(select: default, top: default, skip: default, count: default, cancellationToken: cancellationToken);
}```

instead of:

```csharp
/// <summary> Lists all indexes available for a search service. </summary>
/// <param name="select"> Selects which top-level properties to retrieve. Specified as a comma-separated list of JSON property names, or '*' for all properties. The default is all properties. </param>
/// <param name="cancellationToken"> The cancellation token that can be used to cancel the operation. </param>
[EditorBrowsable(EditorBrowsableState.Never)]
public virtual AsyncPageable<SearchIndexResponse> GetIndexesWithSelectedPropertiesAsync(IEnumerable<string> @select, CancellationToken cancellationToken)
{
    return this.GetIndexesWithSelectedPropertiesAsync($select: default, $top: default, $skip: default, $count: default, cancellationToken: cancellationToken);
}

• We should not generate methods that violate Client method should have an optional CancellationToken called cancellationToken (both name and it being optional matters) or a RequestContext called context as the last parameter.. If the previous method had a cancellationToken param, it should be optional.

[github-actions]

🎯 Issue Triage

Two bugs in the C# client emitter's back compat service method generation cause compilation failures: reserved parameter names (e.g. select, top, skip) are emitted with $ prefix instead of the verbatim @ prefix in named arguments, and CancellationToken parameters are not marked as optional when they should be.

Category: Bug
Area: emitter:client:csharp

⚠️ No reproduction provided. A minimal repro would help diagnose this issue.

🔍 Analysis

• Root cause 1: When generating the forwarding call in back compat methods, reserved C# keyword parameter names (e.g. select, top, skip, count) are prefixed with $ (incorrect) instead of the verbatim identifier prefix @ used in the method signature.
• Root cause 2: Generated back compat methods that include CancellationToken as the last parameter are not marking it as optional, violating the client method contract.
• Affected component: Back compat service operation generation, introduced in commit 72a95c2a46b5703375d200b84a74bc915865dbd2.
• Discovered via: azure-sdk-for-net#59156.

📝 Suggested next steps

• Fix the named argument generation to use @ verbatim prefix (not $) for reserved C# keyword parameter names.
• Ensure CancellationToken parameters in generated back compat methods are marked as optional.
• Add regression tests covering both cases.

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