[CSharp] ClientProvider.BuildApiVersionFields produces duplicate field names for multi-service clients

[ArcturusZhang]

Bug Report

Description

When a multi-service client is created via @client({ service: [ServiceA, ServiceB] }), ClientProvider.BuildApiVersionFields() can produce duplicate field names, causing BuildMethods() to crash with:

An item with the same key has already been added. Key: serviceTestsApiVersion
   at Dictionary2.Add(TKey key, TValue value)
   at Enumerable.ToDictionary(...)
   at ClientProvider.BuildMethods()

Impact

This crashes the Azure management plane generator (@azure-typespec/http-client-csharp-mgmt) when generating multi-service SDKs such as Azure Compute (which combines 4 services via @client({ service: [Compute, ComputeDisk, ComputeGallery, ComputeSku] })).

The mgmt generator does not use the ClientProvider or its API version fields at all — it replaces them with its own ARM resource-based client hierarchy. However, the crash occurs during ClientProvider.BuildMethods() before the mgmt generator gets a chance to override it, so the mgmt generator cannot work around this without patching the base ClientProvider.

Root Cause

In ClientOptionsProvider.BuildVersionProperties(), the property name for each service version is computed via:

string name = _inputClient.IsMultiServiceClient 
    ? ClientHelper.BuildNameForService(inputEnumType.Namespace, "Service", "ApiVersion") 
    : "Version";

When two services have namespaces that reduce to the same identifier (e.g. both share a common namespace segment from the C# output namespace), BuildNameForService produces the same property name for both services.

Then in ClientProvider.BuildApiVersionFields(), when VersionProperties.Count > 1:

string name = (count > 1) ? ("_" + propertyProvider.Name.ToVariableName()) : text;

Both services get the same field name (e.g. _serviceTestsApiVersion), and Fields.ToDictionary() in BuildMethods() crashes on the duplicate key.

Reproduction

Two ARM services combined via @client, with the output C# namespace causing name collisions:

ServiceOne/main.tsp:

@armProviderNamespace
@service(#{ title: "ServiceOne" })
@versioned(ServiceOne.Versions)
namespace ServiceOne;

enum Versions { v2024_01_01: "2024-01-01" }
// ... resource definition ...

ServiceTwo/main.tsp:

@armProviderNamespace
@service(#{ title: "ServiceTwo" })
@versioned(ServiceTwo.Versions)
namespace ServiceTwo;

enum Versions { v2024_06_01: "2024-06-01" }
// ... resource definition ...

client.tsp:

@client({ name: "MultiServiceClient", service: [ServiceOne, ServiceTwo] })
namespace MultiServiceCombine;

tspconfig.yaml with namespace Azure.Generator.MgmtTypeSpec.MultiService.Tests

Expected Behavior

Each service should get a unique field name for its API version field, even when namespace segments collide.

Actual Behavior

Both services produce the same field name serviceTestsApiVersion, causing ToDictionary to crash.

[ArcturusZhang]

This issue is still present as of the latest main (March 23, 2026). We verified by removing our emitter-side workaround (which deduplicates ApiVersionEnum enums sharing the same namespace) and the multi-service test project crashes in MockableResourceProvider.BuildMethods() — same root cause, duplicate API version fields from ClientOptionsProvider.

We are currently carrying a workaround in the mgmt emitter that merges duplicate ApiVersionEnum enums before they reach the base generator: Azure/azure-sdk-for-net#57160

[ArcturusZhang]

Updated crash details — without our workaround, the crash now manifests at a different point after recent main changes:

System.ArgumentException: An item with the same key has already been added. Key: serviceTestsApiVersion
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at Microsoft.TypeSpec.Generator.Providers.ClientProvider.BuildMethods()
   at Microsoft.TypeSpec.Generator.Providers.TypeProvider.get_Methods()
   at Azure.Generator.Management.Providers.MockableResourceProvider.BuildMethods()
     in MockableResourceProvider.cs:line 215
   at Microsoft.TypeSpec.Generator.Providers.TypeProvider.get_Methods()
   at Azure.Generator.Management.ManagementOutputLibrary.ExtractArrayResponseCollectionResults()
     in ManagementOutputLibrary.cs:line 364
   at Azure.Generator.Management.ManagementOutputLibrary.BuildTypeProviders()
     in ManagementOutputLibrary.cs:line 326
   at Microsoft.TypeSpec.Generator.OutputLibrary.get_TypeProviders()
   at Microsoft.TypeSpec.Generator.CSharpGen.ExecuteAsync()

The root cause is the same: two services (ServiceOne and ServiceTwo) both produce a Versions enum with the same namespace, ClientOptionsProvider.BuildVersionProperties() generates duplicate field names, and ClientProvider.BuildMethods() → Fields.ToDictionary() crashes on the duplicate key.