Fix TypeFactory.CreateCSharpType crash on self-referencing models

[Copilot]

CreateCSharpType crashes with ArgumentException (duplicate key in Dictionary.Add) when processing self-referencing models like QueryFilter { and?: QueryFilter[] }. Re-entrant calls from CreateCSharpTypeCore → CreateModel → property resolution → CreateCSharpType(same type) cache the result before the outer call attempts Add.

• Fix: Changed TypeCache.Add(inputType, type) to TypeCache[inputType] = type — idempotent on re-entrant writes. Matches the pattern already used by CreateModel (line 190).
• Test: Added CreateCSharpType_SelfReferencingModel_DoesNotThrow that simulates the re-entrant call path and asserts no exception.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>TypeFactory.CreateCSharpType crashes on self-referencing models (re-entrant Dictionary.Add)</issue_title>
<issue_description>## Bug Description

TypeFactory.CreateCSharpType crashes with System.ArgumentException: An item with the same key has already been added when processing self-referencing models — models that contain an array property of their own type (e.g. QueryFilter with property and: QueryFilter[]).

Root Cause

CreateCSharpType caches resolved types in TypeCache (a Dictionary<InputType, CSharpType?>). When it encounters a self-referencing model:

1. Outer call: CreateCSharpType(QueryFilter) → cache miss → calls CreateCSharpTypeCore
2. CreateCSharpTypeCore → CreateModel → BuildProperties → encounters and: QueryFilter[]
3. Inner (re-entrant) call: CreateCSharpType(QueryFilter) → cache miss again (outer hasn't added yet) → CreateCSharpTypeCore → resolves → TypeCache.Add(QueryFilter, ...) succeeds
4. Outer call resumes: tries TypeCache.Add(QueryFilter, ...) → 💥 throws because the key was already added by the inner call

Affected Models (example from Azure CostManagement)

• QueryFilter (properties: and: QueryFilter[], or: QueryFilter[])
• ForecastFilter (same pattern)
• ReportConfigFilter (same pattern)
• ErrorDetail (property: details: ErrorDetail[])

Any ARM/data-plane service with recursive models will trigger this crash.

Reproduction

1. Create a TypeSpec model with a self-referencing array property:

   model QueryFilter {
     and?: QueryFilter[];
     or?: QueryFilter[];
   }

2. Run the C# emitter/generator against it
3. Observe the ArgumentException crash in TypeFactory.CreateCSharpType</issue_description>

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

• Fixes TypeFactory.CreateCSharpType crashes on self-referencing models (re-entrant Dictionary.Add) #10031

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10060

commit: 580b05d