Address Missing Nullable Value Guard in CreateRequest Method for DateTime Params

[jorgerangel-msft]

See comment. When converting nullable date time params to a string, we are not checking the underlying value is not null first.

[github-actions]

🎯 Issue Triage

The C# client emitter's CreateRequest method does not guard against null before converting a nullable DateTime parameter to a string, causing a potential null reference issue.

Category: Bug
Area: emitter:client:csharp

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

🔍 Analysis

• Affected component: @typespec/http-client-csharp — CreateRequest code generation for DateTime parameters
• Root cause: When generating serialization code for nullable DateTime params, the emitter emits a .ToString() call (or similar string conversion) without first checking that the value is not null, which would throw a NullReferenceException at runtime.
• Reference: Identified in PR #11750 review comment.

📝 Suggested next steps

• Add a null guard before calling ToString() on the nullable DateTime value in the CreateRequest generation logic.
• Add a test case covering nullable DateTime query/path/header parameters.

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