Fix optional request body to wrap Content-Type header in null check

[Copilot]

When the request body is optional, the C# generator was unconditionally setting the Content-Type header, producing invalid code like if ("application/xml" != null). The Content-Type header should be gated on the content parameter being non-null. The request.Content assignment does not need wrapping since setting it to null is a no-op.

Before:

if ("application/xml" != null)
{
    request.Headers.SetValue("Content-Type", "application/xml");
}
request.Content = content;

After:

if (content != null)
{
    request.Headers.SetValue("Content-Type", "application/xml");
}
request.Content = content;

Changes

• RestClientProvider.AppendHeaderParameters: Reordered null check conditions so the Content-Type + optional body check runs before the general header null check. This fixes an edge case where an optional Content-Type header (contentType?: "application/xml") matched the general null check first, wrapping with the literal value instead of the content parameter.
• Tests: Rewrote ContentTypeHeaderWrappedInNullCheckWhenContentIsOptional and ContentTypeHeaderNotWrappedInNullCheckWhenContentIsRequired to use TestData pattern with MockClientProvider + TypeProviderWriter + Helpers.GetExpectedFromFile(), comparing the full generated CreateRequest method against expected output files. Added ContentTypeHeaderWrappedInNullCheckWhenContentTypeIsOptional test covering the edge case where the Content-Type header itself is optional.
• TestData: Added ContentTypeHeaderWrappedInNullCheckWhenContentIsOptional.cs, ContentTypeHeaderNotWrappedInNullCheckWhenContentIsRequired.cs, and ContentTypeHeaderWrappedInNullCheckWhenContentTypeIsOptional.cs TestData files showing the isolated CreateRequest method output for each scenario

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support Optional Request Body & Optional Content Type</issue_title>
<issue_description>Consider the following spec : link

Here the request body and the content type header are optional. The content type header should only be added to the request if the request body is not empty. Currently, we generate invalid code that will always add the content type parameter:

if ("application/xml" != null)
{
    request.Headers.SetValue("Content-Type", "application/xml");
}

Instead, we should check if the request content is null, and if not, apply the content type:

if (content != null)
{
    request.Headers.SetValue("Content-Type", "application/xml");
    request.Content = content;
}
```</issue_description>

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

<comments>
</comments>

• Fixes Support Optional Request Body & Optional Content Type #10078

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[jorgerangel-msft]

@copilot for context, this was suppose to be fixed in a previous change #9521.

[Copilot]

@copilot for context, this was suppose to be fixed in a previous change #9521.

Thanks for the context. PR #9521 wrapped the Content-Type header in if (content != null) but left request.Content = content unconditionally outside that block. This PR (commit 47ad15a) extends that fix by moving the content assignment inside the same null check, so both the header and content are only set when the body is present.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10079

commit: 0a5a7da