Guard exploded string-struct query parameters against null ToString in C# emitter

[Copilot]

Exploded @query array/dictionary parameters backed by a string struct (extensible string enum) generated code that passed .ToString() directly to AppendQuery(..., escape: true). A default-constructed value has a null internal string, so .ToString() returns null and Uri.EscapeDataString throws ArgumentNullException at runtime.

Changes

• Null/empty guard on exploded elements (RestClientProvider.BuildAppendQueryStatement): wraps each AppendQuery in if (!string.IsNullOrEmpty(...)) for array and dictionary explode when the element/value is a string-backed extensible enum. Gated by a new IsExtensibleStringEnum helper (IsEnum && IsStruct && UnderlyingEnumType == typeof(string)) so int/float/double enums and plain values are unaffected.
• Dictionary explode value fix: the enum was serialized from the KeyValuePair (item) rather than item.Value, yielding wrong output (e.g. @param.ToSerialString(), ((int)@param)). Now uses item.Value, correcting the p6/p7 paths as well.
• Tests: extended TestBuildCreateRequestMethodWithQueryParameters with extensible string enum array + dictionary parameters; baseline updated.

Generated output now:

foreach (var @param in include)
{
    if (!string.IsNullOrEmpty(@param.ToString()))
    {
        uri.AppendQuery("include[]", @param.ToString(), true);
    }
}

• Fixes [Bug]: C# emitter does not guard against null ToString() when serializing exploded query parameters backed by string structs #11431

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commit: f473482

[Copilot]

Pull request overview

This PR fixes a runtime crash and a serialization bug in the C# HTTP client emitter when generating exploded query parameters for string-backed extensible enums (struct enums). It ensures null/empty enum string values aren’t passed into AppendQuery(..., escape: true) and corrects dictionary explode serialization to use KeyValuePair.Value rather than the KeyValuePair itself.

Changes:

• Add string.IsNullOrEmpty(...) guards around per-element/per-value AppendQuery calls for exploded arrays/dictionaries when the element type is a string-backed extensible enum.
• Fix dictionary explode enum serialization to serialize item.Value (not the KeyValuePair), correcting generated output for both string and numeric enum dictionary values.
• Extend RestClientProviderTests.TestBuildCreateRequestMethodWithQueryParameters and update the expected baseline to cover extensible string enum exploded array/dictionary cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/RestClientProvider.cs	Guards exploded query serialization for extensible string enums and fixes dictionary explode value serialization.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/RestClientProviders/RestClientProviderTests.cs	Adds new test inputs for extensible string enum exploded array/dictionary query parameters.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/RestClientProviders/TestData/RestClientProviderTests/TestBuildCreateRequestMethodWithQueryParameters.cs	Updates expected generated output to include null/empty guards and corrected dictionary value serialization.

[JoshLove-msft]

This guard also filters out an explicitly constructed empty-string enum value, not just the default struct's null backing value. The generated extensible-enum constructor rejects null but accepts "", and AppendQuery(..., "", true) safely represents that value as an empty query entry; after this change an array/dictionary element created from "" is silently omitted instead. Could this check only for null and add coverage for an empty-string enum element/value?

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