Fix potential NullReferenceException in C# union model discriminator generation

[hobostay]

Summary

• Add null/empty check for mappedType.Key before generating discriminator condition in C# union model factory method

Problem

In WriteFactoryMethodBodyForUnionModel, when iterating over properties of a union/discriminated type:

var mappedType = parentClass.DiscriminatorInformation.DiscriminatorMappings
    .FirstOrDefault(x => x.Value.Name.Equals(propertyType.Name, StringComparison.OrdinalIgnoreCase));
writer.WriteLine($"...\"{mappedType.Key.SanitizeDoubleQuote()}\".Equals(...)");

If FirstOrDefault finds no matching mapping, it returns default(KeyValuePair<string, CodeType>) with a null key. The next line unconditionally accesses mappedType.Key, generating invalid C# code like "".Equals(mappingValue, ...) instead of a proper discriminator check.

Fix

Guard the WriteLine call with a null/empty check:

if (!string.IsNullOrEmpty(mappedType.Key))
    writer.WriteLine(...);

Test plan

• Verify existing C# writer tests pass
• Verify union model generation handles unmapped types gracefully

🤖 Generated with Claude Code

[baywet]

Thanks for the contribution!

[adrian05-ms]

Thank you for the contribution! Some of the integration tests are failing. Please fix them and I'll approve and merge the PR

[Copilot]

Pull request overview

This PR hardens union-model factory-method generation against missing discriminator mappings by avoiding code emission when the discriminator mapping key can’t be resolved, preventing writer-time exceptions and malformed conditional chains across multiple language emitters.

Changes:

• Add null/empty guards around discriminator mapping keys before emitting discriminator-based branches in union factory methods (C#, Python, Dart; skip/continue in Go/Java/PHP).
• Fix conditional-chain state (includeElse) and closing-brace behavior to avoid emitting invalid else if/dangling blocks when no discriminator branches are generated.
• Update CHANGELOG.md under Unreleased to document the fix (but currently includes unrelated entries).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Kiota.Builder/Writers/CSharp/CodeMethodWriter.cs	Skip discriminator branch generation when the mapping key can’t be found; keep else chaining valid.
src/Kiota.Builder/Writers/Python/CodeMethodWriter.cs	Guard mapped discriminator key usage and ensure elif chaining only begins after an emitted branch.
src/Kiota.Builder/Writers/Dart/CodeMethodWriter.cs	Avoid reading discriminator unless needed; skip branches with missing mapping keys and fix else chaining.
src/Kiota.Builder/Writers/Go/CodeMethodWriter.cs	Skip discriminated-type branches when mapping key is null/empty; only close the chain when started.
src/Kiota.Builder/Writers/Java/CodeMethodWriter.cs	Skip discriminated-type branches when mapping key is null/empty; only close the chain when started.
src/Kiota.Builder/Writers/Php/CodeMethodWriter.cs	Skip discriminated-type branches when mapping key is null/empty; only close the chain when started.
CHANGELOG.md	Add an Unreleased changelog entry for the fix (currently also adds unrelated items).