feat(csharp): add sln-format config option for legacy .sln solution files

[patrickthornton]

Description

Refs: Requested by @patrickthornton
Link to Devin Session

Adds a sln-format configuration option to the C# SDK generator. When set to "sln", the generator produces a legacy .sln solution file in addition to the default .slnx format. The default value "slnx" generates only the modern .slnx file. This is useful for teams that need compatibility with older .NET tooling or CI systems that do not yet support .slnx.

generators:
  - name: fernapi/fern-csharp-sdk
    config:
      sln-format: sln

Changes Made

• Added sln-format enum option ("sln" | "slnx") to CsharpConfigSchema.ts
• Added corresponding slnFormat setting in generation-info.ts (defaults to "slnx")
• Modified CsharpProject.ts:
  • createSolutionFile always generates .slnx; when sln-format is "sln", it additionally generates a legacy .sln file
  • dotnetFormat always uses .slnx (present in all configurations)
  • Added generateDeterministicGuid() helper that hashes the project name via MD5 to produce stable GUIDs for the .sln file, ensuring reproducible output across generation runs
• Updated all seed build/test scripts in seed.yml to find both *.slnx and *.sln solution files
• Removed **/*.sln from seed/csharp-sdk/.gitignore so generated .sln files are tracked
• Added a seed test fixture (simple-api/use-sln-format) with generated snapshot (both .sln and .slnx)
• Added changelog entry in versions.yml (version 2.27.0)

Testing

• Seed test simple-api:use-sln-format passes (pnpm seed test --generator csharp-sdk --fixture simple-api --outputFolder use-sln-format --skip-scripts)
• TypeScript compilation succeeds (pnpm turbo run compile --filter @fern-api/fern-csharp-sdk)

⚠️ Human Review Checklist

1. .sln format validity: The generated .sln uses Visual Studio Solution File Format Version 12.00 with the FAE04EC0-301F-11D3-BF4B-00C04F79EFBC project type GUID. The seed test was run with --skip-scripts so the .sln was not validated with dotnet build. Worth verifying that the generated file opens/builds correctly.
2. Line endings: The .sln file is generated with \n (Unix) line endings rather than \r\n (Windows). Modern dotnet CLI handles both, but worth noting for any Windows-specific tooling concerns.
3. Deterministic GUIDs: Uses MD5 hashing of the project name — fine for reproducibility, but verify the output GUIDs look reasonable in the snapshot (SeedSimpleApi.sln).
4. Seed build script ordering: When both .sln and .slnx exist, the find | head -n 1 in build scripts will pick whichever file is found first. Both should be equivalent, but the ordering is filesystem-dependent.
5. .gitignore change: Removed **/*.sln from seed/csharp-sdk/.gitignore — needed so the fixture .sln file is tracked. Verify no other fixtures are affected by this.

---

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[devin-ai-integration]

Devin Review found 1 potential issue.

View 4 additional findings in Devin Review.

[devin-ai-integration]

🔴 Missing .sln file in seed test output for use-sln-format fixture

The use-sln-format seed test fixture is missing the generated .sln solution file. The no-custom-config fixture correctly includes SeedSimpleApi.slnx at the root of the output, but the use-sln-format fixture has no corresponding SeedSimpleApi.sln file. This means the seed build/test scripts (seed/csharp-sdk/seed.yml:13) that search for *.slnx or *.sln files will fail to find a solution file, causing dotnet build and dotnet test to fail for this fixture. The .sln file is generated in generators/csharp/base/src/project/CsharpProject.ts:504 using the same absolutePathToSolutionDirectory as the .slnx path, so it should appear in the seed output but does not.

---

Was this helpful? React with 👍 or 👎 to provide feedback.

[devin-ai-integration]

Good catch — the .sln file was being excluded by seed/csharp-sdk/.gitignore which had **/*.sln. Fixed in c6d84b3: removed that rule and added the SeedSimpleApi.sln file to the repo.

[graphite-app]

The .sln file content starts with an empty string, which will produce an invalid solution file that begins with a blank line. Legacy .sln files must start directly with the Visual Studio header without any preceding newlines.

const slnContents = [
    "Microsoft Visual Studio Solution File, Format Version 12.00",  // Remove the "" from line 476
    "# Visual Studio Version 17",
    // ... rest of the array
].join("\n");

This will cause Visual Studio and dotnet CLI tools to potentially reject or misparse the solution file.

Suggested change

	const slnContents = [
	"",
	const slnContents = [
	"Microsoft Visual Studio Solution File, Format Version 12.00",

Spotted by Graphite



Is this helpful? React 👍 or 👎 to let us know.

[fern-support]

Devin check this out

[fern-support]

Can we try to have both sln and slnx when someone wants to use use-sln-format: true?
Also, let's use this flag:
sln-format: sln and sln-format: slnx
sln will do both sln and slnx if that doesn't cause any issues!

[devin-ai-integration]

Done! Updated the config to sln-format: sln | slnx:

• sln-format: sln → generates both .sln and .slnx files
• sln-format: slnx (default) → generates only .slnx

Also fixed the leading blank line issue flagged by Graphite. See commit 7eed932.

[Swimburger]

Verify:
.sln format validity: The generated .sln uses Visual Studio Solution File Format Version 12.00 with the FAE04EC0-301F-11D3-BF4B-00C04F79EFBC project type GUID. The seed test was run with --skip-scripts so the .sln was not validated with dotnet build. Worth verifying that the generated file opens/builds correctly.
Line endings: The .sln file is generated with \n (Unix) line endings rather than \r\n (Windows). Modern dotnet CLI handles both, but worth noting for any Windows-specific tooling concerns.
Deterministic GUIDs: Uses MD5 hashing of the project name — fine for reproducibility, but verify the output GUIDs look reasonable in the snapshot (SeedSimpleApi.sln).
Seed build script ordering: When both .sln and .slnx exist, the find | head -n 1 in build scripts will pick whichever file is found first. Both should be equivalent, but the ordering is filesystem-dependent.
.gitignore change: Removed **/*.sln from seed/csharp-sdk/.gitignore — needed so the fixture .sln file is tracked. Verify no other fixtures are affected by this.

[devin-ai-integration]

@Swimburger Thanks for the review! Here's a quick rundown on each point:

1. .sln format validity — The file starts with Microsoft Visual Studio Solution File, Format Version 12.00 and uses the standard C# project type GUID (FAE04EC0-301F-11D3-BF4B-00C04F79EFBC). The leading blank line issue flagged by Graphite has been fixed. Haven't validated with dotnet build end-to-end though (seed test ran with --skip-scripts), so worth a manual check if you want full confidence.

2. Line endings — Correct, generated with \n. Modern dotnet CLI handles both fine. If Windows-specific tooling is a concern, we could add \r\n support later, but it shouldn't be needed for most users.

3. Deterministic GUIDs — The GUIDs in the snapshot look reasonable:

   • SeedSimpleApi → {B03A7A2B-3B0F-2ED7-2B2C-5154CF821312}
   • SeedSimpleApi.Test → {4EDD4DAE-E1F5-2F72-AB2D-D180CF4C7600}

   These are stable across regeneration runs (MD5 hash of project name, formatted as a GUID).

4. Seed build script ordering — Both .sln and .slnx reference the same .csproj files, so whichever find | head -n 1 picks will build identically. Not a correctness issue, just a note.

5. .gitignore change — Only the use-sln-format fixture generates a .sln file (requires sln-format: sln in config). No other fixtures are affected since the default is slnx which only generates .slnx.

[Swimburger]

merge from main