> For the complete documentation index, see [llms.txt](https://draekien-industries.wpei.me/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://draekien-industries.wpei.me/reference/analyzers/migration-aids.md).

# Migration aids

The WM3xxx rules. They ship off, and you turn them on while you convert a codebase to the library.

These two rules ship **off**. They report on code that has not adopted the library yet, so in most codebases they fire everywhere. Turn them on while you convert, then turn them off again.

| ID                  | What it reports                                                                                          |
| ------------------- | -------------------------------------------------------------------------------------------------------- |
| [`WM3001`](#wm3001) | A member that returns a nullable type, where `Option<T>` would make the absent case impossible to ignore |
| [`WM3002`](#wm3002) | A `throw`, where returning `Result<TOk, Error>` would state the failure in the signature                 |

Enable one in your `.editorconfig`:

```ini
[*.cs]
dotnet_diagnostic.WM3001.severity = suggestion
dotnet_diagnostic.WM3002.severity = suggestion
```

`WM3002` ignores the throws a `Result` would not improve: `ArgumentException` and its subtypes, `NotImplementedException`, `NotSupportedException`, `ObjectDisposedException`, a bare `throw;` rethrow, and any throw inside a lambda you pass to `Option.Try` or `Result.Try`.

## WM3001

**A nullable return leaves the absent case easy to ignore.** `Option<T>` makes the caller acknowledge it. Off by default, because in a codebase that has not adopted the library this fires on nearly every member.

```diff
-User? FindUser(int id);
+Option<User> FindUser(int id);
```

`WM2012` is the narrower, on-by-default version, for a type that already uses `Option` elsewhere.

## WM3002

**A `throw` states a failure nowhere in the signature.** Returning `Result<TOk, Error>` puts it there, where a caller cannot miss it. Off by default, because it fires on every throw in a codebase that has not adopted `Result`.

```diff
-if (!found) throw new InvalidOperationException("no such user");
+if (!found) return new Error("NoSuchUser", "no such user");
```

It skips the throws listed above, which a `Result` would not improve. `WM2003` is the on-by-default version, for a member that already returns `Result`.