> 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/source-generation.md).

# Source generation

Mark an enum with \[ErrorCodeCatalog] and get its error codes as compile-time constants, generated at build time.

`Waystone.Monads` ships a source generator inside the package. It ships from 6.2.0. You add no reference and configure nothing. It stays silent until you mark an enum with `[ErrorCodeCatalog]`.

## Which page do you want

| You want to                                        | Read                                                                         |
| -------------------------------------------------- | ---------------------------------------------------------------------------- |
| Know why this exists and turn it on                | This page                                                                    |
| Know exactly what it emits                         | [Error code catalogs](/reference/source-generation/error-code-catalogs.md)   |
| Change the shape of the generated strings          | [Code format language](/reference/source-generation/code-format.md)          |
| Review your codes, or fail a build when they drift | [Reviewing generated codes](/reference/source-generation/reviewing-codes.md) |
| Understand a `WMG` build error                     | [Generator diagnostics](/reference/source-generation/diagnostics.md)         |

Hand-written `ErrorCode` values are a different thing, and they are on [Errors](/guides/errors.md). This group covers the generated ones only.

## Why it exists

Up to 6.x, `ErrorCode.FromEnum(InputErrors.Missing)` worked out the string `"InputErrors.Missing"` at run time, by reflecting over the enum. That meant the string your callers saw was not written anywhere you could point at. You could not use it in a `switch`, you could not put it in an attribute, and you could not find every place that read it.

Mark the enum with `[ErrorCodeCatalog]` and `Waystone.Monads` generates those strings as constants when you build.

{% hint style="warning" %}
`ErrorCode.FromEnum` was obsolete from 6.2.0 and 7.0.0 removes it, so this group describes the only supported way to get an error code from an enum. See [Deprecations](/upgrading/deprecations.md) for the migration.
{% endhint %}

## Marking an enum

```csharp
using Waystone.Monads.Results.Errors;

namespace Ordering;

[ErrorCodeCatalog]
public enum OrderErrorCode
{
    NotFound,
    AlreadyShipped,
}
```

That gives you a new class next to the enum, `OrderErrorCodeCatalog`, in the same namespace and with the same accessibility as the enum.

The name is the enum's own name with `Catalog` on the end, and nothing is taken off it. `OrderFailure` gives you `OrderFailureCatalog`; `OrderErrorCode` gives you `OrderErrorCodeCatalog`. If that reads twice, rename the enum — two enums whose names differ always get two catalogs whose names differ.

Next: [what the catalog contains](/reference/source-generation/error-code-catalogs.md).