# LS2019: Use Case Names a Type of Another Use Case

## Intent

LS2019 is a compiler check that reports a use case folder naming a type declared in another use case folder, because a reference between two slices couples work that changes for different reasons. This page states what the rule reports, where the error sits, and how to move the shared type to the folder that owns it.

## Reference

### What the Rule Enforces

LS2019 enforces [Keep Use Case Folders Isolated](https://litespec.dev/4.0.0/backend/dotnet/architecture.md#keep-use-case-folders-isolated) (`litespec/backend/dotnet/architecture#keep-use-case-folders-isolated`). It runs in every layer except Test. Its category is `LiteSpec.Layers`. Severity and enablement follow the [Analyzers overview](https://litespec.dev/4.0.0/analyzers/README.md).

### What the Rule Reports

The rule reports a name in a use case or reaction folder under `UseCases/` that resolves to a type declared in another such folder. The diagnostic sits on the name, and its message names the type and the folder that owns it.

### Failing Example

```csharp
namespace Claimwick.Spending.Expenses.UseCases.SubmitExpense;

internal sealed class SubmitExpenseCommandHandler
{
    public void Handle() => ApproveExpensePolicy.ForExpense();
}
```

with `ApproveExpensePolicy` declared in the `ApproveExpense/` folder.

### Passing Example

```csharp
namespace Claimwick.Spending.Expenses.UseCases.SubmitExpense;

internal sealed class SubmitExpenseCommandHandler(IExpensePolicy policy)
{
}
```

with `IExpensePolicy` declared in the `Expenses/Application/` folder.

### How to Fix

Move the shared type to the aggregate, module, or shared kernel layer folder that holds every consumer naming it. A type two use cases name belongs beside their aggregate, not inside either slice.

## Notes

### What the Rule Ignores

The rule does not report a name in a Test file or in a file the layer rules cannot place. It does not report a type in the same folder or in a layer folder, a name that resolves to no project source file, or a use case message or result: a type whose outermost type ends in `Command`, `Query`, `CommandResult` or `QueryResult`, which a reaction or use case that runs that use case sends and reads.