# Return a conflict when EF Core detects a stale order edit

A concurrency token lets EF put the expected row version into the UPDATE predicate. When another edit won, the application must return a conflict or explicitly resolve it, rather than tell the caller their old decision succeeded.

Save this requirement in Perch and check it after each agent edit. The rule gives your agent a named check to run, a finding to repair, and the same command to recheck.

## Implement the ASP.NET Core boundary

Save this illustrative integration module as `Services/CancelOrder.cs`.

```csharp
using Microsoft.EntityFrameworkCore;

public sealed class CancelOrder(AppDbContext db) {
    public async Task<bool> Apply(Order order, byte[] expectedVersion, CancellationToken cancellationToken) {
        db.Entry(order).Property(x => x.RowVersion).OriginalValue = expectedVersion;
        order.Status = "cancelled";
        try {
            await db.SaveChangesAsync(cancellationToken);
            return true;
        } catch (DbUpdateConcurrencyException) {
            return false;
        }
    }
}
```

The order, inventory, or resource-lifetime scenario is adapted from Perch’s public [order-service source at commit 4af67af](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/auth.py). This ASP.NET Core implementation is an illustrative adaptation.

## Integrate with your application

This example assumes SQL Server rowversion configured through [Timestamp] or IsRowVersion. Other providers need an application-managed concurrency token. Load and authorize the tracked order using this same DbContext before calling Apply, decode the client’s expected version, and map false to HTTP 409. Discard/reload the failed tracked entity before reuse; blindly retrying the same stale command defeats the purpose of optimistic locking.



## Exercise the boundary and failure paths

| Scenario | Expected outcome |
| --- | --- |
| Expected version is current | true after save |
| Version is stale | false and HTTP conflict |
| Another writer wins between load and save | Concurrency exception handled |
| Connection failure | Error propagates rather than being called a version conflict |

Add regression tests for these cases in your existing ASP.NET Core test project. Use separate connections or deliberately reordered completion where the case involves concurrency; a sequential happy-path test does not exercise that failure. Run the test command after creating the matching test file or suite:

```sh
dotnet test --filter CancelOrderConcurrencyTests
```

The scenarios above specify tests to implement. A completed Perch source check below is separate from running these application integration tests.

## Introduce the specific agent regression

In a throwaway copy, replace this source span:

```csharp
} catch (DbUpdateConcurrencyException) {
            return false;
```

with:

```csharp
} catch (DbUpdateConcurrencyException) {
            return true;
```

Run the relevant negative test and the Perch command on that broken copy. Restore the corrected source before committing the application. The supplied mutation reproduces the exact regression for the rule check.

## Preserve the requirement as a Perch rule

Save this in `.perch/rules/perch-guide-aspnet-core-optimistic-concurrency.yaml` at the repository root, keeping your existing `perch.yaml`:

```yaml
- name: stale-update-is-not-success
  where: "Services/CancelOrder.cs"
  ensure: >-
    Apply must return false when SaveChangesAsync throws DbUpdateConcurrencyException. A stale write must not be reported as successful.
```

Install and authenticate the CLI once, then check the current working file:

```sh
npm install -g @lakeday/perch
perch login
perch check Services/CancelOrder.cs --rules stale-update-is-not-success --json
```

Use Node 22 or newer. The framework can use another language; Perch’s CLI remains a Node command. A gated finding exits 3. Exits 1 or 2 mean the check did not complete and must be reported as a blocker.

## Inspect the captured rule check

On 2026-10-14, Perch using jev-latest checked the exact fixed and broken source provided here. The capture records SHA-256 hashes of both sources and the requirement.

| Source control | Exit | Outcome |
| --- | --- | --- |
| Broken | 3 | Rule finding |
| Corrected | 0 | No rule finding |

The rule distinguished these controls in this run. [Raw Perch capture](/optimistic-concurrency/capture.json) · [Exact sources and mutation](/optimistic-concurrency/source.json).

## Install the rule pack and configure your agent

[Download this rule pack ZIP](/optimistic-concurrency/rule-pack.zip) · [Download the YAML rules](/optimistic-concurrency/rules.yaml) · [Agent task](/optimistic-concurrency/agent-task.txt). The pack contains the rule, its example paths and the repair/recheck instructions. Review the `where` selector before using it in an existing project. Keep your root `perch.yaml`; install this rule as a separate file under `.perch/rules/`.

1. Install Perch using Node 22 or newer and sign in once:

```sh
npm install -g @lakeday/perch
perch login
```

2. Download the pack, inspect it, and extract it from your repository root. The extraction preserves any existing files:

```sh
curl -fL 'https://reporules.fyi/aspnet-core/optimistic-concurrency/rule-pack.zip' -o 'perch-guide-aspnet-core-optimistic-concurrency.zip'
unzip -n 'perch-guide-aspnet-core-optimistic-concurrency.zip'
```

The installed rule is `.perch/rules/perch-guide-aspnet-core-optimistic-concurrency.yaml`. Its selector matches `Services/CancelOrder.cs` in this example; adapt the selector to your application file before checking an existing project.

3. Configure the coding agent you use. Run one of these commands:

Claude Code:

```sh
perch setup claude-code
```

Codex:

```sh
perch setup codex
```

Cursor:

```sh
perch setup cursor
```

Pi:

```sh
perch setup pi
```

4. Ask the agent to edit the implementation and check the saved requirement:

> Use the installed Perch skill while editing Services/CancelOrder.cs. Preserve stale-update-is-not-success. Run perch check Services/CancelOrder.cs --rules stale-update-is-not-success --json after the edit. Read each finding and the affected code, fix the implementation, and run the identical check again after the fix. Keep the rule and regression tests intact. Run the application's relevant regression tests and report the commands and actual results. Exit 3 means a rule finding; exit 1 or 2 means the check did not complete. Report an unavailable or incomplete check as blocked. Do not claim the application tests ran if only the Perch check ran.

5. After repairing a finding, run the same check again and inspect the result:

```sh
perch check Services/CancelOrder.cs --rules stale-update-is-not-success --json
```

Commit the downloaded rule and the installed agent configuration with your application. The rule file adds no guarantee of correctness; keep the regression tests from this guide.

With the base reference fetched, use `perch scan --since origin/main --filter type=lint` in CI.

## Framework references and verification scope

- [learn.microsoft.com reference](https://learn.microsoft.com/en-us/ef/core/saving/concurrency)

The dated capture records the source check shown above. Implement the adapters, schema and regression tests described in this guide in your application. Unchanged requests reuse cached answers; fresh model judgments can vary. Model analysis sends relevant source to the configured service.