# System.InvalidOperationException: 在此上下文实例上完成上一个操作之前，又启动了第二个操作。这通常是由不同线程同时使用同一 DbContext 实例引起的。

- **ID:** `dotnet/ef-core-sync-query-on-async-context`
- **领域:** dotnet
- **类别:** runtime_error
- **验证级别:** ai_generated
- **修复率:** 85%

## 根因

DbContext 实例不是线程安全的，多个线程同时使用它，通常是由于同步调用阻塞异步操作或 DI 注册不当（例如，使用 Singleton 代替 Scoped）导致的。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| Entity Framework Core 3.1 | active | — | — |
| Entity Framework Core 5.0 | active | — | — |
| Entity Framework Core 6.0 | active | — | — |
| Entity Framework Core 7.0 | active | — | — |
| Entity Framework Core 8.0 | active | — | — |

## 解决方案

1. ```
   Ensure DbContext is registered as Scoped in DI (default). For ASP.NET Core: services.AddDbContext<MyDbContext>(options => options.UseSqlServer(connectionString));
   ```
2. ```
   Avoid mixing sync and async calls on the same DbContext. Use async methods (e.g., ToListAsync() instead of ToList()) consistently. If you must use sync, create a separate DbContext instance.
   ```
3. ```
   Use DbContext pooling for high-throughput scenarios: services.AddDbContextPool<MyDbContext>(options => options.UseSqlServer(connectionString), poolSize: 128);
   ```

## 无效尝试

- **Changing DbContext registration to Singleton to reuse the same instance globally.** — Singleton DbContext is not thread-safe and will cause the same error under concurrent requests; it also leads to stale data and memory leaks. (95% 失败率)
- **Adding locks around all DbContext usage to serialize access.** — Locks defeat the purpose of async I/O and can cause deadlocks or severe performance degradation under load. (85% 失败率)
- **Setting DbContext.PoolSize to a large number without configuring AddDbContextPool.** — PoolSize only applies when using DbContext pooling; without AddDbContextPool, it has no effect. (90% 失败率)