c# - 身份用户管理器 DeleteAsync DbUpdateConcurrencyException

Question

我正在尝试通过 webapi 后面的 aspnetcore.identity UserManager 删除用户。

    [HttpPost("Delete", Name = "DeleteRoute")]
    [Authorize(Roles = "SuperUser")]
    public async Task<IActionResult> DeleteAsync([FromBody] User user)
    {
        Console.WriteLine("Deleting user: " + user.Id);
        try {
            await _userManager.DeleteAsync(user);
            return Ok();
        } catch(Exception e) {
            return BadRequest(e.Message);
        }

    }

这抛出一个DbUpdateConcurrencyException

   Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException: Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ThrowAggregateUpdateConcurrencyException(Int32 commandIndex, Int32 expectedRowsAffected, Int32 rowsAffected)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeResultSetWithoutPropagationAsync(Int32 commandIndex, RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeAsync(RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.ExecuteAsync(IRelationalConnection connection, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.Internal.BatchExecutor.ExecuteAsync(DbContext _, ValueTuple`2 parameters, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(IReadOnlyList`1 entriesToSave, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException: Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ThrowAggregateUpdateConcurrencyException(Int32 commandIndex, Int32 expectedRowsAffected, Int32 rowsAffected)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeResultSetWithoutPropagationAsync(Int32 commandIndex, RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.ConsumeAsync(RelationalDataReader reader, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.ExecuteAsync(IRelationalConnection connection, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Update.Internal.BatchExecutor.ExecuteAsync(DbContext _, ValueTuple`2 parameters, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(IReadOnlyList`1 entriesToSave, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)

我知道这个异常通常表示竞争条件，但我不明白为什么会发生这种情况。

难道我做错了什么？

编辑

我发布的用户对象如下所示：

"User": {
 "Email": "",
 "FirstName": "",
 "LastName": "",
 "Gender": "",
 "Affiliation": {
     "isStudent": true,
     "isEmployee": false
   }
   ...
}

标签： c#asp.net-coreentity-framework-coreasp.net-core-identity

Answer 1

Entity Framework Core 使用乐观并发：

在乐观并发模型中，如果在用户从数据库接收到值之后，另一个用户在第一个用户尝试修改该值之前修改了该值，则认为发生了违规。

将此与悲观并发进行对比：

...在悲观并发模型中，更新行的用户会建立锁。在用户完成更新并释放锁之前，没有其他人可以更改该行。

为了实现乐观并发，IdentityUser该类包含一个ConcurrencyStamp属性（以及数据库中的相应列），它是 GUID 的字符串表示形式：

public virtual string ConcurrencyStamp { get; set; } = Guid.NewGuid().ToString();

每次将用户保存到数据库时，ConcurrencyStamp都会将其设置为新的 GUID。

以删除用户为例，DELETE发送到服务器的 SQL 语句的简化版本可能如下所示：

DELETE FROM dbo.AspNetUsers
WHERE Id = '<USER_ID>' AND ConcurrencyStamp = '<CONCURRENCY_STAMP>'

CONCURRENCY_STAMP当上述 SQL 语句中的值与给定用户在数据库中存储的值不匹配时，您会收到错误消息。这确保了如果您从数据库中检索用户（其中​​包含特定的ConcurrencyStamp），则只有在其他地方没有进行其他更改时（因为您提供的ConcurrencyStamp值与数据库中存在的相同），您才能将更改保存到数据库中。

从ConcurrencyStamp上面的定义可以看出，该属性默认为一个新的GUID——每次IdentityUser创建一个（或子类）时，它都会获得一个新ConcurrencyStamp值。在您的示例中，通过将User传递给您的DeleteAsync操作，ASP.NET Core 模型绑定首先创建一个新实例，User然后设置 JSON 有效负载中存在的属性。ConcurrencyStamp由于有效负载中没有值，User最终将得到一个与数据库中的值不匹配的新值。 ConcurrencyStamp

为避免此问题，您可以将该ConcurrencyStamp值添加到从客户端发送的有效负载中。但是，我不会推荐这个。解决此问题的最简单和最安全的方法是将Id的User作为有效负载发送，使用该实例检索User自身_userManager.FindByIdAsync，然后使用该实例执行删除。这是一个例子：

[HttpPost("Delete/{id}", Name = "DeleteRoute")]
[Authorize(Roles = "SuperUser")]
public async Task<IActionResult> DeleteAsync(string id)
{
    Console.WriteLine("Deleting user: " + id);
    try {
        var user = await _userManager.FindByIdAsync(id);

        if(user == null)
            // ... 

        await _userManager.DeleteAsync(user);
        return Ok();
    } catch(Exception e) {
        return BadRequest(e.Message);
    }
}