引用 ITVF 会引发 "second operation started on this context before a previous operation completed" 异常

Question

我正在尝试在 Linq 查询中引用内联 Table 值函数 (ITVF):

var results = await (
    from v in _context.Vehicles
    from r in _context.UnitRepairStatus(v.VehicleNumber) <-- ITVF reference
    orderby v.VehicleNumber
    select new FooViewModel { 
            ID = v.ID, 
            VehicleNumber = v.VehicleNumber,
            InRepair = Convert.ToBoolean(r.InRepair) <-- ITFV field
        }
    ).ToListAsync();

查询运行时，生成错误：

InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread safe.

提及代码：

System.Linq.AsyncEnumerable.Aggregate_(IAsyncEnumerable source, TAccumulate seed, Func accumulator, Func resultSelector, CancellationToken cancellationToken) MyApplication.Controllers.VehiclesController.Foo() in VehiclesController.cs
                var results = await (

如果我删除 ITFV 引用，查询将按预期工作

var results = await (
    from v in _context.Vehicles
    orderby v.VehicleNumber
    select new FooViewModel { 
            ID = v.ID, 
            VehicleNumber = v.VehicleNumber,
            InRepair = False <-- dummy value
        }
    ).ToListAsync();

添加 ITVF 引用时为什么会出现这种情况？我该如何解决？

代码

UnitRepairStatus ITVF:

CREATE FUNCTION dbo.UnitRepairStatus(
  @unit_number varchar(18)
)
RETURNS TABLE
AS

RETURN

  SELECT  h.InRepair
  -- connects to a second database on same server
  --  shouldn't be an issue, but mentioning it in case it might be
  FROM    Schema2..Unit u
  INNER JOIN Schema2..History h on u.ID = h.UnitID
  WHERE   u.UnitNumber = @unit_number

UnitRepairStatus 型号：

public class UnitRepairStatus
{
    public string UnitNumber { get; set; }
    public int? InRepair { get; set; }
}

MyDatabaseDbContext DbContext:

public class MyDatabaseDbContext : DbContext
{

    public MyDatabaseDbContext(DbContextOptions<MyDatabaseDbContext> options) : base(options) {}
...

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        ...
        modelBuilder.Query<UnitRepairStatus>();
    }

    public IQueryable<UnitRepairStatus> UnitRepairStatus(string unitNumber) =>
        Query<UnitRepairStatus>().FromSql($"SELECT * FROM UnitRepairStatus({unitNumber})");

}

FooViewModel 视图模型：

public class FooViewModel
{
    public int ID { get; set; }
    public string VehicleNumber { get; set; }
    public bool InRepair { get; set; }
}

VehiclesController构造函数：

public VehiclesController(
    ILogger<VehiclesController> logger, 
    MyDatabaseDbContext context
)
{
    _logger = logger;
    _context = context;
}

VehiclesControllerFoo方法：

public async Task<IActionResult> Foo()
{

    List<FooViewModel> model = null;

    try
    {
        var results = await (  <-- line referenced in error message
            from v in _context.Vehicles
            from r in _context.UnitRepairStatus(v.VehicleNumber)
            orderby v.VehicleNumber
            select new FooViewModel { 
                    ID = v.ID, 
                    VehicleNumber = v.VehicleNumber,
                    InRepair = Convert.ToBoolean(r.InRepair)
                }
            ).ToListAsync();

    }
    catch (Exception e)
    {
        _logger.LogError(e.Message);
        throw;
    }

    return View(model);

}

参考：

Use a Inline Table-Valued Functions with Linq and Entity Framework

Answer 1

对不起，我的错。您上一个问题的答案中的技术适用于使用 constant/variable 参数调用 ITVF，但不适用于像您的情况（以及我的错误示例）中的相关子查询。

解决方案是删除 ITVF 参数并将结果扩展为也包含该列（有效地将其变成无参数视图）：

CREATE FUNCTION dbo.UnitRepairStatus()
RETURNS TABLE
AS
RETURN
  SELECT u.UnitNumber, h.InRepair
  FROM    Schema2.Unit u
  INNER JOIN Schema2.History h on u.ID = h.UnitID

也从上下文方法中删除参数

EF 核心 2.x:

public IQueryable<UnitRepairStatus> UnitRepairStatus() =>
    Query<UnitRepairStatus>().FromSql("SELECT * FROM UnitRepairStatus()");

EF 核心 3.x:

public IQueryable<UnitRepairStatus> UnitRepairStatus() =>
    Set<UnitRepairStatus>().FromSqlRaw("SELECT * FROM UnitRepairStatus()");

并将 LINQ 查询更改为使用连接：

var results = await (
    from v in _context.Vehicles
    join r in _context.UnitRepairStatus() on v.VehicleNumber equals r.UnitNumber // <---
    orderby v.VehicleNumber
    select new FooViewModel { 
        ID = v.ID, 
        VehicleNumber = v.VehicleNumber,
        InRepair = Convert.ToBoolean(r.InRepair)
    }
).ToListAsync();

现在应该翻译并执行服务器端，并在客户端成功实现。

原始方法的问题是 EF Core 悄悄地将查询执行切换到客户端评估（讨厌那样），然后在同一个上下文中执行多个异步操作时触发保护。

引用 ITVF 会引发 "second operation started on this context before a previous operation completed" 异常

Reference to an ITVF raises a "second operation started on this context before a previous operation completed" exception

c#

linq

asp.net-core

asp.net-core-2.1

entity-framework-core-2.1

代码