Blazor Server 和 EF Core：在上一个操作完成之前，在此上下文实例上启动了第二个操作

Question

我的 ef 核心有问题。我有两个从数据库读取数据的服务。在一页上称为第一个服务，在第二页上称为第二个服务。当我点击按钮创建一个新程序时，我得到了错误。我通常从带有注入服务的页面调用它。有人可以帮我吗？

Show in application

builder.Services.AddDbContextPool<Context>(options =>
{ 
options.UseSqlServer(builder.Configuration.GetConnectionString("Connection"));
});

测试服务 1:

public class TestService1 : ITestService1
{
    private readonly Context _context;
    private readonly IMapper _mapper;

    public TestService1(Context context, IMapper mapper)
    {
        _kreativgangContext = kreativgangContext;
        _mapper = mapper;
    }

    public virtual async Task<AllProgramViewModel> HandleAsync(AllProgramFilterViewModel filter)
    {
        var model = new AllProgramViewModel();

        var data = _context.Programs.Where(x => (EF.Functions.Like(x.Name ?? "", "%" + filter.Name + "%") || string.IsNullOrEmpty(filter.Name)))
            .Select(x => new Core.Models.Program() { ID = x.ID, Name = x.Name, Order = x.Order });

        result.Model.TotalCount = await data.CountAsync();

        result.Model.Items = data.Select(x => _mapper.Map<AllProgramItemViewModel>(x));
    
        return model;
    }
}

public interface ITestService1
{
    public Task<AllProgramViewModel> HandleAsync(AllProgramFilterViewModel filter);
}

测试服务 2：

    public class TestService2 : ITestService2
{
    private readonly Context _context;

    public TestService2(Context context)
    {
        _context = context;
    }

    public virtual async Task<NewProgramViewModel> HandleAsync()
    {
        var model = new NewProgramViewModel();

        List<ProgramOrderViewModel> items = _context.Programs.Select(x => new Core.Models.Program() { Order = x.Order, ID = x.ID })
            .Select(x => new ProgramOrderViewModel()
            {
                ID = x.ID,
                Order = x.Order
            }).ToList();

        return await Task.FromResult(model);
    }
}

public interface ITestService2
{
    public Task<NewProgramViewModel> HandleAsync();
}

错误：

Error: System.InvalidOperationException: A second operation was started on this context instance before a previous operation completed. This is usually caused by different threads concurrently using the same instance of DbContext. For more information on how to avoid threading issues with DbContext, see https://go.microsoft.com/fwlink/?linkid=2097913.
   at Microsoft.EntityFrameworkCore.Infrastructure.Internal.ConcurrencyDetector.EnterCriticalSection()
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Mitar.Kreativgang.Admin.Handlers.TestService2.HandleAsync() in D:\Programming\Kreativgang\Src\Mitar.Kreativgang.Admin\Handlers\TestService2.cs:line 26
   at Mitar.Kreativgang.Admin.Pages.Program.ProgramNew.OnInitializedAsync() in D:\Programming\Kreativgang\Src\Mitar.Kreativgang.Admin\Pages\Program\ProgramNew.razor:line 114
   at Microsoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync()

Answer 1

这是一个已知的记录在案的陷阱，在 ASP.NET Core Blazor Server with Entity Framework Core (EFCore) 中进行了解释。在 Blazor Server 中，DI 范围是用户电路 - 本质上是用户会话。这意味着像 TestService2 或 DbContext 这样的 scoped 服务将在内存中保留很长时间，并最终被多种方法和操作重用。

正如文档所解释的：

Blazor Server is a stateful app framework. The app maintains an ongoing connection to the server, and the user's state is held in the server's memory in a circuit. One example of user state is data held in dependency injection (DI) service instances that are scoped to the circuit. The unique application model that Blazor Server provides requires a special approach to use Entity Framework Core.

您需要注册并使用 DbContextFactory（或 PooledDbContextFactory）而不是 DbContextPool，并在使用它的地方创建一个新的 DbContext 实例。

builder.Services.AddDbContextFactory<ContactContext>(opt =>
    opt.UseSqlServer(...));

或

builder.Services.AddPooledDbContextFactory<ContactContext>(opt =>
    opt.UseSqlServer(...));

服务构造函数应该接受工厂而不是上下文：

    public TestService2(AddDbContextFactory<ContactContext> factory)
    {
        _factory = factory;
    }

    public virtual async Task<NewProgramViewModel> HandleAsync()
    {
        
        using var context=_factory.CreateContext())
        {
        ...
        }

    }

组件范围

要将 DbContext 的范围限制为单个组件，仅注入 DbContextFactory 是不够的。当用户导航离开组件时，需要显式释放 DbContext 实例。为此，组件需要实现 IDisposable。 Scope to the component lifetime

部分对此进行了解释

@implements IDisposable
@inject IDbContextFactory<ContactContext> DbFactory
...

@code 
{

    ContactContext? Context;

    public void Dispose()
    {
        Context?.Dispose();
    }

    protected override async Task OnInitializedAsync()
    {
        Context = DbFactory.CreateDbContext();
        ...
    }

}

Blazor Server 和 EF Core：在上一个操作完成之前，在此上下文实例上启动了第二个操作

Blazor Server and EF Core: A second operation was started on this context instance before a previous operation completed

c#

entity-framework-core

blazor-server-side