c# - Blazor:在此之前的操作完成之前，在此上下文中开始了第二次操作

Question

我正在创建服务器端Blazor应用程序。以下代码在Startup.cs中。

services.AddDbContext<MyContext>(o => o.UseSqlServer(Configuration.GetConnectionString("MyContext")), ServiceLifetime.Transient);
services.AddTransient<MyViewModel, MyViewModel>();

并在ViewModel中:

public class MyViewModel : INotifyPropertyChanged
{
    public MyViewModel(MyContext myContext)
    {
        _myContext = myContext;
    }

    public async Task<IEnumerable<Dto>> GetList(string s)
    {
        return await _myContext.Table1.where(....)....ToListAsync();
    }

并在 Razor 文件中。

@inject ViewModels.MyViewModel VM
<input id="search" type="text" @bind="search" />
<input id="search" type="button" value="Go" @onclick="SearchChanged" />   
@code {
    string search = "";
    int currentCount = 0;
    async void SearchChanged() {
        currentCount++;
        dtos = GetList(search);
    }
}

但是，有时在单击搜索按钮时会发生以下错误？

System.InvalidOperationException: 'A second operation started on this context before a previous operation completed. This is usually caused by different threads 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.'

Answer 1

编辑过 2020年8月
官方指导:https://docs.microsoft.com/ca-es/aspnet/core/blazor/blazor-server-ef-core?view=aspnetcore-3.1有几种解决方案。我认为，发布的最佳方法是“创建新的DbContext实例”:

The recommended solution to create a new DbContext with dependencies is to use a factory.

//The factory
using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;

namespace BlazorServerDbContextExample.Data
{
    public class DbContextFactory<TContext> 
        : IDbContextFactory<TContext> where TContext : DbContext
    {
        private readonly IServiceProvider provider;

        public DbContextFactory(IServiceProvider provider)
        {
            this.provider = provider;
        }

        public TContext CreateDbContext()
        {
            if (provider == null)
            {
                throw new InvalidOperationException(
                    $"You must configure an instance of IServiceProvider");
            }

            return ActivatorUtilities.CreateInstance<TContext>(provider);
        }
    }
}

注塑厂:

services.AddDbContextFactory<ContactContext>(opt =>
    opt.UseSqlite($"Data Source={nameof(ContactContext.ContactsDb)}.db")
    .EnableSensitiveDataLogging());

使用工厂:

private async Task DeleteContactAsync()
{
    using var context = DbFactory.CreateDbContext();

    Filters.Loading = true;

    var contact = await context.Contacts.FirstAsync(
        c => c.Id == Wrapper.DeleteRequestId);

    if (contact != null)
    {
        context.Contacts.Remove(contact);
        await context.SaveChangesAsync();
    }

    Filters.Loading = false;

    await ReloadAsync();
}

我不赞成的答案:
您可以尝试为每个请求创建一个新范围:

public class MyViewModel : INotifyPropertyChanged
{
    
    protected readonly IServiceScopeFactory _ServiceScopeFactory;

    public MyViewModel(IServiceScopeFactory serviceScopeFactory)
    {
        _ServiceScopeFactory = serviceScopeFactory;
    }

    public async Task<IEnumerable<Dto>> GetList(string s)
    {
        using (var scope = _ServiceScopeFactory.CreateScope())
        {
            var referenceContext = scope.ServiceProvider.GetService<MyContext>();    
            return await _myContext.Table1.where(....)....ToListAsync();
        }
    }

在以下屏幕截图中，您可以看到此问题的示例案例。用户在几个分页元素中快速单击。在上一个请求结束之前，将开始一个新请求。

丹尼尔·罗斯(Blazor产品经理)在这里谈论 Using Entity Framework Core with Blazor