在上一个异步操作完成之前，在此上下文上启动了第二个操作

Question

留言：

"System.NotSupportedException was unhandled
Message: An unhandled exception of type 'System.NotSupportedException' occurred in mscorlib.dll
Additional information: A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe."

代码：

public async Task<IEnumerable<UserLangDTO>> ImportLang(int userId)
{
    var userLangs = new List<UserLangDTO>();
    using (FirstContext ctx = new FirstContext())
    {
        if (await (ctx.UserLang.AnyAsync(u => u.UserId == userId)) == false)
            //some exception here

        userLangs = await ctx.UserLang.AsNoTracking()
                                .Where(ul => ul.UserId == userId)
                                .Join(ctx.Language,
                                    u => u.LangID,
                                    l => l.LangID,
                                    (u, l) => new { u, l })
                                .Join(ctx.Level,
                                    ul => ul.u.LevelID,
                                    le => le.LevelID,
                                    (ul, le) => new { ul, le })
                                .Select(r => new UserLangDTO
                                {
                                UserId = r.ul.u.UserId,
                                Language = r.ul.l.Language,
                                Level = r.le.Level,
                                }).ToListAsync().ConfigureAwait(false);

    }
    using (SecondContext ctx = new SecondContext())
    {
        if ( await (ctx.UserLangs.AnyAsync(u => u.UserId == userId)) == true && userLangs.Any())
            ctx.UserLangs.RemoveRange(ctx.UserLangs.Where(u => u.UserId == userId));
        if (await hasUserLangs && userLangs.Any())
        {
            userLangs.ForEach(async l =>
            {
                var userLanguage = new UserLang();
                userLanguage.UserId = userId;
                userLanguage.LanguageId = await ctx.Languages.AsNoTracking()
                                                 .Where(la => la.NameEn == l.Language)
                                                 .Select(la => la.Id).FirstOrDefaultAsync().ConfigureAwait(false);
                userLanguage.LevelId = await ctx.Levels.AsNoTracking()
                                                .Where(la => la.NameEn == l.Language)
                                                .Select(la => la.Id).FirstOrDefaultAsync().ConfigureAwait(false);

                ctx.UserLangs.Add(userLanguage);
            });
        }
        await ctx.SaveChangesAsync().ConfigureAwait(false);
    }
    return userLangs;
}

我尝试过的：

我不确定我做错了什么，我尝试了不同的东西，例如：

1.

await Task.Run(() => Parallel.ForEach(strings, s =>
{
    DoSomething(s);
})); 

2.

var tasks = userLangs.Select(async l =>
{
    //rest of the code here
}
await Task.WhenAll(tasks); 

3.

var tasks = userLangs.Select(async l =>
{
    //rest of the code here
}
await Task.WhenAll(tasks);

await ctx.SaveChangesAsync().ConfigureAwait(false); 

4. 其他尝试和错误的尝试，我现在不记得了

我做错了什么？

Answer 1

这是你的问题：

userLangs.ForEach(async

这是创建一个

async void

方法，因为

ForEach

不理解异步委托。所以

ForEach

的主体会并发运行，而Entity Framework不支持并发异步访问。

将

ForEach

更改为

foreach

，你应该会很好：

foreach (var l in userLangs)
{
  var userLanguage = new UserLang();
  userLanguage.UserId = userId;
  userLanguage.LanguageId = await ...
}

有关更多信息，请参阅我的异步最佳实践文章中的“避免异步无效”指南。

Answer 2

对于那些有能力使用 .NET 6 的人来说，现在有一个

Parallel.ForEachAsync()

https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.parallel.foreachasync?view=net-6.0 https://www.hanselman.com/blog/parallelforeachasync-in-net-6