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

消息：

"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;
}

我尝试的：

我不知道我做错了什么，我尝试了不同的方法，比如：

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); 

我做错了什么？

Answer 1

对于那些有能力使用的人。NET 6，现在有一个并行。ForEachAsync（）

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

Answer 2

这是你的问题：

userLangs.ForEach(async

这将创建一个异步void方法，因为ForEach不理解异步委托。因此，ForEach的主体将并发运行，实体框架不支持并发异步访问。

将ForEach更改为ForEach，您应该表现良好：

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”指导。