c#-EF 6-在SaveChangesAsync方法中异步执行SQL命令会引发NotSupportedException

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我已经重写了DbContext的SaveChangesAsync方法来调用一堆存储过程.首先,我调用DbContext的SaveChangesAsync,然后为每个更改的实体执行一个存储过程.

等待所有异步方法调用.

这是EF抛出的异常：

System.NotSupportedException: 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.
   at System.Data.Entity.Internal.ThrowingMonitor.EnsureNotEntered()
   at System.Data.Entity.Core.Objects.ObjectContext.ExecuteStoreCommandAsync(TransactionalBehavior transactionalBehavior, String commandText, CancellationToken cancellationToken, Object[] parameters)
   at System.Data.Entity.Internal.InternalContext.ExecuteSqlCommandAsync(TransactionalBehavior transactionalBehavior, String sql, CancellationToken cancellationToken, Object[] parameters)
   at System.Data.Entity.Database.ExecuteSqlCommandAsync(TransactionalBehavior transactionalBehavior, String sql, CancellationToken cancellationToken, Object[] parameters)
   at System.Data.Entity.Database.ExecuteSqlCommandAsync(String sql, CancellationToken cancellationToken, Object[] parameters)
   at System.Data.Entity.Database.ExecuteSqlCommandAsync(String sql, Object[] parameters)
   at Common.Dal.AuditDbContext.<>c__DisplayClass20_0.<WriteAuditsParallelAsync>b__0(Audit x) in 
   at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()
   at System.Threading.Tasks.Task.WhenAll[TResult](IEnumerable`1 tasks)
   at Common.Dal.AuditDbContext.<WriteAuditsParallelAsync>d__20.MoveNext() in
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at Common.Dal.AuditDbContext.<SaveToDatabaseAsync>d__18.MoveNext() in

这是我的代码：

public override Task<Int32> SaveChangesAsync( CancellationToken cancellationToken )
{
    var modified = this.GetModifiedOrDeletedEntities();
    var added = this.GetAddedEntities();

    var audits = AuditService.GetAudits( GetObjectStateManager(), modified );

    //Call SaveChangesAsync
    var result = await base.SaveChangesAsync( cancellationToken );

    audits.AddRange( AuditService.GetAudits( GetObjectStateManager(), added ) );

    //Call stored prcedures
    await WriteAuditsAsync( audits, user );

    return result;
}

private async Task WriteAuditsAsync(List<Audit> audits, String user)
{
    foreach ( var audit in audits)
    {
        try
        {
            ...
            //Execute SQL command 
            await Database.ExecuteSqlCommandAsync(myCommand, myParameters);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
    }
}

EF为什么会引发此异常？

解决方法:

查看异常,您可以看到正在WriteAuditsParallelAsync中调用Task.WhenAll,这意味着您正在同时启动多个异步操作,并且异步等待所有这些操作完成.

在EF上下文中,您不能同时执行多个操作,这就是抛出该错误的原因.

您可以通过一次开始并等待一次依次执行这些操作来解决此问题.

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