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尝试获取结果列表时出现等待/异步错误

asynchronous c#

Craig · 技术社区 · 6 年前

var accountId = context.Account.OrderBy(x => x.LastUpdateDate).ThenBy(x=>x.LastUpdateDate).FirstOrDefaultAsync(x => x.UserAccount.ExternalId == _jwt.HomeAccountId).ExternalId;
var transactionLineId = context.TransactionLine.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.Transaction.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId).ExternalId;
var transactionId = context.Transaction.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId).ExternalId;
var budgetId = context.Budget.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.UserAccount.ExternalId == _jwt.HomeAccountId).ExternalId;
var scheduleId = context.Schedule.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId).ExternalId;

然后,等待每次重复并传递给一个方法。

var guids = new List<Guid> { await accountId, await transactionLineId, await transactionId, await budgetId, await scheduleId };

var checkGuid = MungeTwoGuids(guids);

找不到“Task”类型的参数(是否缺少

这发生在第一行。我想我可以把所有的选择。。。。一下子,基本上。。。然后等待所有的回应。而不是打电话,等等,打电话,等等。。。。

这不是实现我想要做的事情的正确方法吗?不是的,因为我有一个错误,但是我对等待的理解是不是走错了方向?

await accountId.Result.ExternalId,

0 回复 | 直到 6 年前

Stefan 6 年前

你的 FirstOrDefaultAsync 返回一个 Task<T> ,不是 T 因此没有 ExternalId .

通常有两种选择:

选择 Select 然后等待任务,或者
任务<T> 首先,结果将是 T型 ,包含您的属性以便您可以访问它。

但是,正如@GSerg所指出的:第一种选择并不适用,因为 executing parralell queries will result in a concurrency exception .

所以,在你的情况下,等待 FirstOrDefaultAsync 选择 , ?? 或者一个空检查来克服可能的空引用异常。

尽量避免 accountId.Result.ExternalId 因为它会使代码同步运行,而且会失去异步调用的好处。请参见: What is the difference between await Task<T> and Task<T>.Result?

Johan Donne 6 年前

您应该这样更改代码:

 var accountId = context.Account.OrderBy(x => x.LastUpdateDate).ThenBy(x=>x.LastUpdateDate).FirstOrDefaultAsync(x => x.UserAccount.ExternalId == _jwt.HomeAccountId);
 var transactionLineId = context.TransactionLine.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.Transaction.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId);
 var transactionId = context.Transaction.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId);
 var budgetId = context.Budget.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.UserAccount.ExternalId == _jwt.HomeAccountId);
 var scheduleId = context.Schedule.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId);


var guids = new List<Guid> { 
          (await accountId).ExternalId,
          (await transactionLineId).ExternalId,
          (await transactionId).ExternalId,
          (await budgetId).Externalid,
          (await scheduleId).EternalId };

var checkGuid = MungeTwoGuids(guids);

:上面的更改将解决最初的问题,但正如GSerg在下面的注释中指出的,您将得到一个异常,因为EF不允许在同一上下文上同时执行异步操作。

您可以通过以下方法解决此问题:

a) 立即等待每项任务

b) 如果您真的想要并行执行:为每个任务创建一个单独的上下文

Binara Thambugala 6 年前

请照下面的做。

var accountId = (await context.Account.OrderBy(x => x.LastUpdateDate).ThenBy(x=>x.LastUpdateDate).FirstOrDefaultAsync(x => x.UserAccount.ExternalId == _jwt.HomeAccountId)).ExternalId;
var transactionLineId = (await context.TransactionLine.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.Transaction.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId)).ExternalId;
var transactionId = (await context.Transaction.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId)).ExternalId;
var budgetId = (await context.Budget.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.UserAccount.ExternalId == _jwt.HomeAccountId)).ExternalId;
var scheduleId = (await context.Schedule.OrderBy(x => x.LastUpdateDate).ThenBy(x => x.LastUpdateDate).FirstOrDefaultAsync(x => x.CreditAccount.UserAccount.ExternalId == _jwt.HomeAccountId)).ExternalId

var guids = new List<Guid> {accountId, transactionLineId, transactionId, budgetId, scheduleId};
var checkGuid = MungeTwoGuids(guids);