如何从返回“的Guid'';的Guid＆GT;可空＆LT”？Guid、LT、GT
我想返回一个的Guid 值以下。但是数据库（和我的dbml 的）有一个列的可空的Guid ，它是产生于例外。选择部分说，它不能从的IQueryable＆LT转换;的System.Guid＆GT; 到的System.Guid 。
我猜我需要让我的返回值的的具体的的第一????真的吗？ 如果是的话我怎么做，与的GUID？
公共静态的Guid GetCurrentWorkerByType（INT enrollmentID，INT staffTypeID）
{
使用（VAR上下文= CmoDataContext.Create（））
{
IQueryable的＆LT; tblWorkerHistory＆GT; tWorkHist = context.GetTable＆其中; tblWorkerHistory＆GT;（）;
返程（tWorkHist.Where（workHist =＆GT;
（workHist.EnrollmentID == enrollmentID）及＆安培;
（workHist.tblStaff.StaffTypeID == staffTypeID）及＆安培;
（workHist.EndDate == NULL || workHist.EndDate＆GT; DateTime.Now））
。选择（workHist =＆GT; workHist.Worker））;
}
}
}
解决方案
//获取的GUID？本身，对于为了举例从IQueryable的＆所述;的Guid＆GT？;
//（可能是`null`从DB值，或因为可查询的是空）
GUID？ maybeGuid = queryable.FirstOrDefault（）;
//需要有* A * GUID或默认情况下它的东西
//因为一个GUID是一个价值型和需求* A *值。
GUID theGuid = maybeGuid？ Guid.Empty;
另请参见可空＆LT; T＆GT; .HasValue /值 - 较长，但等效，方法是：
的Guid theGuid = maybeGuid.HasValue？ maybeGuid.Value：Guid.Empty;
观察到 hasValue的可适合一般如果语句来更改逻辑和还指出，值如果maybeGuid是没有价值，将抛出一个异常 - 是空 - 这就是为什么保护是必需的
快乐编码。
迂腐的细节：等效方法是不是线程安全。也就是说，假设 maybeGuid 是共享的，它可以被分配空 hasValue的和值。有一些做题，涵盖了线程安全 ?? （聚结运营商） - 产生IL有效地使用一个临时变量，因此该数值可能陈旧但异常不能被抛出。
I am trying to return a Guid value below. However the database(and my dbml) has that column as a nullable Guid and it is generating an exception on the .Select portion saying it can't convert from an IQueryable<System.Guid?> to a System.Guid.
I am guessing I need to make my return value "concrete" first???? True? If so how do I do that with Guid's?
public static Guid GetCurrentWorkerByType(int enrollmentID, int staffTypeID)
{
using (var context = CmoDataContext.Create())
{
IQueryable<tblWorkerHistory> tWorkHist = context.GetTable<tblWorkerHistory>();
return (tWorkHist.Where(workHist =>
(workHist.EnrollmentID == enrollmentID) &&
(workHist.tblStaff.StaffTypeID == staffTypeID) &&
(workHist.EndDate == null || workHist.EndDate > DateTime.Now))
.Select(workHist => workHist.Worker));
}
}
}
解决方案
// Get the Guid? itself, for sake of example from IQueryable<Guid?>
// (could be `null` from DB value or because queryable was empty)
Guid? maybeGuid = queryable.FirstOrDefault();
// Need to have *a* GUID or default it to something
// because a Guid is a value-type and needs *a* value.
Guid theGuid = maybeGuid ?? Guid.Empty;
Also see Nullable<T>.HasValue/Value -- A longer, but equivalent, method would be:
Guid theGuid = maybeGuid.HasValue ? maybeGuid.Value : Guid.Empty;
Observe that HasValue may be suitable in general if statements to change logic and also note that Value will throw an exception if maybeGuid is "has no value" -- is null -- which is why the guard is required.
Happy coding.
Pedantic detail: The equivalent method is not "thread safe". That is, assuming maybeGuid was shared, it could be assigned null between HasValue and Value. There are a number of SO questions that cover the "thread safety" of ?? (the coalesce operator) -- the generated IL effectively uses a temporary variable so the value may be stale but an exception can't be thrown.
