C#LINQ查询-无隐式转换错误
我已经为我的LINQ查询定义了以下实体类:C#LINQ查询-无隐式转换错误,c#,linq,entity-framework,C#,Linq,Entity Framework,我已经为我的LINQ查询定义了以下实体类: public class Application { public Application() { } public int Id { get; set; } public string Name { get; set; } public string Description { get; set; } public System.DateTime DateTimeCreated { get; set; }
public class Application
{
public Application() { }
public int Id { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public System.DateTime DateTimeCreated { get; set; }
public System.DateTime? DateTimeModified { get; set; }
public Employee CreatedBy { get; set; }
public Employee ModifiedBy { get; set; }
}
public class Employee
{
public Employee() { }
public string EmployeeID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
我创建了以下查询以创建应用程序
对象,并尝试为CreatedBy
和“ModifiedBy”属性创建Employee
实体。有时,ModifiedBy
列可能包含null,我想将ModifiedBy
属性设置为null
var query = from a in context.Applications
join u1 in context.Employees on a.CreatedBy equals u1.Employee_ID.Trim()
join u2 in context.Employees on a.ModifiedBy equals u2.Employee_ID.Trim()
where a.ApplicationId == applicationId
select new Entity.Application
{
Id = a.ApplicationId,
Name = a.ApplicationName,
Description = a.ApplicationDesc,
DateTimeCreated = a.DateTimeCreated,
CreatedBy = new Entity.Employee{ EmployeeID = a.CreatedBy, FirstName = u1.First_Name, LastName = u1.Last_Name },
DateTimeModified = a.DateTimeModified ?? null,
ModifiedBy = (a.ModifiedBy != null) ? new Entity.Employee { EmployeeID = a.ModifiedBy, FirstName = u2.First_Name, LastName = u2.Last_Name } : (Entity.Employee) null,
};
调试上面的查询时,出现以下错误:
无法确定条件表达式的类型,因为存在
“员工”和“应用程序”之间没有隐式转换
如何解决此错误?它可能不会直接解决您遇到的错误,但您根本不需要该查询,因为您已经设置了导航属性。改用
Include
,它应该可以正常工作-EF将为您加入必要的连接:
var query context.Applications
.Include("ModifiedBy")
.Include("CreatedBy")
.Where(a => a.ApplicationId == applicationId);
我注意到了几件事
join u1 in context.Employees on a.CreatedBy equals u1.Employee_ID.Trim()
join u2 in context.Employees on a.ModifiedBy equals u2.Employee_ID.Trim()
您不能这样加入,因为CreateBy
和ModifiedBy
属于Employee
类型,而不是string
另外,请看以下内容:
(Entity.Employee) null
您不能将null强制转换为员工
。您可能希望将来使用该类型的默认值:
default(Entity.Employee)
更新
正如评论中所指出的,将null强制转换为
Entity.Employee
,是合法的,但由于最终的结果是null,因此该操作没有多大意义default(Entity.Employee)
也会导致null,因为这是引用类型的默认值,但是default
可以为各种其他类型提供不同的值,这有时是有用的。经过一些额外的研究后,下面是经过修改的代码片段,它最终对我有用:
var result = (from a in context.Applications
join u1 in context.Employee on a.CreatedBy equals u1.Employee_ID.Trim()
join u2 in context.Employee on a.ModifiedBy equals u2.Employee_ID.Trim() into us
from u2 in us.DefaultIfEmpty()
where a.ApplicationId == applicationId
select new Entity.Application()
{
Id = a.ApplicationId,
Name = a.ApplicationName,
Description = a.ApplicationDesc,
DateTimeCreated = a.DateTimeCreated,
CreatedBy = new Entity.Employee
{
EmployeeID = a.CreatedBy,
FirstName = u1.First_Name,
LastName = u1.Last_Name
},
DateTimeModified = a.DateTimeModified ?? null,
ModifiedBy = new Entity.Employee
{
EmployeeID = a.ModifiedBy ?? string.Empty,
FirstName = u2.First_Name ?? string.Empty,
LastName = u2.Last_Name ?? string.Empty
}
}).Single();
什么是字段/属性
ApplicationId
——应该是Id
?因此,如果删除这一行,上面的查询可以工作吗<代码>ModifiedBy=(a.ModifiedBy!=null)?new Entity.Employee{EmployeeID=a.ModifiedBy,FirstName=u2.First\u Name,LastName=u2.Last\u Name}:(Entity.Employee)null,@nate-是的,当我删除了你评论中引用的行时,我已经克服了这个错误。我同意加入,但你肯定可以将null
转换为实体。Employee
。的确如此!我一定是在C之前的日子里提出了一些假设!上下文应用程序表中的CreatedBy
和ModifiedBy
列定义为字符串。我正在执行连接以从上下文employee
view@MichaelKniskern为什么在您的应用程序
模型中会出现Employee
类型的字段?当我尝试查看此查询的结果视图时,我在调试模式下收到以下消息:无法计算子项。