Action lambda 中的逆变 - C#
我有一个像这样的类层次结构，
public abstract class CalendarEventBase{}
public class TrainingEvent : CalendarEventBase{}
public class AuditEvent : CalendarEventBase{}
我想创建一个动作 Action lamda，它有一个 CalendarEventBase 类型的通用类型参数，我可以将其分配给以下不同的方法：
public void EmailCancelation(TrainingEvent trainingEvent)
public void EmailCancelation(AuditEvent auditEvent)
我创建了以下非法分配：
Action<CalendarEventBase> emailCancelation = _trainingService.EmailTrainingCancellation;
编译器抱怨它需要一个方法以 void(CalendarEventBase) 作为签名。我对此感到惊讶，因为我认为它会接受更派生的类型。
为了解决这个问题，我创建了以下委托来完成我的任务：
public delegate void EmailCancelation<in T>(T calendarEvent) where T : CalendarEventBase;
我的问题是，我是否可以在不创建额外委托的情况下完成任务？我想我可以创建一个 Action 实例。
任何帮助或指示，非常感谢。
I have a class hierarchy like this
public abstract class CalendarEventBase{}
public class TrainingEvent : CalendarEventBase{}
public class AuditEvent : CalendarEventBase{}
I wanted to create an action Action lamda that had a generic type paramater of type CalendarEventBase that I could assign to the following different methods:
public void EmailCancelation(TrainingEvent trainingEvent)
public void EmailCancelation(AuditEvent auditEvent)
I created the following illegal assignment:
Action<CalendarEventBase> emailCancelation = _trainingService.EmailTrainingCancellation;
The compiler complains that it was expecting a method with void(CalendarEventBase) as a signature. I was surprised by this as I thought it would accept a more derived type.
To get round this, I created the following delegate that allows me to complete my task:
public delegate void EmailCancelation<in T>(T calendarEvent) where T : CalendarEventBase;
My question is, could I have completed the task without having to create an additional delegate? I thought I could just create an Action instance.
Any help or pointers, greatly appreciated.
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该行：
实际上期望的是协方差，而不是逆变。但这在逻辑上是没有意义的；该方法需要
TrainingEvent作为输入 - 如何将更通用类型 (CalendarEventBase) 传递给它？这是不合法的：
但这很好：
The line:
is actually expecting covariance, not contravariance. But that logically doesn't make sense; the method expects a
TrainingEventas input - how can you pass a more general type (CalendarEventBase) to it?This isn't legal:
but this is fine:
lambda 无法支持这一点，因为您声明的 emailCancelation 变量将接受 CalendarEventBase，但实际实现将仅接受 TrainingEvent。如果有人使用 AuditEvent 参数调用 emailCancelation 会发生什么？
The lambda can't support this, because the emailCancelation variable you've declared will accept a CalendarEventBase, but the actual implementation will only accept the TrainingEvent. What would happen if someone were to call emailCancelation with an AuditEvent parameter?