将其转换为泛型基类失败 [英] Casting to Generic base class failing

问题描述

我无法理解为什么我试图转换为泛型基类不工作。

代码的基本结构如下。

 接口ICmd 
 {
} 
 
类上下文
 {
} 
 
 class Cmd< TContext> ：ICmd 
 TContext：Context 
 {
} 
 
 class MyContext：Context 
 {
} 
 
 class MyCmd：Cmd< MyContext> 
 {
} 
  

现在我有一个ICmd实例，将它转换为Cmd，如下所示：

  var base = cmd as Cmd< Context> 
  

在执行此行后base始终为空。

$ b $

  var base = cmd as Cmd< MyContext> b 
改变转换只针对上下文而定。 - 这个作品？ 
  

希望我已经提供了足够的信息，这是一个协变变量问题吗？$ / $>

谢谢

解决方案

您可能想要使用共同与反变化。

问题在于 Cmd< Context> 不是 Cmd< / base>的基类; MyContext> 。

有关更详细的答案，请参阅最近的问题： C＃泛型继承问题

I am failing to see why my attempt to cast to a generic base class is not working.

The basic structure of the code is as follows.

interface ICmd
{
}

class Context
{
}

class Cmd<TContext> : ICmd
    where TContext : Context
{
}

class MyContext : Context
{
}

class MyCmd : Cmd<MyContext>
{
}

So now I have an instance of ICmd and want to cast it to Cmd as follows

var base = cmd as Cmd<Context>

base is always null after this line is executed.

changing cast to be specific for the context only and it works.

var base = cmd as Cmd<MyContext>       -- this works ???

Hope I have provided enough information, is this a covariance\contravariance issue?

Thanks

解决方案

You may want to use co- and contravariance.

The problem is that Cmd<Context> is not the base class of Cmd<MyContext>.

See this recent question for a more detailed answer: C# Generics Inheritance Problem

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