如何在C＃中的类型安全集合中存储具有约束泛型类型的Action代理？ [英] How to store Action delegates with constrained generic type in a type safe collection in C#?

问题描述

假设我想将代理存储在这样的集合中：

  public void AddDelegate< T> ; action）where T：ISomething 
 {
 _delegates.Add（action）; 
} 
  

什么类型的 _delegates be？

尝试使用 IList< Action< ISomething>> _delegates;
导致错误消息 参数类型'System.Action< T>'不能分配给参数类型System.Action< ISomething> 为上述添加调用。但为什么它不起作用？编译器应该知道T必须是 ISomething 。
如果我不想通过使用例如，不想放松类型安全，我有什么选择一个 IList< object> 或参数化泛型类型 T ？

解决方案

_delegates 必须是类型 IList< Action< T> 。

因此，您必须添加< T>其中T：ISomething 到类。

或者，不要使用泛型和支持 Action< ISomething> / code>直接。

所以你的两个选项是：

  public class Delegates< T>其中T：ISomething 
 {
 private List< Action< T>> _delegates; 
 
 public void AddDelegate（Action< T> action）
 {
 _delegates.Add（action）; 
} 
} 
  

或

  public class Delegates 
 {
 private List< Action< ISomething>> _delegates; 
 
 public void AddDelegate（Action< ISomething> action）
 {
 _delegates.Add（action）; 
} 
} 
  

修改
正如Sehnsucht指出的，还有第三个选项：将 Action< T> 委托在一个 Action< ISomething> 委托中，然后OP可以实现他原本想要的。

原因是，虽然 T 是 ISomething 的子类型（实现者）， Action< T> 不是 Action< ISomething> 的子类型，实际上正如dcastro在他的回答中所解释的那样，实际上是相反的（尽管看起来是直觉的）。即使使用转换，也不能将 Action< T> 的实例添加到 Action< ISomething> 列表中。 / p>

Let's say I want to store delegates in a collection like this:

public void AddDelegate<T>(Action<T> action) where T : ISomething
{
   _delegates.Add(action);
}

What should the type of _delegates be?

Trying with IList<Action<ISomething>> _delegates; results in error message Argument type 'System.Action<T>' is not assignable to parameter type 'System.Action<ISomething>' for the Add call above. But why doesn't it work? The compiler should 'know' that T must be an ISomething. What are my options if I don't want to loose type safety by using e.g. a IList<object> or parameterize the whole class on the generic type T?

解决方案

_delegates has to be of type IList<Action<T>>.

Therefore you have to add the <T> where T : ISomething to the class.

Alternatively, do away with the generics and support Action<ISomething> directly.

So your two options are:

public class Delegates<T> where T : ISomething
{
    private List<Action<T>> _delegates;

    public void AddDelegate(Action<T> action)
    {
        _delegates.Add(action);
    }
}

Or

public class Delegates
{
    private List<Action<ISomething>> _delegates;

    public void AddDelegate(Action<ISomething> action)
    {
        _delegates.Add(action);
    }
}

Edit As Sehnsucht points out, there's a third option: wrap the Action<T> delegate inside an Action<ISomething> delegate and then the OP can achieve what he originally wanted.

The reason for this is that whilst T is a "subtype" (implementer) of ISomething, Action<T> is not a subtype of Action<ISomething>, in fact as dcastro explains in his answer, the opposite is actually true (despite that seeming counter-intuitive). Even with casting, you cannot add an instance of Action<T> to a list of Action<ISomething>.

这篇关于如何在C＃中的类型安全集合中存储具有约束泛型类型的Action代理？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！