泛型上的InvalidCastException [英] InvalidCastException on Generics

问题描述

从Java世界开始，泛型和C＃编程经常令人头疼。像这样：

$ $ p $接口ISomeObject {
类SomeObjectA：ISomeObject {}
类SomeObjectB：ISomeObject {}


interface ISomething< T>其中T：ISomeObject
{
T GetObject（）;
}
class SomethingA：ISomething< SomeObjectA>
{
public SomeObjectA GetObject（）{return new SomeObjectA（）; }
}
class SomethingB：ISomething< SomeObjectB>
{
public SomeObjectB GetObject（）{return new SomeObjectB（）; }
}


类SomeContainer
{

private ISomething< ISomeObject>东西;

public void SetSomething< T>（ISomething< T> s）其中T：ISomeObject
{
Something =（ISomething< ISomeObject>）s;



$ b class TestContainerSomething
{
static public Test（）
{
SomeContainer Container = new SomeContainer（）;
Container.SetSomething< SomeObjectA>（new SomethingA（））;


$ / code>

其结果为 InvalidCastException at Something =（ISomething< ISomeObject>）s; 。在Java中，这是可行的，我甚至可以使用泛型通配符<？> 。这在C＃中是不可能的。



虽然这只是我解释有问题的一个例子，但如何消除这种例外情况？唯一的主要限制是 SomeContainer 不能是一个泛型类



** Note **：有很多关于这个的问题，但是他们都没有（我能找到）在非泛型类中寻址泛型类成员。

** 更新 **

在方法 SetSomething 这些行：

  Console.WriteLine（s.GetType（）。IsSubclassOf（typeof（ISomething< SomeObjectA>）））; 
 Console.WriteLine（s.GetType（）。ToString（）+：+ s.GetType（）。BaseType.ToString（））; 
 foreach（var in s.GetType（）。GetInterfaces（））
 {
 Console.WriteLine（i.ToString（））; 
} 
  

这令我惊喜输出

  False 
 SomeThingA：System.Object 
 ISomething`1 [SomeObjectA] 
  

这就是为什么我得到这个异常的原因吗？

解决方案

Out 关键字将是一个修复，如果您的 ISomething 只有返回T的方法

  interface ISomething< out T>其中T：ISomeObject 
  

在创建通用接口时，可以指定接口之间是否存在隐式转换实例有不同类型的参数。

它被称为协方差和反变换

Eric Lippert有一个很好的 interface差异被使用

这里是我的代码，它对我的​​预期工作

 接口ISomeObject {} 
类SomeObjectA：ISomeObject {} $ b $ class SomeObjectB：ISomeObject {} 
 
 
 interface ISomething< out T>其中T：ISomeObject 
 {
 T GetObject（）; 
} 
 class SomethingA：ISomething< SomeObjectA> 
 {
 public SomeObjectA GetObject（）{return new SomeObjectA（）; } 
} 
 class SomethingB：ISomething< SomeObjectB> 
 {
 public SomeObjectB GetObject（）{return new SomeObjectB（）; } 
} 
 
 
类SomeContainer 
 {
 
 private ISomething< ISomeObject>东西; 
 
 public void SetSomething< T>（ISomething< T> s）其中T：ISomeObject 
 {
 Something =（ISomething< ISomeObject>）s; 
 
 
 
 $ b class TestContainerSomething 
 {
 static public Test（）
 {
 SomeContainer Container = new SomeContainer（）; 
 Container.SetSomething< SomeObjectA>（new SomethingA（））; 
} 
} 
  

Coming from the Java world, programming with generics and C# is often a headache. Like this one:

interface ISomeObject { }
class SomeObjectA : ISomeObject { }
class SomeObjectB : ISomeObject { }


interface ISomething<T> where T : ISomeObject
{
    T GetObject();
}
class SomethingA : ISomething<SomeObjectA>
{
    public SomeObjectA GetObject() { return new SomeObjectA(); }
}
class SomethingB : ISomething<SomeObjectB>
{
    public SomeObjectB GetObject() { return new SomeObjectB(); }
}


class SomeContainer
{

    private ISomething<ISomeObject> Something;

    public void SetSomething<T>(ISomething<T> s) where T : ISomeObject
    {
        Something = (ISomething<ISomeObject>)s;
    }
}


class TestContainerSomething
{
    static public void Test()
    {
        SomeContainer Container = new SomeContainer();
        Container.SetSomething<SomeObjectA>(new SomethingA());
    }
}

Which results into an InvalidCastException at Something = (ISomething<ISomeObject>)s;. In Java, this would work, and I could even use (if all else fails) the generics wildcard <?>. This is not possible in C#.

While this is just an example that I put together to explain the problematic, how can this exception be eliminated? The only main constraint is that SomeContainer cannot be a generic class

** Note ** : there are many questions about this, but none of them (that I could find) address a generic class member inside a non generic class.

** Update **

Inside the method SetSomething, I added these lines :

Console.WriteLine(s.GetType().IsSubclassOf(typeof(ISomething<SomeObjectA>)));
Console.WriteLine(s.GetType().ToString() + " : " + s.GetType().BaseType.ToString());
foreach (var i in s.GetType().GetInterfaces())
{
    Console.WriteLine(i.ToString());
}

which to my surprise output

False
SomeThingA : System.Object
ISomething`1[SomeObjectA]

Is this why I get this exception?

解决方案

Out keyword will be a fix, if your ISomething only have methods that return T

interface ISomething<out T> where T : ISomeObject

when creating a generic interface, you can specify whether there is an implicit conversion between interface instances that have different type arguments.

It is called Covariance and Contravariance

Eric Lippert have a good series of articles why we need to think about this, here interface variance is used

Here is my code, which works as expected for me

interface ISomeObject { }
class SomeObjectA : ISomeObject { }
class SomeObjectB : ISomeObject { }


interface ISomething<out T> where T : ISomeObject
{
    T GetObject();
}
class SomethingA : ISomething<SomeObjectA>
{
    public SomeObjectA GetObject() { return new SomeObjectA(); }
}
class SomethingB : ISomething<SomeObjectB>
{
    public SomeObjectB GetObject() { return new SomeObjectB(); }
}


class SomeContainer
{

    private ISomething<ISomeObject> Something;

    public void SetSomething<T>(ISomething<T> s) where T : ISomeObject
    {
        Something = (ISomething<ISomeObject>)s;
    }
}


class TestContainerSomething
{
    static public void Test()
    {
        SomeContainer Container = new SomeContainer();
        Container.SetSomething<SomeObjectA>(new SomethingA());
    }
}

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