无法转换源类型到目标类型编译错误 [英] Cannot convert source type to target type compilation error

问题描述

我有这个简单的类

public class Program
{
    private static void Main(string[] args)
    { 
        ClassB<ClassA> objA = new ClassB<ClassA>();

        ClassB<ITestA<MyDTO>> objB = new ClassB<ClassA>();

    }
}


public class ClassB<T>
{
    ///some code here
}


public interface ITestA<T>
{
    ///some code here
}

public class MyDTO
{
    ///some code here
}

public class ClassA : ITestA<MyDTO>
{
    ///some code 
}

这行代码

ClassB<ITestA<MyDTO>> objB = new ClassB<ClassA>();

是给编译错误

is giving compilation error

Cannot implicitly convert type 'ClassB<ClassA>' to 'ClassB<ITestA<MyDTO>> 

由于ClassA的实现ITestA，我不知道为什么会这样给出一个编译错误。请帮我明白我做错了。

Since ClassA implements ITestA, I don't know why would this give a compilation error. Please help me understand what am I doing wrong.

谢谢，
也先

推荐答案

这是由于所谓的仿制药的一个相当复杂的功能的方差

It's due to a rather complex feature of generics called variance.

类是不变的，这意味着如果你声明 ClassB的< T> ，创建实例然后当：

Classes are invariant, which means that if you declare ClassB<T>, then when creating an instance:

ClassB<T1> obj = new ClassB<T2>

然后 T1 具有完全相同类 T2 。

您可以使用接口来解决这个问题，比如你的代码更改为以下，这会编译：

You can use interfaces to get around this, eg change your code to the following and it'll compile:

...
public class Program
{
    private static void Main(string[] args)
    {
        ClassB<ClassA> objA = new ClassB<ClassA>();

        IClassB<ITestA<MyDTO>> objB = new ClassB<ClassA>();

    }
}


public interface IClassB<out T>  // <- note the out before T
{
    //some code here
}
public class ClassB<T> : IClassB<T>
{
    //some code here
}
...

在这种情况下， IClassB 被声明为协变的，这意味着它可以处理被赋予一个派生类 T的，而不需要 T 本身。周围有使用协方差（和逆变）虽然，这就是为什么一般类是默认不变风险。

In this case, IClassB is declared as covariant, which means that it can handle being given a derived class of T, rather than needing T itself. There are risks around using covariance (and contravariance) though, which is why generic classes are invariant by default.

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