Question

我使用variance in delegate的{{3}}示例。但是下面的代码给了我一个编译错误。根据我的理解，它应该接受First作为参数。我究竟做错了什么？代码示例。

SampleGenericDelegate<Second, First> secdel = AFirstRSecond;
secdel(new First()); //compilation error here.

public class First { }
public class Second : First { }
public delegate R SampleGenericDelegate<A, R>(A a);
public static Second AFirstRSecond(First first)
{ 
    return new Second(); 
}

错误

Delegate 'ConsoleApplication1.SampleGenericDelegate<ConsoleApplication1.Second,
ConsoleApplication1.First>' has some invalid arguments

Argument 1: cannot convert from 'ConsoleApplication1.First' to    
'ConsoleApplication1.Second'

Answer 1

secdel的参数类型为Second，因此您需要传递一个实例：

SampleGenericDelegate<Second, First> secdel = AFirstRSecond;
secdel(new Second());

Answer 2

泛型定义中的第一种类型是用于定义委托参数的类型，而不是返回类型。改变你的代码，你应该做得很好。

SampleGenericDelegate<First, Second> secdel = AFirstRSecond;
secdel(new First()); //compilation error here.

public class First { }
public class Second : First { }
public delegate R SampleGenericDelegate<A, R>(A a);
public static Second AFirstRSecond(First first)
{ 
    return new Second(); 
}

通用委托逆向编译错误

2 个答案: