无法将类型 'app.a1<<string>>' 隐式转换为 'app.a1<<object>>'

Question

当我尝试编译时，我试图从以下代码中理解泛型和协方差。我收到类似

的错误

Error 1 Cannot implicitly convert type 'ConsoleApplication1.a1<<string>>' to 'ConsoleApplication1.a1<<object>>'

class a1<T>
{
    public   void Fmethod( T a)
    {
        Console.WriteLine(a.GetType().ToString());
    }
}

class Program
{
    static void Main(string[] args)
    {

        a1<string> aa1 = new a1<string>();
        a1<object> aa2 = new a1<object>();
        object b;
        aa2.Fmethod(b);

        aa1.Fmethod("aa");
        aa2 = aa1;
         Console.ReadLine();

    }
}

Answer 1

根据 docs:

Generic classes are invariant. In other words, if an input parameter specifies a List<BaseClass>, you will get a compile-time error if you try to provide a List<DerivedClass>.

应用于您的示例，因为您将 a1<string> 类型的对象分配给 a1<object> 类型的变量，所以会出现错误。

无法将类型 'app.a1<<string>>' 隐式转换为 'app.a1<<object>>'

Cannot implicitly convert type 'app.a1<<string>>' to 'app.a1<<object>>'

c#

generics

covariance

c#-4.0