C#泛型方法无法创建List <T>

Question

我有一个接口和两个类.

public interface IMyInterface { }
public class A : IMyInterface { }
public class B : IMyInterface { }

我有一个通用的方法,

private List<T> GetList<T>(DataTable table) 
            where T : class, IMyInterface
{
    ...
}

应该根据DataTable中的数据返回一个对象列表.所以,我在这个方法中创建了一个列表,我想在最后返回.我以为我可以做到以下几点,

private List<T> GetList<T>(DataTable table)
           where T : class, IMyInterface
{
   List<T> myList = new List<T>;

   // Now I thought I could easily add Objects based on T because,
   // both classes implement the interface

   if (typeof(T) == typeof(B))
   {
      myList.Add(new B());
   }
   else
   {
      myList.Add(new A());
   }

   return myList;
}

但编译器告诉我"参数类型A(B)不是可以指定的"!为什么不可分配？

---

好吧,或者我可以做以下事情,

private List<T> GetList<T>(DataTable table)
           where T : class, IMyInterface
{
   List<IMyInterface> myList = new List<IMyInterface>;

   // Now I can assign the Object's :)
   if (typeof(T) == typeof(B))
   {
      myList.Add(new B());
   }
   else
   {
     myList.Add(new A());
   }

   return myList as List<T>;
}

编译器没有抱怨,但return子句的结果始终为null.肯定有值myList.演员似乎失败了.有人请帮我更优雅地解决这个问题.

Answer 1

一种方法是添加new()约束.限制是您需要类型参数T的公共无参数构造函数.

private static List<T> GetList<T>(DataTable table) where T : class, IMyInterface, new()
{
    List<T> myList = new List<T>();
    T instance = new T();
    //access anything defined in `IMyInterface` here
    myList.Add(instance);
    return myList;
}