此代码:
public interface IInter { }
public class Concrete : IInter { /*... body ...*/ }
var t = (List<IInter>)new List<Concrete>();
产生此错误:
Cannot convert type 'System.Collections.Generic.List<Concrete>' to 'System.Collections.Generic.List<IInter>'
为什么?我该如何克服它?我的目标是:
var t = new List<List<IInter>>()
{
new List<ConcreteA>(){/* ... data ... */},
new List<ConcreteB>(){/* ... data ... */},
// ...
new List<ConcreteX>(){/* ... data ... */},
};
感谢您的帮助。啊,我有点抽象的东西,让它更容易阅读......但我真正的问题是:
public class SingletonFactory<T> where T : IToken
{
private SingletonFactory() { }
private static SingletonFactory<T> _instance = new SingletonFactory<T>();
public static SingletonFactory<T> Instance { get { return _instance; } }
public T Produce(int position) { return (T)Activator.CreateInstance(typeof(T), position); }
public T Produce(int position, string token) { return (T)Activator.CreateInstance(typeof(T), position, token); }
}
然后:
var keywords = new Dictionary<string, SingletonFactory<IToken>>()
{
{ "abc", SingletonFactory<Abc>.Instance },
{ "xyz", SingletonFactory<Xyz>.Instance },
{ "123", SingletonFactory<Num>.Instance }
};
所以我觉得它要复杂得多......
我正在使用c#4.0
答案 0 :(得分:2)
var res = new List<Concrete>().Cast<IInter>().ToList();
答案 1 :(得分:2)
虽然List<Concrete>
实施List<IInter>
,但Concrete
不是IInter
。
您可以使用:
List<IInter> t = ConcreteList.Cast<IInter>().ToList();
答案 2 :(得分:2)
这称为 covariance ,C#支持通用接口而非泛型类的协方差(和逆变)。这是以下工作,但您的示例不是:
IEnumerable<IInter> e = new List<Concrete>();
ICollection<IInter> c = new List<Concrete>();
阵列也支持协方差:
IInter[] a = new Concrete[3];
答案 3 :(得分:2)
因为您无法将List<ConcreteA>
分配给List<IInter>
,否则您可以这样做:
concreteAList = newList<ConcreteA>();
List<Inter> interList = concreteAList as List<Inter>; // seems harmless
interList.Add(new ConcreteB()); // not allowable
你可以做:
var t = new List<List<IInter>>()
{
new List<IInter>(){/* ... fill with ConcreteAs ... */},
new List<IInter>(){/* ... fill with ConcreteBs ... */},
// ...
new List<IInter>(){/* ... fill with ConcreteXs ... */},
};
但我不知道这是否能达到你想要的效果。
答案 4 :(得分:1)
一种方法是:
IEnumerable<IInter> t = new List<Concrete>();
另一个是:
List<IInter> t = new List<Concrete>().ConvertAll(x => (IInter) x);
编辑,添加更好的样本以显示它有效(测试通过):
[Test]
public void CovarianceTest()
{
var concrete = new Concrete();
IEnumerable<IInter> t = new List<Concrete> { concrete };
Assert.IsTrue(new[] { concrete }.SequenceEqual(t));
List<IInter> t2 = new List<Concrete> { concrete }.ConvertAll(x => (IInter)x);
Assert.IsTrue(new[] { concrete }.SequenceEqual(t2));
}
答案 5 :(得分:1)
为您需要从SingletonFactory<T>
访问的方法创建一个协作界面。这将确保您的类型T输出类型安全。
public interface IToken { }
public class Abc : IToken { }
public class Xyz : IToken { }
public class Num : IToken { }
public interface ISingletonFactory<out T> where T : IToken
{
T Produce(int position);
T Produce(int position, string token);
}
public class SingletonFactory<T> : ISingletonFactory<T> where T : IToken
{
private SingletonFactory() { }
private static SingletonFactory<T> _instance = new SingletonFactory<T>();
public static SingletonFactory<T> Instance { get { return _instance; } }
public T Produce(int position) { return (T)Activator.CreateInstance(typeof(T), position); }
public T Produce(int position, string token) { return (T)Activator.CreateInstance(typeof(T), position, token); }
}
var keywords = new Dictionary<string, ISingletonFactory<IToken>>()
{
{ "abc", SingletonFactory<Abc>.Instance },
{ "xyz", SingletonFactory<Xyz>.Instance },
{ "123", SingletonFactory<Num>.Instance }
};