在下面的代码中,我将一个struct传递给一个期望一个类的构造函数.为什么编译运行没有错误(并产生所需的输出)?
class Program { static void Main() { var entity = new Foo { Id = 3 }; var t = new Test<IEntity>(entity); // why doesn't this fail? Console.WriteLine(t.Entity.Id.ToString()); Console.ReadKey(); } } public class Test<TEntity> where TEntity : class { public TEntity Entity { get; set; } public Test(TEntity entity) { Entity = entity; } public void ClearEntity() { Entity = null; } } public struct Foo : IEntity { public int Id { get; set; } } public interface IEntity { int Id { get; set; } }
如果我更改我的Main()方法,使其包含对ClearEntity()的调用,如下所示,它仍然不会产生错误.为什么?
static void Main() { var entity = new Foo { Id = 3 }; var t = new Test<IEntity>(entity); Console.WriteLine(t.Entity.Id.ToString()); t.ClearEntity(); // why doesn't this fail? Console.ReadKey(); }
解决方法
其中TEntity:class强制TEntity为引用类型,但IEntity等接口是引用类型.
看这里:
http://msdn.microsoft.com/en-us/library/d5x73970(v=vs.80).aspx
where T : class | The type argument must be a reference type,including any class,interface,delegate,or array type
关于你的第二个问题,你可能会认为t.ClearEntity()会失败,因为它为类型为值类型的变量赋值null,但事实并非如此.编译时类型的Entity是引用类型IEntity,运行时类型(赋值后)是null类型.所以你永远不会有Foo类型的变量但值为null.