如何从运行solr备份solrconfig文件(how to backup solrconfig file from running solr)

 假设您可以执行以下操作： 
 
// NOT REAL CODE
public interface IMyInterface { } 
public class MyRealClass : IMyInterface { } 

...

public class Domain
{
    private List<MyRealClass> myList;

    public IList<IMyInterface> Interfaces { get { return (IList<IMyInterface>)this.myList; } }
}
 
 什么阻止此Domain类的用户执行此操作？ 
 
public class MyFakeClass : IMyInterface { } 

...

domain.Interfaces.Add(new MyFakeClass());
 
 显然这会有问题，因为myList实际上是一个List<MyRealClass> ，但是编译器无法确保只将MyRealClass实例添加到列表中（它需要运行时检查）。 换句话说，这种行为不是类型安全的，因此编译器不允许它。 
 
 您可以公开MyRealClass的IList / ICollection - 这是类型安全的，但不能确保只将MyRealClass添加到列表中。 
 
public class Domain
{
    private List<IMyInterface> myList;

    public IList<IMyInterface> Interfaces { get { return this.myList; } }
}
 
 或者，您可以公开IEnumerable的IMyInterface （或者像Zmbq建议的IReadOnlyList ） - 因为类型参数是协变的（参见泛型中的协方差和反演 ）。 虽然这不允许您向集合添加新项目。 
 
public class Domain
{
    private List<MyRealClass> myList;

    public IEnumerable<IMyInterface> Interfaces { get { return this.myList; } }
}
 
 另一种解决方案是实现您自己的实现IList<IMyInterface>的集合类，但如果用户尝试插入除MyRealClass之外的任何内容，则抛出异常。 这不是一个特别优雅的解决方案，实际上，它与简单地暴露IList<MyRealClass>没有任何不同。 

Suppose you could do something like this:
 
// NOT REAL CODE
public interface IMyInterface { } 
public class MyRealClass : IMyInterface { } 

...

public class Domain
{
    private List<MyRealClass> myList;

    public IList<IMyInterface> Interfaces { get { return (IList<IMyInterface>)this.myList; } }
}
 
What's to stop a user of this Domain class from doing this?
 
public class MyFakeClass : IMyInterface { } 

...

domain.Interfaces.Add(new MyFakeClass());
 
Obviously this would be problematic because myList is in reality a List<MyRealClass>, but the compiler cannot ensure that only instances of MyRealClass are added to the list (it would require a run-time check). In other words, this kind of behavior is not type-safe, so the compiler won't allow it.
 
You can expose a IList / ICollection of IMyInterface—which is type-safe, but doesn't ensure that only your MyRealClass is added to the list.
 
public class Domain
{
    private List<IMyInterface> myList;

    public IList<IMyInterface> Interfaces { get { return this.myList; } }
}
 
Alternatively, you can expose an IEnumerable of IMyInterface (or IReadOnlyList, as zmbq suggests)—since the type parameter is covariant (see Covariance and Contravariance in Generics). Although this would not allow you add new items to the collection.
 
public class Domain
{
    private List<MyRealClass> myList;

    public IEnumerable<IMyInterface> Interfaces { get { return this.myList; } }
}
 
Another solution would be to implement your own collection class that actually implements of IList<IMyInterface> but throws an exception if a user tries to insert anything other than a MyRealClass. This isn't a particularly elegant solution and in practical terms, it's not any different from simply exposing an IList<MyRealClass>.