问题
Often when including namespaces or assemblies into my code, I often run into strange cases where a namespace is inherited from another, yet classes from the parent namespace are not available. For example, when using List<> or Dictionary<>, I use the System.Collections.Generic namespace. However, if I also want to use an IEnumerator, I also have to include the System.Collections namespace. Shouldn't System.Collections already be referenced by any member of System.Collections.Generic, as it is a child? Both namespaces also share the same assembly, mscorlib.dll.
Why is the parent namespace not included when the child is?
回答1:
Shouldn't System.Collections already be referenced by any member of System.Collections.Generic, as it is a child?
No. There's no inheritance between namespaces. A using directive only imports types from that namespace - it doesn't import types from namespaces starting with the given namespace, or namespaces included within the given namespace's name.
For example, after:
using System.Collections;
... that doesn't let you use List<T> as a simple name for System.Collections.Generic.List<T>, nor does it let you use Guid as a simple name for System.Guid.
The only exception to this is that if you're writing code within a namespace declaration of X.Y.Z, that implicitly imports namespaces X.Y and X (as well as X.Y.Z, of course). (In that example, X.Y is the enclosing namespace if X.Y.Z, but it's not an inheritance relationship.)
来源：https://stackoverflow.com/questions/23613887/namespace-assembly-and-inheritance-hierarchy-when-including