在C＃中是子类继承的基类的`using`指令？

Question

Let's say we have a base class Rectangle and a derived class Square :

namespace Shapes {
    using System.Foo;

    public class Rectangle {
        public Rectangle(int l, int w){}
    }
}

namespace Shapes {
   public class Square : Rectangle

   public Square(int l, int w){}
}

Does the Square class have to explicitly say that it is using System.Foo ? I'm getting erratic results. In one project the using directives seem to be inherited and in a web application they aren't.

Answer 1

using statements, in this context, don't compile to code -- they are helpers to make your code read cleaner for others. As a result, they are not "inherited".

So, to answer your question, your Square class needs to reference System.Foo - either with a using statement, or by using a fully qualified class name.

Answer 2

A using statement will only propagate to the next set of closing braces ( } ) from the level it was declared on within the same the file.

//From File1.cs
using System.Baz;
namespace Example
{
    using System.Foo;
    //The using statement for Foo and Baz will be in effect here.

    partial class Bar
    {
        //The using statement for Foo and Baz will be in effect here.
    }
}

namespace Example
{
    //The using statement for Baz will be in effect here but Foo will not.

    partial class Bar
    {
        //The using statement for Baz will be in effect here but Foo will not.
    }
}

//From File2.cs
namespace Example
{
    //The using statement for Foo and Baz will NOT be in effect here.
    partial class Bar
    {
        //The using statement for Foo and Baz will NOT be in effect here.
    }
}

Answer 3

using directives are only shared if the classes are in the same file and they are not nested in the classes themselves like in your example.

For instance:

using System.Foo;
namespace N
{
    class A {}
    class B {}
}

If this is all in one file, A and B can both use Foo .

Answer 4

using System.Foo;

namespace  Shapes {...

Importing should always be top most and not within a namespace. This will allow the entire structure of the class to rely on that import when needed.

Answer 5

I think everyone is missing the point when it comes to using directives. They really have nothing to do with the class at all. using directives in a code file (.cs, .vb, etc...) are not part of the classes defined within the file. They are used by the compiler to resolve namespaces when compiling.