Nested namespaces

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This is a deliberate rule of C#. If you do this:

namespace Frobozz
{
    namespace Magic
    {
        class Lamp {}
    }

    class Foo
    {
        Magic.Lamp myLamp; // Legal; Magic means Frobozz.Magic when inside Frobozz
    }
}

That is legal. But this is not:

namespace Frobozz
{
    namespace Magic
    {
        class Lamp {}
    }
}

namespace Flathead
{
    using Frobozz;
    class Bar
    {
        Magic.Lamp myLamp; // Illegal; merely using Frobozz does not bring Magic into scope
    }
}

The rule of C# that describes this is in section 7.6.2 of the C# 4 spec. This is a very confusing section; the bit you want is the paragraph near the end that says

Otherwise, if the namespaces imported by the using-namespace-directives of the namespace declaration contain exactly one type having name I…

The key point is that it says “exactly one type”, not “exactly one type or namespace“. We deliberately disallow you “slicing” a namespace name like this when you are outside of that namespace because it is potentially confusing. As others have said, if you want to do that sort of thing, fully qualify it once in a using-alias directive and then use the alias.

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