C# Language Specification

Uniqueness of aliases

Each compilation unit and namespace body has a separate declaration space for extern aliases and using aliases. Thus, while the name of an extern alias or using alias must be unique within the set of extern aliases and using aliases declared in the immediately containing compilation unit or namespace body, an alias is permitted to have the same name as a type or namespace as long as it is used only with the :: qualifier.

In the example

namespace N
{
public class A {}

public class B {}

}

namespace N

{
using A = System.IO;

class X
{

A.Stream s1; // Error, A is ambiguous

A::Stream s2; // Ok

}
}

the name A has two possible meanings in the second namespace body because both the class A and the using alias A are in scope. For this reason, use of A in the qualified name A.Stream is ambiguous and causes a compile-time error to occur. However, use of A with the :: qualifier is not an error because A is looked up only as a namespace alias.

10.