Compiler Error CS1681

You cannot redefine the global extern alias

The global alias is already defined to include all unaliased references and therefore cannot be redefined.

Example

The following sample generates CS1681.

// CS1681.cs
// compile with: /reference:global=System.dll
// CS1681 expected

// try this instead: /reference:System.dll
class A
{
   static void Main() {}
}