A using clauses must precede all other namespace elements
Statements with a using clause must appear first in a namespace.
The following sample generates CS1529:
namespace x { namespace subspace { using Microsoft; class SomeClass { }; using Microsoft; // CS1529, place before class definition } using System.Reflection; // CS1529, place before namespace 'subspace' } using System; // CS1529, place at the beginning of the file