Use the OutputNamespace property for generating the top-level namespace.

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -103,7 +103,7 @@ namespace Cxxi.Generators.CSharp
 
             if (Options.GenerateLibraryNamespace)
             {
-                WriteLine("namespace {0}", SafeIdentifier(Driver.Options.LibraryName));
+                WriteLine("namespace {0}", SafeIdentifier(Driver.Options.OutputNamespace));
                 WriteStartBraceIndent();
             }