'reference' is an ambiguous reference
Your program contains using directives for two namespaces and your code references a name that appears in both namespaces.
The following sample generates CS0104:
using x; using y; namespace x { public class Test { } } namespace y { public class Test { } } public class a { public static void Main() { Test test = new Test(); // CS0104, is Test in x or y namespace? // try the following line instead // y.Test test = new y.Test(); } }