I am not certain, but I think you were doing this in the session. It is not possible to create strong type variables in the session, only in functions or classes.
In your example if you had checked the value of String after the assignment I think you would have found that you had overwritten the class with a character value. We do not require the parenthesis enclosing for assigning to multiple values.
If you create your example in a function, you will get the results you expected. We need to a webcast on using the session. We have a major update to the session coming, probably right between Christmas and New Years.
As for the using and reference, our original design required the using, but we are changing this to match the C# expectation of being able to explicitly specify a class or method.
This will also be in the Christmas to New Years release.