.net – How to change exception message of Exception object?

You create a new Exception (or – better – specific subtype) that has the new message (and pass the original exception as InnerException).

Eg.

throw new MyExceptionType("Some interesting message", originalException);

NB. If you really want to use Activator.CreateInstance you can use an overload that can be passed parameters, but different Exception derived types cannot be relied on to have an constructor overload for (message, innerException).