Question

Will both catch blocks be hit?

try
{
    DoSomething();
}
catch (AuthenticationException e)
{
    throw;
}
catch (Exception e)
{
    throw new AuthenticationException("Hello ");
}

Answer 1

It’s valid (in that it will compile, build and run), but it’s not good practice.

Catching the general exception only to rethrow a specific exception will cause problems. If nothing else you are destroying the original exception call stack.

In response to your edit, both catch blocks won’t be hit.

If AuthenticationException is raised the first block will be executed, for any other exception the second block will be executed.