IS NOT operator in C#

In this case, wrap and check the boolean opposite:

if (!(err is ThreadAbortException))

Just change the catch block to:

catch(ThreadAbortException ex)
{
}
catch(Exception ex)
{
}

so you can handle ThreadAbortExceptions and all others separately.

More than likely what you ought to do in this circumstance is:

try
{
   // Do Something
}
catch (ThreadAbortException threadEx)
{
   // Do something specific
}
catch (Exception ex)
{
   // Do something more generic
}

You can have multiple catch blocks for a try. Always make sure to order them such that the most specific is on top, and the most generic (catch (Exception ex)) is last because the lookup order is from top to bottom, so if you put the catch (Exception ex) first, it will always be the only one to run.