SSIS Script Task not changing values

I have a SSIS package that I am testing to see if a File Exists and if it does, then I do not want to continue the package.   I have written a Script Task inside a For each Loop that connects to my Data Flow task.   All works well for a new file.   But, when I am checking for an existing file, I dont get an error from the Script task but it does not return the correct value from what I learned in the Message Box window.  And, the Data Flow tasks kicks off, but this is the weird part, the For each Loop and the last task called File System task fails as they should!   I need to know how to make the Data Flow task fail.  

I have set up a User::Flag as a ReadWriteVariables in the script task to be a 1 when the file exists and 0 when it does not.   I also have a variable User::VarArchiveFullPath as a ReadOnlyVariables.   ( I used the C# 2010 version of the script)

Here is my script.   What am I doing wrong that causes the flag to not change and always send a success.  It never fails!


        public void Main()
        {

            string filefullpath = Dts.Variables["User::VarArchiveFullPath"].Value.ToString();
            try
            {
                if (File.Exists(filefullpath))
                {
                    Dts.Variables["User::Flag"].Value = true;
                }
                else
                {
                    Dts.Variables["User::Flag"].Value = false;
                }

                Dts.TaskResult = (int)ScriptResults.Success;
            }
            catch (Exception Ex)
            {
                MessageBox.Show(Ex.ToString());
                Dts.TaskResult = (int)ScriptResults.Failure;
            }
        }


Thanks in advance!