如果抛出自定义异常，则重定向asp.net mvc

I need to globally redirect my users if a custom error is thrown in my application. I have tried putting some logic into my global.asax file to search for my custom error and if it's thrown, perform a redirect, but my application never hits my global.asax method. It keeps giving me an error that says my exception was unhandled by user code.I need to globally redirect my users if a custo