what are PayPal CallBack parameters - asp.net

Question

Update 2:
I have found the right QueryString.
Update 1:
I think, QueryString should be the right type to use in this case.
But I tried to use it and found it's not the ASP.NET WebForm Response.QueryString instead, it is
Microsoft.Owin.QueryString.
Do I need to add some references in order to use the Response.QueryString?
Thank you for any feedback and help
I'm very close to the right direction, but need you help.
I have a PayPal callback method in my ASp.NET MVC controller called: PayPalCallBack(FormCollection form)
The problem is I cannot get any values(e.g. cm(custom), st(payment_status) etc.) from this form variable.
But when the method is finished, I did see the return url came with those values
https://www.mywebsite.net/Event/PayPalCallBack?tx=4KT34448LJ0584918&st=Completed&amt=0%2e11&cc=AUD&cm=3%2f23%2f2014%2011%3a00%3a00%20AM%7c3%2f23%2f2014%2012%3a00%3a00%20PM%7cWinTech%7cmr%7cWinston%7cFan%7cfranva008%40gmail%2ecom%7c0425620666%7c1&item_number=
So, I guess, I am using a wrong type of variable to receive its return values.
My question are
1. what should be the right type of parameter used in my PayPalCallBack( values)?
what parameters can the return_url have?
I am so close to the last step, my friends please help me out.
Note: I am asking the return values/parameters in return_url, not the notify_url which is used in PayPal IPN. Thanks
Thank you!