Storing UserID And Password Values From MainString To Variables

I have this string
mainString="/MyApp/ViewPage.aspx?userId=admin&password=1&id=975"

I am getting value in mainString using this code
Dim mainString As String = Request.QueryString("dest")

And I have two sting variables
1)strTempUsername
2)strTempPassword
In these variables I want to store userId and password values from mainString.

So for this requirement how I write logic in code
Dim arrArray() As String
'/MyApp/ViewPage.aspx?userId=admin&password=1&id=975"
strIndexq = mainString.IndexOf("?") ' Check QuestionMark Exist or not in mainString
If strIndexq > 0 Then
Dim strTemp As String
[Code] .....

Here I am getting userId and password but in this code I will get problem when after ? if userId and password starts wit capital letters and password maybe contain characters like "1&a"

And after getting userid and password. I want to remove userid and password from mainstring. So I want mainstring look like:
"/MyApp/ViewPage.aspx?id=975"