vbscript代码一段，谁帮忙写成c#

[Ts23]

function DecodeStr(str)
        return1 = "abcdef"
        Dim return2
        return2 = split("50, 70, 80, 90, 100, 110",",")
        returnstr = ""
        Dim k 
        Dim j 
        
        k=0
        j=1
        for i=1 to len(str)
            if k>ubound(return2) then k=0
            returnstr = returnstr & cstr(asc(mid(str,i,1)) - cint(return2(k)))
            k=k+1
        next
        
        dim returnstr1
        returnstr1 = ""
        for i=1 to len(returnstr) step 3
            if j>len(return1) then j=1
            returnstr1 = returnstr1 & chr(cint(mid(returnstr,i,3)) - Asc(mid(return1,j,1)))
            j = j+1
        next

        DecodeStr = returnstr1    
end function