The following should work (you need to reference the "Microsoft Scripting Runtime" COM object of course):

    public byte[] GetReport(int Report_Type_Requested, object Report_Params)
    {
        Scripting.Dictionary dic = (Scripting.Dictionary)Report_Params;
        foreach (string key in dic)
        {
            object value = dic.get_Item(key);
        }
        ...
    }