Convert Boolean to Int64

Hi, 
I trying to convert number to string, but it is giving error as "Input string is not in correct format". Please advice below is my code..
foreach (var item in uniqueEventIds) { string _return = string.Empty; // _return = Convert.ToInt64(1.12862898112862E+17).ToString(); // if i give value directly it is giving expected out : 112862898112862000 _return = Convert.ToInt64(item).ToString(); // if i give var item it is throwing error } Thanks