I need to keep in the program number which is longer than long or Int64. Because if I use long it will return just 0.
You can use BigInteger in .Net 4.0
The BigInteger type is an immutable type that represents an arbitrarily large integer whose value in theory has no upper or lower bounds. The members of the BigInteger type closely parallel those of other integral types (the Byte, Int16, Int32, Int64, SByte, UInt16, UInt32, and UInt64 types). This type differs from the other integral types in the .NET Framework, which have a range indicated by their MinValue and MaxValue properties.
If you don't have the luxury of moving to .Net 4. then you can use an open source project to help you out named as IntX.
IntX is an arbitrary precision integers library written in pure C# 2.0 with fast - about O(N * log N) - multiplication/division algorithms implementation. It provides all the basic arithmetic operations on integers, comparing, bitwise shifting etc. It also allows parsing numbers in different bases and converting them to string, also in any base. The advantage of this library is fast multiplication, division and from base/to base conversion algorithms - all the fast versions of the algorithms are based on fast multiplication of big integers using Fast Hartley Transform which runs for O(N * log N * log log N) time instead of classic O(N^2).