Hi gokuldas,
Add the System.Numerics.dll reference to the project.
Right click your project, click Add Reference.
Then from Reference Manager dialog window, expand Assemblies node and look for System.Numerics.
Then check the CheckBox and click Ok.
Then use this in your code.
C#
System.Numerics.BigInteger i = System.Numerics.BigInteger.Parse("11111111111111111111111111111111111111");
VB.Net
Dim i As System.Numerics.BigInteger = System.Numerics.BigInteger.Parse("11111111111111111111111111111111111111")