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For example, to calculate factorial 100! we can use data type BigInteger (which
is new as of .NET Framework 4.0 and is missing in the older .NET versions). 
This type represents an integer, which can be very large (for example 100,000
digits). There is no limit on the size of the numbers recorded in the class
BigInteger (as long as you have enough RAM).

In order to use BigInteger, add a reference from our project to the assembly
System.Numerics.dll (this is a standard .NET library for working with very 
large integers, which is not referenced by default by our VS projects). Adding 
a reference to it is done by right-clicking on the current project references 
in the Solution Explorer window of Visual Studio.

Search and choose the assembly System.Numerics.dll from the list.

Then we need to add "using System. Numerics; " before the beginning of the 
class of our program and replace decimal with BigInteger.

By BigInteger you can calculate 1000!, 10000! and even 100000! It will take
some time, but OverflowException will not occur. The BigInteger class is
very powerful but it works many times slower than int and long. For our
unpleasant surprise there is no class "big decimal" in .NET Framework, only
"big integer".