Compiler Error CS1021

Integral constant is too large

A value represented by an integer literal is greater than UInt64.MaxValue.

The following sample generates CS1021:

// CS1021.cs  
class Program
{
    static void Main(string[] args)
    {
        int a = 18_446_744_073_709_552_000;
    }
}  

The following code also generates CS1021:

using System.Numerics;

class Program
{
    static void Main(string[] args)
    {
        var a = new BigInteger(18_446_744_073_709_552_000);
    }
}

For information about how to instantiate a System.Numerics.BigInteger instance whose value exceeds the range of the built-in numeric types, see the Instantiating a BigInteger Object section of the BigInteger reference page.