using System.Numerics; 

计算 斐波那契数列（Fibonacci sequence），不受长整型位数限制。

编写  fibonacci.cs  如下

// C# program for Fibonacci Series
// using Dynamic Programming
using System;
using System.Numerics;

class fibonacci {

	static BigInteger fib(int n)
	{	
        BigInteger a = new BigInteger(0);
		BigInteger b = new BigInteger(1);
		BigInteger c = new BigInteger(0);

		int i;
		for (i = 2; i <= n; i++) {
			c = a + b;
            a = b;
            b = c;
		}
		return b;
	}

	// Fibonacci Series test
	public static void Main(string[] args)
	{
        if (args.Length <1){
            Console.WriteLine(" usage: fib.exe n ");
            return;
        }       
		int n;
        if (int.TryParse(args[0], out n)){
            if (n >1) Console.WriteLine(fib(n));
        } else {
            Console.WriteLine(" input n must be +int ");
        }
	}
}

where csc
C:\Windows\Microsoft.NET\Framework\v4.0.30319\csc.exe

编译  csc fibonacci.cs
fibonacci.cs(8,9): error CS0246: 未能找到类型或命名空间名称“BigInteger”

编译  csc  /r:System.Numerics.dll  fibonacci.cs

运行 fibonacci.exe 1000
参阅：https://www.geeksforgeeks.org/program-for-nth-fibonacci-number/