Function that returns random BigInteger between min and max [duplicate]

I need c# code that generates random BigInteger number between 2^64 and 99999999999999999999 Something like that:

BigInteger maxLimit = BigInteger.Parse("99999999999999999999");
BigInteger minLimit = BigInteger.Parse("18446744073709551617");
BigInteger bigInteger = maxLimit - minLimit;
Random randNum = new Random();
randNum.Next(minLimit, maxLimit); //cannot convert from 'System.Numerics.BigInteger' to 'int'