using System;

​

public class Program

{

  public static void Main()

  {

    const double MAX_SAFE_INT = 9007199254740991;

    Decimal maxD = Convert.ToDecimal(MAX_SAFE_INT.ToString());

    var rng = new Random(Environment.TickCount);

    for (var i = 0; i < 1000; i++) 

    { 

        double random = rng.NextDouble();

        double product = MAX_SAFE_INT * random;

        // converting via string to workaround the "15 significant digits" limitation for Decimal(Double)

        Decimal randomD = Decimal.Parse(String.Format("{0:F18}", random));

        Decimal productD = maxD * randomD;

        double converted = Convert.ToDouble(productD);

        if (Math.Floor(converted) != Math.Floor(product)) 

        {

          Console.WriteLine($"{maxD}, {randomD, 22}, products: decimal {productD, 32}, converted {converted, 20}, original {product, 20}");

        }

    }

  }

}

​