using System;

public class Program

{

    public static void Main()

    {

        var a = decimal.MaxValue - 1;

        Console.WriteLine(a.Add(1m));

        Console.WriteLine(a.Add(0.1m));

    }

}

​

public static class DecimalExtensions

{

    public static decimal Add(this decimal a, decimal b)

    {

        var result = a + b;

        if (b != 0 && a == result)

            throw new InvalidOperationException("Precision loss!");

        return result;

    }

}