Question

I’m having a hard time understand the process of rounding when I convert a decimal to a long.

For example

decimal pi = Convert.ToDecimal(Math.PI);
long d = 2534254324524352;
long dpi = Convert.ToInt64(pi * Convert.ToDecimal(d));
//I'd like to do the reverse to get the value of d as dd
long dd = Convert.ToInt64(Convert.ToDecimal(dpi) /pi);

In this particular example it works but sometimes when I try to get to number back it doesn’t work. When you convert a decimal to a long is there an exact way it’s rounded? Is there a way to control that behavior?

Thanks

Answer 1

Convert.ToInt64, like most default conversion methods, uses banker’s rounding, i.e. rounding to the nearest multiple of two. Here’s a demo – both 6.5 and 5.5 are rounded to 6.

Console.WriteLine("Rounding 5.5 to {0}", Convert.ToInt64(5.5D)); // 6
Console.WriteLine("Rounding 6.5 to {0}", Convert.ToInt64(6.5D)); // 6

You can change this behaviour by using Math.Floor or Math.Ceil on a decimal before converting it to a long, as seen here. Math.Floor is probably what you need.

Console.WriteLine("Rounding 5.5 to {0}", Convert.ToInt64(Math.Floor(5.5D))); // 5
Console.WriteLine("Rounding 6.5 to {0}", Convert.ToInt64(Math.Floor(6.5D))); // 6

So:

decimal pi = Convert.ToDecimal(Math.PI);
long d = 2534254324524352;
long dpi = Convert.ToInt64(Math.Floor(pi * Convert.ToDecimal(d)));
long dd = Convert.ToInt64(Math.Floor(Convert.ToDecimal(dpi) / pi));