[FIXED] How *exactly* does C# convert double to decimal?

Issue

The following C# code prints 0.1 - why?

var s = "0.1";
var dbl = Double.Parse(s);
var dcml = Convert.ToDecimal(dbl);
Console.WriteLine(dcml.ToString());

Isn't 0.01 not representable in binary, therefore it should print 0.100000001490116 ?

Solution

The value of dbl is precisely 0.1000000000000000055511151231257827021181583404541015625.

That's 0.1 to 17 significant digits.

The documentation for Convert.ToDecimal(Double) states:

The Decimal value returned by this method contains a maximum of 15 significant digits. If the value parameter contains more than 15 significant digits, it is rounded using rounding to nearest.

The conversion from Single (aka float) is documented to truncate earlier:

The Decimal value returned by this method contains a maximum of seven significant digits. If the value parameter contains more than seven significant digits, it is rounded using rounding to nearest.

If you call Convert.ToDecimal(Double) with a value initially converted from 0.1f, it will display 0.100000001490116:

double dbl = 0.1f;        
decimal dcml = (decimal) dbl;
Console.WriteLine(dcml);

Answered By - Jon Skeet

Answer Checked By - Gilberto Lyons (PHPFixing Admin)