c# - Convert float to double loses precision but not via ToString

Question

I have the following code:

float f = 0.3f;
double d1 = System.Convert.ToDouble(f);
double d2 = System.Convert.ToDouble(f.ToString());

The results are equivalent to:

d1 = 0.30000001192092896;
d2 = 0.3;

I'm curious to find out why this is?

Answer 1

Its not a loss of precision .3 is not representable in floating point. When the system converts to the string it rounds; if you print out enough significant digits you will get something that makes more sense.

To see it more clearly

float f = 0.3f;
double d1 = System.Convert.ToDouble(f);
double d2 = System.Convert.ToDouble(f.ToString("G20"));

string s = string.Format("d1 : {0} ; d2 : {1} ", d1, d2);

output

"d1 : 0.300000011920929 ; d2 : 0.300000012 "