.NET rounding error in ToString("f2")

Question

Hello I have this code in C#:

float n = 2.99499989f;
MessageBox.Show("n = " + n.ToString("f2", CultureInfo.InvariantCulture));

And this code in C++:

float n = 2.99499989f;
printf("n = %.2f", n);

First one outputs 3.00.
Second one outputs 2.99.

I have no clue why this is happening.

Update:

I also tried Objective-C NSLog and the output is 2.99.

I needed to fix it fast so I used following method:

float n = 2.99499989f;
float round = (float)Math.Round(n, 2);
MessageBox.Show("round = " + round.ToString(CultureInfo.InvariantCulture));

This code shows 2.99, but computes round in double precision. I can't find Math.RoundF.

Answer 1

Using BitConverter.GetBytes and printing out the actual bytes produced shows that this is not a compiler difference - in both cases, the actual float value stored is 0x403FAE14, which this handy calculator tells me is the exact value

2.99499988555908203125

The difference therefore must lie in differing behaviours of printf and ToString. More than that I cannot immediately say.