Math.Round issue in C#

Question

Below is my program with some sample values;

using System; 

class Geeks 
{ 

	public static void Main() 
	{ 
	
		double[] val = {18527.475,28527.475,38527.475}; 
		

		Console.WriteLine("Rounded values are:"); 
		
		foreach(double value in val) 
		
		
			Console.WriteLine("{0} == {1}", value, Math.Round(value, 2, 
											MidpointRounding.AwayFromZero)); 
			
	} 
}

I got the results below

Rounded values are:

18527.475 == 18527.47

28527.475 == 28527.48

38527.475 == 38527.48.

My question is why 18527.475 is rounding as .47 while others are rounding as .48?

Answer 1

Since the double type stores binary numbers, it may not be possible to accurately hold values below the decimal point. Let's add each value 1000 times.

double[] val = { 18527.475, 28527.475, 38527.475 };
foreach (double value in val) {
    double sumValue = 0;
    for (int i = 0; i < 1000; i++) {
        sumValue += value;
    }
    Console.WriteLine(sumValue);
}

18527474.999999903
28527475.00000046
38527475.00000072

Use decimal type.

decimal[] val = { 18527.475m, 28527.475m, 38527.475m };
Console.WriteLine("Rounded values are:");
foreach (decimal value in val)
    Console.WriteLine("{0} == {1}", value, Math.Round(value, 2,
                                    MidpointRounding.AwayFromZero));


Rounded values are:
18527.475 == 18527.48
28527.475 == 28527.48
38527.475 == 38527.48

Answer 2

A Double is stored in base 2. Decimal numbers can not accurately stored in base 2. Much like 1/3 can not be converted to decimal, several decimal values can not be converted to base 2.