How Can I Accurately Output the Exact Decimal Value of a Double in C#?

Formatting Doubles in C#

When outputting doubles, C# automatically rounds them to 15 significant digits before applying any formatting. This can lead to discrepancies between the formatted output and the actual value of the double.

Problem with String.Format:

The String.Format method in C# only affects the display of the formatted number. It does not adjust the precision of the underlying double value. As a result, String.Format("{0:F20}", i) will display 20 decimal places, but the actual value of the double may still have been rounded to 15 decimal places internally.

Example:

Consider the following code:

double i = 10 * 0.69;
Console.WriteLine(String.Format("  {0:F20}", i));

The output will be 6.90000000000000000000 , even though the actual value of i is closer to 6.89999999999999946709.

Solution:

To output the exact decimal value of a double, you can use Jon Skeet's DoubleConverter class. This class has a ToExactString method that returns the exact decimal representation of a double. You can then format the string as needed.

For example:

double i = 10 * 0.69;
Console.WriteLine(DoubleConverter.ToExactString(i));

This will output 6.89999999999999946709294817992486059665679931640625 , which is the exact decimal representation of i.

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