Why Do C# Double Formatting and Debugger Values Differ?

What's Happening When Formatting Doubles in C

When formatting doubles for output in C#, it's important to note that the output differs from the value displayed in the debugger. Here's why:

The Underlying Issue

Unlike C, where the output precision is determined by the format specifier, C# rounds all doubles to 15 significant decimal digits before applying any formatting. This discrepancy arises because C# prioritizes accuracy over the requested precision.

Visual Studio Debugger's Behavior

The Visual Studio debugger directly displays the internal binary representation of doubles, hence the discrepancy with the formatted output.

Solution: Custom Formatting

While C# lacks a built-in solution for exact decimal formatting, you can manually construct a string representation from the internal binary data. Alternatively, you can use third-party libraries like DoubleConverter from Jon Skeet, which provides a ToExactString method for precise decimal output.

Example

Using DoubleConverter to format a double to 20 decimal places:

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

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