C# Decimal ("D") Format Specifier

The "D" (decimal) format specifier in C# is used to format integer types as a string of decimal digits (0-9). It displays the number with optional zero-padding to achieve a specified minimum length.

Syntax

Following is the syntax for the "D" format specifier −

number.ToString("D")        // Basic decimal format
number.ToString("Dn")       // Decimal with minimum n digits

Where n is the minimum number of digits. If the number has fewer digits, it will be padded with leading zeros.

Parameters

• Precision specifier − An optional integer that specifies the minimum number of digits in the result string. If omitted, uses the minimum digits needed.

• Supported types − Integer types only (int, long, short, byte, etc.). Does not work with floating-point types.

Using Basic Decimal Format

The basic "D" format displays the number as-is without any padding −

using System;

class Program {
    static void Main() {
        int number = 467;
        Console.WriteLine("Basic D format: " + number.ToString("D"));
        
        long bigNumber = 123456789;
        Console.WriteLine("Long number: " + bigNumber.ToString("D"));
        
        short smallNumber = 42;
        Console.WriteLine("Short number: " + smallNumber.ToString("D"));
    }
}

The output of the above code is −

Basic D format: 467
Long number: 123456789
Short number: 42

Using Decimal Format with Padding

When you specify a precision, the result is padded with leading zeros to meet the minimum length −

using System;

class Program {
    static void Main() {
        int val = 877;
        Console.WriteLine("No padding: " + val.ToString("D"));
        Console.WriteLine("4 digits minimum: " + val.ToString("D4"));
        Console.WriteLine("8 digits minimum: " + val.ToString("D8"));
        
        int smallVal = 5;
        Console.WriteLine("Small number with D6: " + smallVal.ToString("D6"));
    }
}

The output of the above code is −

No padding: 877
4 digits minimum: 0877
8 digits minimum: 00000877
Small number with D6: 000005

Working with Negative Numbers

The "D" format specifier also works with negative integers, preserving the negative sign −

using System;

class Program {
    static void Main() {
        int negativeNumber = -123;
        Console.WriteLine("Negative with D: " + negativeNumber.ToString("D"));
        Console.WriteLine("Negative with D6: " + negativeNumber.ToString("D6"));
        Console.WriteLine("Negative with D8: " + negativeNumber.ToString("D8"));
    }
}

The output of the above code is −

Negative with D: -123
Negative with D6: -000123
Negative with D8: -00000123

Conclusion

The "D" format specifier in C# formats integer values as decimal digits with optional zero-padding. It's useful for creating fixed-width number representations, especially for IDs, codes, or when consistent formatting is required. Remember that it only works with integer types and preserves the negative sign when padding negative numbers.