Introduction

If you want a numeric format string that always shows a sign, you need to define separate patterns for positive and negative values. In .NET custom numeric formatting, the positive section normally omits the plus sign unless you add it explicitly.

The Basic Pattern in .NET

A custom numeric format string can have up to three sections separated by semicolons:

• positive format
• negative format
• zero format

A simple sign-always pattern is:

"+0;-0;0"

Example:

1using System;
2
3Console.WriteLine((12).ToString("+0;-0;0"));
4Console.WriteLine((-12).ToString("+0;-0;0"));
5Console.WriteLine((0).ToString("+0;-0;0"));

Output:

+12
-12
0

This is the core solution when the environment supports .NET custom numeric format strings.

Add Decimal Places When Needed

If you want fixed decimals, include them in each section.

1using System;
2
3double a = 12.34;
4double b = -12.34;
5double c = 0.0;
6
7Console.WriteLine(a.ToString("+0.00;-0.00;0.00"));
8Console.WriteLine(b.ToString("+0.00;-0.00;0.00"));
9Console.WriteLine(c.ToString("+0.00;-0.00;0.00"));

Output:

+12.34
-12.34
0.00

This is especially useful for finance, deltas, and scientific output where sign visibility matters.

Thousands Separators Work Too

You can combine sign display with grouping separators.

1using System;
2
3int value = 1234567;
4Console.WriteLine(value.ToString("+#,0;-#,0;0"));

Output:

+1,234,567

The same idea works with more detailed patterns such as "+#,0.00;-#,0.00;0.00".

Why the Positive Section Must Be Explicit

Without a positive section, .NET uses the default positive formatting behavior, which does not include a plus sign.

For example:

Console.WriteLine((12).ToString("0;-0"));

Output:

12

That is correct behavior, but it is not what you want if explicit sign display is required.

So the plus sign belongs in the positive format section itself.

Zero Is Its Own Decision

The third section is optional. If you omit it, zero uses the positive format rules.

Example:

Console.WriteLine((0).ToString("+0;-0"));

This may output:

+0

If you want plain 0 instead, keep the third section:

"+0;-0;0"

That detail matters in dashboards and reports where showing +0 may look odd.

Culture Still Affects Separators

Custom numeric format strings still respect culture for decimal and thousands separators unless you override the culture.

1using System;
2using System.Globalization;
3
4double value = 1234.56;
5Console.WriteLine(value.ToString("+#,0.00;-#,0.00;0.00", CultureInfo.InvariantCulture));

If your output must always use . for decimals regardless of system locale, pass an explicit culture.

This Is Formatting, Not Arithmetic

Displaying a sign does not change the numeric value. It only changes the string representation.

That may sound obvious, but it matters when developers start trying to use formatted strings in later numeric processing. Keep numeric computation and text formatting separate.

Useful Scenarios

Always-show-sign formatting is common in:

• percentage deltas
• financial profit and loss displays
• scientific measurements
• debugging output for signed values

In these contexts, +5 communicates more clearly than plain 5.

Common Pitfalls

• Forgetting that the positive section must explicitly include +.
• Omitting the zero section and then being surprised by +0 output.
• Mixing culture-dependent separators with output that needs fixed formatting.
• Using the formatted string in later calculations instead of the underlying numeric value.
• Assuming standard numeric format strings will expose the plus sign automatically.

Summary

• In .NET custom numeric formatting, use separate sections for positive, negative, and optionally zero values.
• A simple sign-always pattern is "+0;-0;0".
• Extend the same idea for decimals and thousands separators.
• Add a third section if zero should not display with a plus sign.
• Treat this as display formatting only, not as part of the numeric computation itself.