C# String Format() Method

What You’ll Learn

The static method string.Format() builds a new string from a template with numbered placeholders like {0} and {1}. Each placeholder is replaced by a formatted value—perfect for messages, reports, currency, dates, and any output that mixes fixed text with variables.

Definition and Usage

In C#, string.Format() uses a composite format string. Curly braces mark where values go: {0} for the first argument, {1} for the second, and so on. The method returns one new string with every placeholder filled in.

You can add format specifiers inside placeholders to control how numbers and dates appear—for example {0:C} for currency or {0:N2} for a number with two decimal places. This is more readable than chaining + operators or manual rounding.

📝 Syntax

Common overloads on System.String:

public static string Format(string format, params object[] args);
public static string Format(IFormatProvider provider, string format, params object[] args);

Parameters

• format — template with placeholders {index} or {index:format}. Use {{ and }} for literal braces.
• args — values inserted at matching indices. null becomes an empty string.
• provider — optional culture/rules (e.g. CultureInfo.InvariantCulture) for number and date formatting.

Return Value

A new string with all placeholders replaced by formatted argument values.

Examples Gallery

Run with dotnet run. Each example shows a practical use of Format()—from simple placeholders to numbers, dates, interpolation comparison, and culture-aware output.

using System;

class Program {
    static void Main() {
        string name = "John";
        int age = 30;

        string message = string.Format(
            "Hello, my name is {0} and I'm {1} years old.",
            name, age);

        Console.WriteLine(message);
    }
}

Hello, my name is John and I'm 30 years old.

using System;

class Program {
    static void Main() {
        decimal price    = 49.5m;
        double completion = 0.875;

        string line1 = string.Format("Price:    {0:C}", price);
        string line2 = string.Format("Rounded:  {0:N2}", price);
        string line3 = string.Format("Progress: {0:P0}", completion);

        Console.WriteLine(line1);
        Console.WriteLine(line2);
        Console.WriteLine(line3);
    }
}

Price:    $49.50
Rounded:  49.50
Progress: 88%

using System;

class Program {
    static void Main() {
        DateTime due = new DateTime(2026, 12, 25);

        string shortDate = string.Format("Due date: {0:d}", due);
        string longDate  = string.Format("Full:     {0:D}", due);
        string timeStamp = string.Format("Logged:   {0:yyyy-MM-dd HH:mm}", due);

        Console.WriteLine(shortDate);
        Console.WriteLine(longDate);
        Console.WriteLine(timeStamp);
    }
}

Due date: 12/25/2026
Full:     Thursday, December 25, 2026
Logged:   2026-12-25 00:00

using System;

class Program {
    static void Main() {
        string lang = "C#";
        int version = 12;

        string viaFormat = string.Format(
            "Language: {0}, Version: {1}", lang, version);

        string viaInterp = $"Language: {lang}, Version: {version}";

        Console.WriteLine(viaFormat);
        Console.WriteLine(viaInterp);
    }
}

Language: C#, Version: 12
Language: C#, Version: 12

using System;
using System.Globalization;

class Program {
    static void Main() {
        decimal amount = 1234.56m;

        string us = string.Format(
            CultureInfo.GetCultureInfo("en-US"),
            "US:  {0:C}", amount);

        string de = string.Format(
            CultureInfo.GetCultureInfo("de-DE"),
            "DE:  {0:C}", amount);

        string invariant = string.Format(
            CultureInfo.InvariantCulture,
            "Inv: {0:N2}", amount);

        Console.WriteLine(us);
        Console.WriteLine(de);
        Console.WriteLine(invariant);
    }
}

US:  $1,234.56
DE:  1.234,56 €
Inv: 1,234.56

🚀 Common Use Cases

• User messages — combine labels with dynamic names, counts, or status text.
• Financial output — format currency and percentages in invoices or reports.
• Log lines — structured text with timestamps and values: "[{0:HH:mm:ss}] {1}".
• Resource strings — templates in .resx files filled at runtime with Format.
• Console output — Console.WriteLine(format, arg0, arg1) uses the same composite formatting rules.

📝 Notes

• Placeholders use zero-based indices: {0}, {1}, …
• Escape literal braces by doubling them: {{ and }}.
• null arguments are rendered as empty strings, not the text "null".
• Wrong or missing indices throw FormatException at runtime.
• Alignment syntax: {0,10} right-align in 10 chars; {0,-10} left-align.
• Console.WriteLine("Hi {0}", name) follows the same composite formatting rules.

⚡ Optimization

Format() and string interpolation are both efficient for typical output. For many small concatenations in a tight loop, consider StringBuilder or collecting lines first. When the format string is constant and arguments are simple, interpolation is usually the most readable choice with no performance penalty. Use InvariantCulture when culture-independent output avoids repeated locale lookups in server code.

💡 Best Practices