How to Use Format in C# String: Simple Guide

In C#, use string.Format to insert variables into a string by placing placeholders like {0}, {1}, etc., inside the string. You pass the values as additional arguments, and string.Format replaces the placeholders with those values.

Syntax

The string.Format method uses a format string with placeholders and a list of values to insert. Each placeholder is written as {index[,alignment][:formatString]}, where:

• index: The zero-based position of the argument to insert.
• alignment (optional): Specifies the minimum width and alignment of the inserted value.
• formatString (optional): Defines how to format the value (like number or date formats).

csharp

string formatted = string.Format("Hello, {0}! Today is {1:dddd}.", "Alice", DateTime.Now);

Example

This example shows how to use string.Format to insert a name and a number formatted as currency into a string.

csharp

using System;

class Program
{
    static void Main()
    {
        string name = "Bob";
        double price = 123.45;
        string message = string.Format("Hello, {0}! Your total is {1:C2}.", name, price);
        Console.WriteLine(message);
    }
}

Output

Hello, Bob! Your total is $123.45.

Common Pitfalls

Common mistakes include:

• Using placeholders with indexes that don't match the number of arguments, causing runtime errors.
• Forgetting to use string.Format and trying to use placeholders directly in strings.
• Incorrect format strings causing unexpected output.

Always ensure the number of placeholders matches the arguments passed.

csharp

using System;

class Program
{
    static void Main()
    {
        // Wrong: Missing argument for {1}
        // string message = string.Format("Name: {0}, Age: {1}", "Alice"); // Throws exception

        // Right:
        string message = string.Format("Name: {0}, Age: {1}", "Alice", 30);
        Console.WriteLine(message);
    }
}

Output

Name: Alice, Age: 30

Quick Reference

Placeholder Syntax	Description	Example
{0}	Insert first argument	"{0}" => "Alice"
{1:C2}	Second argument as currency with 2 decimals	"{1:C2}" => "$123.45"
{0,10}	First argument right-aligned in 10 spaces	"{0,10}" => " Alice"
{0,-10}	First argument left-aligned in 10 spaces	"{0,-10}" => "Alice "

Key Takeaways

Use string.Format with placeholders like {0}, {1} to insert values into strings.

Placeholders must match the number and order of arguments passed to string.Format.

Format specifiers like :C2 or :dddd control how values appear (currency, date, etc.).

Avoid runtime errors by ensuring all placeholders have corresponding arguments.

Use alignment options to control spacing and text alignment inside formatted strings.