Topic 5424 min readStructured Tutorial

C# String Formatting Tutorial

Quick Answer

String formatting in C# allows you to create well-structured and readable text by embedding variables and controlling their display format. Common techniques include composite formatting with String.Format, interpolated strings, and format specifiers for numbers, dates, and more.

Learning Objectives

Explain the purpose of String Formatting in a practical learning context.
Identify the main ideas, terms, and decisions involved in String Formatting.
Apply String Formatting in a simple real-world scenario or practice task.

Introduction to C# String Formatting

In C#, strings are a fundamental way to represent text. Often, you need to insert variable values into strings or format them for better readability.

String formatting helps you create dynamic and well-structured text outputs by embedding variables and controlling how they appear.

Good formatting makes your output clear and professional.

Composite Formatting with String.Format

The String.Format method allows you to create formatted strings by specifying placeholders and providing values to replace them.

Placeholders are indicated by curly braces with an index, like {0}, {1}, etc., which correspond to the arguments passed.

Syntax: String.Format("format string", args...)
Placeholders use zero-based indexes: {0}, {1}, etc.
Supports format specifiers to control output appearance.

Example of String.Format

Here is a simple example demonstrating String.Format usage.

Interpolated Strings

Introduced in C# 6, interpolated strings provide a more readable and concise way to format strings.

You prefix the string with a $ symbol and embed expressions directly inside curly braces.

Syntax: $"Text {expression} more text"
Expressions inside braces can be variables, method calls, or calculations.
Supports format specifiers after a colon inside braces.

Example of Interpolated Strings

This example shows how to use interpolated strings for cleaner code.

Common Format Specifiers

Format specifiers control how values like numbers, dates, and currencies appear in strings.

They are used after a colon inside placeholders or interpolated expressions.

N or n: Number with commas and decimals (e.g., N2 for 2 decimals)
C or c: Currency format
D or d: Decimal integer format
F or f: Fixed-point format
P or p: Percent format
X or x: Hexadecimal format
Date/time specifiers like yyyy, MM, dd for dates

Common Format Specifiers in C#
Specifier	Description	Example
N2	Number with 2 decimal places	1234.5678 -> 1,234.57
C	Currency format	1234.5 -> $1,234.50
D4	Decimal with at least 4 digits	42 -> 0042
F1	Fixed-point with 1 decimal	3.1415 -> 3.1
P

Choosing the Right Formatting Method

Both String.Format and interpolated strings achieve similar results, but interpolated strings are generally preferred for readability and ease of use.

Use format specifiers to tailor the output to your needs, especially for numbers and dates.

Use String.Format when working with older C# versions (<6).
Use interpolated strings for cleaner and more maintainable code.
Apply format specifiers to control precision and style.

Practical Example

This example inserts variables into a string using String.Format with placeholders.

This example uses an interpolated string with a currency format specifier.

Examples

Using String.Format

string name = "Alice";
int age = 30;
string message = String.Format("Name: {0}, Age: {1}", name, age);
Console.WriteLine(message);

This example inserts variables into a string using String.Format with placeholders.

Using Interpolated Strings

string name = "Bob";
double price = 1234.56;
string message = $"Customer: {name}, Total: {price:C}";
Console.WriteLine(message);

This example uses an interpolated string with a currency format specifier.

Best Practices

Prefer interpolated strings for clarity and simplicity.
Always use format specifiers to ensure consistent output formatting.
Avoid concatenating strings with + for complex formatting; use formatting methods instead.
Test formatted output with different data to avoid runtime errors.

Common Mistakes

Forgetting to match the number of placeholders with arguments in String.Format.
Using incorrect format specifiers causing runtime exceptions.
Not using format specifiers leading to inconsistent or unreadable output.
Concatenating strings inefficiently instead of using formatting.

Hands-on Exercise

Format a Date and Number

Write a C# program that outputs the current date in yyyy-MM-dd format and a double value formatted as currency.

Expected output: Current date in yyyy-MM-dd format and a currency formatted number.

Hint: Use DateTime.Now and the "yyyy-MM-dd" and "C" format specifiers.

Create a Greeting Message

Use interpolated strings to create a greeting message that includes a user's name and age.

Expected output: A greeting message with the user's name and age.

Hint: Use $"Hello {name}, you are {age} years old." syntax.

Interview Questions

What is the difference between String.Format and interpolated strings in C#?

Interview

String.Format uses placeholders and arguments to format strings, while interpolated strings embed expressions directly within the string using a $ prefix, making code more readable and concise.

How do you format a number as currency in C#?

Interview

You can format a number as currency using format specifiers like "C" in String.Format or interpolated strings, for example: $"{price:C}".

What is String Formatting, and why is it useful?

Beginner

String formatting in C# allows you to create well-structured and readable text by embedding variables and controlling their display format.