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C# String Interpolation

Introduction​

String interpolation is a feature in C# that allows you to insert variable values and expressions directly into a string literal. Introduced in C# 6.0, string interpolation provides a more readable and convenient syntax for creating formatted strings compared to traditional string concatenation or string formatting methods.

With string interpolation, you can embed expressions inside string literals using the $ symbol and curly braces {}. This makes your code more concise and easier to understand, especially when working with complex string construction.

Basic String Interpolation Syntax​

To use string interpolation:

  1. Prefix the string with the $ character
  2. Place expressions within curly braces {}
csharp
string name = "John";
int age = 30;

// Using string interpolation
string message = $"Hello, my name is {name} and I am {age} years old.";

Console.WriteLine(message);

Output:

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

Comparing String Interpolation with Other String Operations​

Let's compare string interpolation with other methods of combining strings:

csharp
string name = "Sarah";
int age = 25;

// String concatenation
string concatString = "Hello, my name is " + name + " and I am " + age + " years old.";

// String.Format method
string formatString = string.Format("Hello, my name is {0} and I am {1} years old.", name, age);

// String interpolation
string interpolatedString = $"Hello, my name is {name} and I am {age} years old.";

Console.WriteLine(concatString);
Console.WriteLine(formatString);
Console.WriteLine(interpolatedString);

All three approaches produce the same output, but string interpolation is generally more readable and less error-prone, especially with multiple variables.

Using Expressions in String Interpolation​

You can include any valid C# expression within the curly braces, not just variables:

csharp
int a = 5;
int b = 3;

Console.WriteLine($"The sum of {a} and {b} is {a + b}");
Console.WriteLine($"The product of {a} and {b} is {a * b}");
Console.WriteLine($"Is {a} greater than {b}? {a > b}");
Console.WriteLine($"The lowercase of {"HELLO"} is {"HELLO".ToLower()}");

Output:

The sum of 5 and 3 is 8
The product of 5 and 3 is 15
Is 5 greater than 3? True
The lowercase of HELLO is hello

Formatting Expressions in String Interpolation​

String interpolation supports formatting expressions using the same format specifiers used in string.Format(). The format is specified after a colon inside the curly braces:

csharp
double price = 123.456;
DateTime today = DateTime.Now;

// Currency formatting
Console.WriteLine($"The price is {price:C}");

// Number formatting with decimal places
Console.WriteLine($"The value with 2 decimal places: {price:F2}");

// Date formatting
Console.WriteLine($"Today is {today:d}");
Console.WriteLine($"The full date and time: {today:F}");

// Percentage
Console.WriteLine($"Percentage: {0.75:P}");

// Custom numeric format
Console.WriteLine($"Phone number: {5551234567:###-###-####}");

Output (may vary based on your regional settings):

The price is $123.46
The value with 2 decimal places: 123.46
Today is 7/15/2023
The full date and time: Saturday, July 15, 2023 2:30:45 PM
Percentage: 75.00%
Phone number: 555-123-4567

Alignment and Spacing in String Interpolation​

You can control the alignment and spacing of interpolated values using the format specifier's alignment component:

csharp
string name1 = "John";
string name2 = "Elizabeth";

// Right-align in a 10-character field
Console.WriteLine($"[{name1,10}]");
Console.WriteLine($"[{name2,10}]");

// Left-align in a 10-character field
Console.WriteLine($"[{name1,-10}]");
Console.WriteLine($"[{name2,-10}]");

Output:

[      John]
[Elizabeth]
[John      ]
[Elizabeth ]

Combining Alignment and Format Specifiers​

You can combine alignment with format specifiers:

csharp
double[] values = { 123.456, 78.9, 0.1234 };

Console.WriteLine("Value Table:");
Console.WriteLine("---------------------------------");
Console.WriteLine($"{"Number",-10}{"Currency",12}{"Percent",10}");
Console.WriteLine("---------------------------------");

foreach (double value in values)
{
    Console.WriteLine($"{value,-10:F2}{value,12:C}{value,10:P0}");
}

Output:

Value Table:
---------------------------------
Number      Currency    Percent
---------------------------------
123.46       $123.46       12%
78.90         $78.90        8%
0.12           $0.12        0%

Escaping Braces in String Interpolation​

If you need to include curly braces in your interpolated string, you can escape them by doubling them:

csharp
int value = 42;

// To display a single { or }, double it
Console.WriteLine($"The value is {{in brackets}}: {value}");

Output:

The value is {in brackets}: 42

Verbatim String Interpolation​

You can combine verbatim string literals (prefixed with @) and string interpolation:

csharp
string filePath = "C:\\Program Files\\MyApp";
string fileName = "data.txt";

// Without verbatim string
Console.WriteLine($"File: {filePath}\\{fileName}");

// With verbatim string
Console.WriteLine($@"File: {filePath}\{fileName}");

Output:

File: C:\Program Files\MyApp\data.txt
File: C:\Program Files\MyApp\data.txt

The verbatim string version ($@"...") doesn't require double backslashes, making it more readable for file paths.

Real-World Applications​

Building a Simple User Profile​

csharp
string BuildUserProfile(string name, int age, string occupation)
{
    return $@"
User Profile
------------------
Name: {name}
Age: {age}
Occupation: {occupation}
Created: {DateTime.Now:yyyy-MM-dd}
------------------";
}

string profile = BuildUserProfile("Alex Johnson", 28, "Software Developer");
Console.WriteLine(profile);

Output:

User Profile
------------------
Name: Alex Johnson
Age: 28
Occupation: Software Developer
Created: 2023-07-15
------------------

Generating HTML Content​

csharp
string GenerateHtmlCard(string title, string content, string imageUrl)
{
    return $@"<div class=""card"">
  <img src=""{imageUrl}"" alt=""{title}"">
  <div class=""container"">
    <h4><b>{title}</b></h4>
    <p>{content}</p>
  </div>
</div>";
}

string card = GenerateHtmlCard(
    "Mountain View", 
    "Beautiful scenery of mountains at sunset", 
    "https://example.com/mountains.jpg"
);

Console.WriteLine(card);

Building a SQL Query​

csharp
string BuildSqlQuery(string tableName, string[] columns, int limit)
{
    string columnList = string.Join(", ", columns);
    return $"SELECT {columnList} FROM {tableName} LIMIT {limit};";
}

string query = BuildSqlQuery("Users", new[] { "Id", "Name", "Email" }, 10);
Console.WriteLine(query);

Output:

SELECT Id, Name, Email FROM Users LIMIT 10;

Summary​

String interpolation in C# provides a clean, readable way to embed expressions within string literals. Key points to remember:

  • Prefix the string with $ to enable interpolation
  • Place expressions within curly braces {}
  • You can include any valid C# expression inside the braces
  • Format specifiers can be added after a colon (e.g., {value:C})
  • Alignment can be controlled with comma and a number (e.g., {value,10})
  • Combine with verbatim strings ($@"...") for multiline strings without escaping
  • Double curly braces to escape them ({{ and }})

String interpolation makes your code more readable and maintainable compared to string concatenation or the string.Format() method, especially when dealing with complex string construction.

Exercises​

  1. Create a program that asks a user for their name, age, and favorite color, then displays a formatted message using string interpolation.

  2. Write a method that takes a decimal price and a quantity, then returns a receipt-like string showing the item price, quantity, and total with proper currency formatting.

  3. Create a temperature converter that takes a temperature in Celsius and displays it in both Celsius and Fahrenheit using string interpolation with appropriate formatting.

  4. Build a method that generates a simple table of multiplication results for a given number, with proper alignment using string interpolation.

Additional Resources​


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