String interpolation in C Sharp (C#)

Introduction

String interpolation helps you easily put values inside text without extra steps.

When you want to show a message with numbers or names inside it.

When you build sentences that change based on data.

When you want your code to be easier to read and write.

When you combine text and variables in one line.

When you want to avoid confusing plus signs (+) for joining text.

Syntax

C Sharp (C#)

string message = $"Hello, {name}! You have {count} new messages.";

Start the string with a $ sign before the opening quote.

Put variables or expressions inside curly braces { } where you want their values.

Examples

Insert a simple variable inside the text.

C Sharp (C#)

string name = "Anna";
string greeting = $"Hi, {name}!";

Show a number inside a sentence.

C Sharp (C#)

int apples = 5;
string message = $"You have {apples} apples.";

You can put simple math inside the braces.

C Sharp (C#)

int a = 3, b = 4;
string result = $"Sum is {a + b}.";

Format dates inside the string using a colon and format code.

C Sharp (C#)

DateTime today = DateTime.Now;
string date = $"Today is {today:MMMM dd, yyyy}.";

Sample Program

This program greets the user by name and tells how many new messages they have using string interpolation.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        string name = "Sam";
        int messages = 3;
        string output = $"Hello, {name}! You have {messages} new messages.";
        Console.WriteLine(output);
    }
}

Important Notes

You can put any expression inside the braces, not just variables.

Use @ before $ for multi-line interpolated strings: @$"..."

Be careful with curly braces if you want to show them literally: use double braces {{ or }}.

Summary

String interpolation makes combining text and values easy and clear.

Use $ before the string and put variables inside { }.

It works with variables, expressions, and formatting.

Practice

1. What does string interpolation in C# allow you to do?

easy

A. Use variables only outside of strings

B. Insert variables directly inside a string using { } with a $ prefix

C. Create strings without any variables

D. Concatenate strings using the + operator only

Solution

1. Step 1: Understand string interpolation syntax

   String interpolation uses a $ before the string and curly braces { } to insert variables inside the string.

2. Step 2: Compare options with this syntax

   Only Insert variables directly inside a string using { } with a $ prefix describes inserting variables inside strings using { } with a $ prefix.

3. Final Answer:

   Insert variables directly inside a string using { } with a $ prefix -> Option B

4. Quick Check:

   String interpolation = $ + {variable} [OK]

Hint: Look for $ and { } to spot string interpolation [OK]

Common Mistakes:

• Confusing concatenation (+) with interpolation
• Forgetting the $ before the string
• Using variables outside the string only

2. Which of the following is the correct syntax for string interpolation in C#?

easy

A. string message = $"Hello, {name}!";

B. string message = "Hello, {name}!";

C. string message = $'Hello, {name}!';

D. string message = 'Hello, {name}!';

Solution

1. Step 1: Identify correct string interpolation syntax

   In C#, string interpolation requires a $ before a double-quoted string and variables inside { }.

2. Step 2: Check each option

   string message = $"Hello, {name}!"; uses $ before double quotes and {name} inside, which is correct. The remaining options lack $ or use single quotes which is invalid for interpolation.

3. Final Answer:

   string message = $"Hello, {name}!"; -> Option A

4. Quick Check:

   $ + "string" + {variable} = correct syntax [OK]

Hint: Use $ before double quotes for interpolation [OK]

Common Mistakes:

• Using single quotes instead of double quotes
• Omitting the $ symbol
• Placing variables outside { }

3. What is the output of this C# code?

int apples = 5;
int oranges = 3;
string result = $"I have {apples + oranges} fruits.";
Console.WriteLine(result);

medium

A. I have 8 fruits.

B. I have {apples + oranges} fruits.

C. I have apples + oranges fruits.

D. I have 53 fruits.

Solution

1. Step 1: Evaluate the expression inside the interpolation

   The expression {apples + oranges} adds 5 + 3, which equals 8.

2. Step 2: Substitute the result into the string

   The string becomes "I have 8 fruits." and is printed.

3. Final Answer:

   I have 8 fruits. -> Option A

4. Quick Check:

   Expression inside { } is calculated before output [OK]

Hint: Calculate expressions inside { } before printing [OK]

Common Mistakes:

• Printing the expression text instead of its value
• Concatenating numbers as strings incorrectly
• Ignoring the + operator inside { }

4. Identify the error in this code snippet:

int age = 30;
string message = $"Your age is {age";
Console.WriteLine(message);

medium

A. age variable is not declared

B. Missing $ before the string

C. Missing closing brace } in the interpolation

D. Console.WriteLine is misspelled

Solution

1. Step 1: Check the interpolation braces

   The string has an opening { but no closing } after age, causing a syntax error.

2. Step 2: Verify other parts

   The $ is present, age is declared, and Console.WriteLine is correct.

3. Final Answer:

   Missing closing brace } in the interpolation -> Option C

4. Quick Check:

   Every { must have a matching } [OK]

Hint: Count { and } to avoid missing braces [OK]

Common Mistakes:

• Forgetting to close interpolation braces
• Removing $ accidentally
• Assuming variable is undeclared without checking

5. You want to display a price with two decimal places using string interpolation. Which code correctly formats the price variable?

double price = 12.5;
// Fill in the blank:
string output = $"Price: {___}";
Console.WriteLine(output);

hard

A. price:.2

B. price.ToFixed(2)

C. "{price:2}"

D. price:F2

Solution

1. Step 1: Understand C# string interpolation formatting

   Inside { }, you can add :F2 to format a number with 2 decimals.

2. Step 2: Check each option

   price:F2 uses {price:F2} which is correct. price:.2 is an invalid format specifier (requires :F2 for fixed decimals). price.ToFixed(2) uses ToFixed which is JavaScript, not C#. "{price:2}" is invalid syntax.

3. Final Answer:

   price:F2 -> Option D

4. Quick Check:

   Use :F2 inside { } to format decimals [OK]

Hint: Use :F2 inside { } to show 2 decimals [OK]

Common Mistakes:

• Using JavaScript methods like ToFixed in C#
• Trying to format outside the interpolation braces
• Incorrect format string syntax