String interpolation and formatting in C Sharp (C#) - Mini Project: Build & Apply

String interpolation and formatting

📖 Scenario: You are creating a simple program to display product information in a store. You want to show the product name, quantity, and price in a clear and friendly way.

🎯 Goal: Build a program that uses string interpolation and formatting to display product details with price shown as currency and quantity as a number.

📋 What You'll Learn

Create variables for product name, quantity, and price

Use a format string with string interpolation to combine these variables

Format the price as currency

Format the quantity as a number without decimals

Print the final formatted string

💡 Why This Matters

🌍 Real World

Displaying product information clearly in shopping apps or inventory systems helps customers and staff understand prices and stock easily.

💼 Career

Knowing how to format strings and numbers is essential for creating user-friendly interfaces and reports in software development.

1

Create product variables

Create a string variable called productName and set it to "Chocolate Bar". Create an integer variable called quantity and set it to 5. Create a double variable called price and set it to 1.99.

C Sharp (C#)

// Your code here

Hint

Use string for text, int for whole numbers, and double for decimal numbers.

2

Create a formatted message variable

Create a string variable called message that uses string interpolation to combine productName, quantity, and price. Format price as currency using :C and quantity as a number with no decimals using :N0 inside the interpolation.

C Sharp (C#)

string productName = "Chocolate Bar";
int quantity = 5;
double price = 1.99;
// Your code here

Hint

Use $"...{variable:format}..." to insert variables with formatting.

3

Add a method to create the message

Create a method called CreateMessage that returns a string. Inside it, return the same formatted string using productName, quantity, and price with the same formatting as before. Use the existing variables from the outer scope.

C Sharp (C#)

string productName = "Chocolate Bar";
int quantity = 5;
double price = 1.99;
string message = $"Product: {productName}, Quantity: {quantity:N0}, Price: {price:C}";
// Your code here

Hint

Define a method with string CreateMessage() and return the formatted string inside.

4

Print the formatted message

Use Console.WriteLine to print the message variable. Then print the result of calling the CreateMessage() method.

C Sharp (C#)

string productName = "Chocolate Bar";
int quantity = 5;
double price = 1.99;
string message = $"Product: {productName}, Quantity: {quantity:N0}, Price: {price:C}";

string CreateMessage()
{
    return $"Product: {productName}, Quantity: {quantity:N0}, Price: {price:C}";
}
// Your code here

Hint

Use Console.WriteLine(message); and Console.WriteLine(CreateMessage()); to print.

Practice

1. What does the following C# code do?
int age = 25;
string message = $"I am {age} years old.";

easy

A. Concatenates the string and integer without formatting.

B. Inserts the value of age into the string at the placeholder.

C. Causes a syntax error because of the dollar sign.

D. Creates a string with the text including curly braces literally.

Solution

1. Step 1: Understand string interpolation syntax

   The dollar sign $ before the string allows inserting variables inside curly braces.

2. Step 2: Identify variable insertion

   The variable age is inserted where {age} appears, replacing the placeholder with its value.

3. Final Answer:

   Inserts the value of age into the string at the placeholder. -> Option B

4. Quick Check:

   String interpolation = Insert variable value [OK]

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

Common Mistakes:

• Thinking $ means string concatenation
• Expecting curly braces to print literally
• Confusing interpolation with format method

2. Which of the following is the correct syntax for formatting a double value to show two decimal places using string interpolation in C#?

easy

A. double price = 9.99; string s = $"Price: {price:0.00}";

B. double price = 9.99; string s = $"Price: {price,2}";

C. double price = 9.99; string s = $"Price: {price:.2f}";

D. double price = 9.99; string s = $"Price: {price.ToString("0.00")}";

Solution

1. Step 1: Recognize C# format specifier syntax

   In C#, inside interpolation braces, :0.00 formats numbers to two decimals.

2. Step 2: Check each option's correctness

   double price = 9.99; string s = $"Price: {price:0.00}"; uses correct format {price:0.00}. double price = 9.99; string s = $"Price: {price,2}"; uses comma which is for alignment, not decimals. double price = 9.99; string s = $"Price: {price:.2f}"; uses Python style .2f which is invalid in C#. double price = 9.99; string s = $"Price: {price.ToString("0.00")}"; calls ToString inside interpolation but with escaped quotes incorrectly.

3. Final Answer:

   double price = 9.99; string s = $"Price: {price:0.00}"; -> Option A

4. Quick Check:

   Format decimals with :0.00 inside {} [OK]

Hint: Use colon and format code inside braces for formatting [OK]

Common Mistakes:

• Using Python or other language format codes
• Confusing alignment comma with format colon
• Trying to call methods inside interpolation incorrectly

3. What is the output of the following code?

int x = 5;
int y = 3;
string result = $"Sum: {x + y}, Product: {x * y}";
Console.WriteLine(result);

medium

A. Sum: 53, Product: 15

B. Sum: {x + y}, Product: {x * y}

C. Sum: 8, Product: 15

D. Sum: 8 Product: 15

Solution

1. Step 1: Evaluate expressions inside interpolation

   The expressions {x + y} and {x * y} calculate 5 + 3 = 8 and 5 * 3 = 15 respectively.

2. Step 2: Check output formatting

   The string inserts these values with a comma and space exactly as written, so output is "Sum: 8, Product: 15".

3. Final Answer:

   Sum: 8, Product: 15 -> Option C

4. Quick Check:

   Expressions inside {} are evaluated before output [OK]

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

Common Mistakes:

• Thinking expressions print literally
• Concatenating numbers as strings
• Missing commas or spaces in output

4. Identify the error in this C# code snippet:

int count = 10;
string message = $"Count is {count,2.0}";
Console.WriteLine(message);

medium

A. Variable count is not declared.

B. Missing dollar sign for string interpolation.

C. No error; code runs fine.

D. Incorrect format specifier; cannot combine alignment and decimal format like that.

Solution

1. Step 1: Understand alignment and format syntax

   In interpolation, {variable,alignment:format} is correct. Here, {count,2.0} mixes alignment and format incorrectly without colon.

2. Step 2: Identify correct syntax

   It should be {count,2:0} or similar. The dot without colon causes syntax error.

3. Final Answer:

   Incorrect format specifier; cannot combine alignment and decimal format like that. -> Option D

4. Quick Check:

   Use comma for alignment, colon for format separately [OK]

Hint: Use comma for alignment, colon for format inside {} [OK]

Common Mistakes:

• Mixing alignment and format without colon
• Forgetting dollar sign for interpolation
• Assuming no error when syntax is wrong

5. You want to display a date in the format "Year: 2024, Month: 06, Day: 15" using string interpolation. Which code snippet correctly formats the DateTime object date to achieve this?

hard

A. DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date:yyyy}, Month: {date:MM}, Day: {date:dd}";

B. DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date.Year}, Month: {date.Month}, Day: {date.Day}";

C. DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date:Year}, Month: {date:Month}, Day: {date:Day}";

D. DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date.ToString("yyyy")}, Month: {date.ToString("MM")}, Day: {date.ToString("dd")}";

Solution

1. Step 1: Understand date format strings in interpolation

   Inside interpolation, {date:format} applies the format string to the DateTime object.

2. Step 2: Compare options

   DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date:yyyy}, Month: {date:MM}, Day: {date:dd}"; uses correct format codes yyyy, MM, dd inside interpolation. DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date.Year}, Month: {date.Month}, Day: {date.Day}"; uses properties but month/day will not have leading zeros. DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date:Year}, Month: {date:Month}, Day: {date:Day}"; uses invalid format names. DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date.ToString("yyyy")}, Month: {date.ToString("MM")}, Day: {date.ToString("dd")}"; calls ToString with escaped quotes incorrectly.

3. Final Answer:

   DateTime date = new DateTime(2024, 6, 15); string s = $"Year: {date:yyyy}, Month: {date:MM}, Day: {date:dd}"; -> Option A

4. Quick Check:

   Use :format inside {} for DateTime formatting [OK]

Hint: Use :yyyy, :MM, :dd inside {} for date formatting [OK]

Common Mistakes:

• Using property names as format strings
• Forgetting leading zeros for month/day
• Overusing ToString inside interpolation unnecessarily