String interpolation in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is string interpolation in C#?

String interpolation is a way to insert variables or expressions directly inside a string literal using a special syntax, making the code easier to read and write.

beginner

How do you start a string literal to use string interpolation in C#?

You start the string with a $ symbol before the opening double quote, like this: $"Your text {variable}".

beginner

What is the output of this code?

int age = 25;
string message = $"I am {age} years old.";
Console.WriteLine(message);

The output will be: I am 25 years old.

intermediate

Can you use expressions inside the curly braces in string interpolation?

Yes, you can put any valid expression inside the curly braces, like calculations or method calls, and it will be evaluated and inserted into the string.

intermediate

How do you format numbers or dates inside string interpolation?

You can add a colon and a format string after the expression inside the braces, for example: $"{price:C}" to format as currency or $"{date:yyyy-MM-dd}" for a date format.

Which symbol is used to start a string literal for interpolation in C#?

A $

B #

C @

D %

In C#, string interpolation requires a $ symbol before the opening quote.

What will this code print?

int x = 3;
int y = 4;
Console.WriteLine($"Sum is {x + y}");

A Sum is x + y

B Sum is {x + y}

C Sum is 34

D Sum is 7

The expression inside braces is evaluated, so 3 + 4 = 7.

How do you include a literal curly brace '{' in an interpolated string?

A Use single quotes '{'

B Escape with backslash '\{'

C Use double braces like '{{'

D You cannot include curly braces

Double braces '{{' or '}}' are used to include literal curly braces in interpolated strings.

Which of these is a valid interpolated string in C#?

A $"Hello {name}"

B "Hello {name}"

C @"Hello {name}"

D #"Hello {name}"

Only the string starting with $ supports interpolation.

How can you format a date inside an interpolated string?

A "{date}"

B $"{date:MM/dd/yyyy}"

C "{date:MM/dd/yyyy}"

D $"{date.format('MM/dd/yyyy')}"

You use a colon and format string inside the braces to format dates.

Explain how string interpolation works in C# and give a simple example.

Think about how to insert a variable value directly inside a string.

Describe how to format numbers or dates inside an interpolated string in C#.

Remember the syntax {expression:format} inside the string.

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