Lambda with captures (closures) in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of lambda capturing a variable

What is the output of this C# code that uses a lambda capturing a variable?

C Sharp (C#)

int x = 5;
Func<int> f = () => x * 2;
x = 10;
Console.WriteLine(f());

A 20

B Compilation error

C 10

D 5

💡 Hint

Remember that lambdas capture variables by reference, not by value.

The lambda captures the variable x by reference. When f() is called, x is 10, so the output is 20.

❓ Predict Output

intermediate

Output of multiple lambdas capturing loop variable

What is the output of this C# code with lambdas capturing a loop variable?

C Sharp (C#)

var actions = new List<Action>();
for (int i = 0; i < 3; i++)
{
    actions.Add(() => Console.Write(i + " "));
}
foreach (var action in actions) action();

A 0 1 2

B Compilation error

C 0 0 0

D 3 3 3

💡 Hint

Consider how the loop variable i is captured by the lambdas.

The lambdas capture the variable i by reference. After the loop, i is 3, so all lambdas print 3.

🔧 Debug

advanced

Why does this lambda capture cause unexpected output?

This code intends to print 0, 1, 2 but prints 3, 3, 3 instead. Why?

C Sharp (C#)

var actions = new List<Action>();
for (int i = 0; i < 3; i++)
{
    actions.Add(() => Console.Write(i + " "));
}
foreach (var action in actions) action();

A The list actions is cleared before printing, so output is from a default value.

B The lambda syntax is incorrect and causes all lambdas to share the same output.

C The variable i is captured by reference, so all lambdas see the final value 3.

D The loop variable i is copied inside the lambda, but the copy is overwritten.

💡 Hint

Think about how variables in loops are captured by lambdas in C#.

In C#, the loop variable i is captured by reference by all lambdas, so they all see the final value after the loop ends.

📝 Syntax

advanced

Which lambda correctly captures a local variable inside a loop?

Which option correctly captures the loop variable so that each lambda prints 0, 1, and 2 respectively?

C Sharp (C#)

var actions = new List<Action>();
for (int i = 0; i < 3; i++)
{
    // capture i correctly here
}
foreach (var action in actions) action();

A actions.Add(() => Console.Write(i + " "));

B

int copy = i;
actions.Add(() => Console.Write(copy + " "));

C actions.Add(delegate { Console.Write(i + " "); });

D actions.Add(() => Console.Write((i + 1) + " "));

💡 Hint

Try creating a new variable inside the loop to hold the current value.

Creating a new variable copy inside the loop captures the current value correctly for each lambda.

🚀 Application

expert

How many unique values are captured by these lambdas?

Consider this code snippet. How many unique values are printed when invoking all lambdas in the list?

C Sharp (C#)

var funcs = new List<Func<int>>();
int x = 1;
for (int i = 0; i < 3; i++)
{
    int y = i;
    funcs.Add(() => x + y);
}
x = 5;
var results = funcs.Select(f => f()).ToList();
Console.WriteLine(string.Join(",", results));

A 3 unique values: 5,6,7

B 3 unique values: 2,3,4

C 1 unique value: 8

D 1 unique value: 6

💡 Hint

Remember which variables are captured by reference and which by value.

x is captured by reference by all lambdas and is changed to 5 before invocation. Each y is a distinct local variable per iteration (y=0,1,2), so the lambdas return 5+0=5, 5+1=6, 5+2=7. Three unique values: 5,6,7.

Practice

1.

What does it mean when a lambda expression captures a variable in C#?

easy

A. The lambda deletes the variable after execution.

B. The lambda remembers and uses the variable from outside its own code block.

C. The lambda creates a new variable with the same name inside its code.

D. The lambda can only use variables declared inside it.

Solution

1. Step 1: Understand lambda captures

   A lambda can use variables declared outside its own code block by capturing them.

2. Step 2: Effect of capturing

   This means the lambda remembers the variable's value and can use or modify it later.

3. Final Answer:

   The lambda remembers and uses the variable from outside its own code block. -> Option B

4. Quick Check:

   Lambda captures = remembers outside variables [OK]

Hint: Captured variables come from outside the lambda [OK]

Common Mistakes:

• Thinking lambda creates new variables instead of capturing
• Assuming variables are deleted after lambda runs
• Believing lambdas can't use outside variables

2.

Which of the following is the correct syntax to declare a lambda that captures an external variable count and increments it?

int count = 0;
var increment = ?;

easy

A. () => count++;

B. (int count) => count++;

C. () => { int count = 0; count++; };

D. count => count + 1;

Solution

1. Step 1: Identify lambda capturing external variable

   The lambda should use the external variable count without redeclaring it.

2. Step 2: Check syntax correctness

   () => count++; uses an empty parameter list and increments the external count variable correctly.

3. Final Answer:

   () => count++; -> Option A

4. Quick Check:

   Empty params and external variable increment [OK]

Hint: Use empty parentheses to capture external variables [OK]

Common Mistakes:

• Trying to redeclare captured variable inside lambda
• Using lambda parameters instead of capturing
• Incorrect lambda syntax with parameters

3.

What is the output of this C# code?

int x = 5;
Func<int> f = () => x * 2;
x = 10;
Console.WriteLine(f());

medium

A. Compile error

B. 10

C. 5

D. 20

Solution

1. Step 1: Understand variable capture behavior

   The lambda captures the variable x by reference, not by value.

2. Step 2: Evaluate the lambda call after changing x

   Since x was changed to 10 before calling f(), the lambda returns 10 * 2 = 20.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   Captured variable reflects latest value [OK]

Hint: Captured variables reflect current value when lambda runs [OK]

Common Mistakes:

• Assuming lambda captures variable value at creation time
• Expecting output 5 or 10 instead of 20
• Thinking code causes compile error

4.

Identify the error in this code snippet involving a lambda with captures:

int total = 0;
var actions = new List<Action>();
for (int i = 0; i < 3; i++) {
    actions.Add(() => total += i);
}
actions.ForEach(a => a());
Console.WriteLine(total);

medium

A. The variable i is captured incorrectly, causing unexpected results.

B. The variable total cannot be captured by the lambda.

C. The lambda syntax is invalid and causes a compile error.

D. The list actions cannot store lambdas.

Solution

1. Step 1: Analyze variable capture in loop

   The lambda captures the loop variable i by reference, so all lambdas share the same i.

2. Step 2: Effect on final output

   After the loop, i is 3, so each lambda adds 3 to total, resulting in total = 9 instead of expected 0+1+2=3.

3. Final Answer:

   The variable i is captured incorrectly, causing unexpected results. -> Option A

4. Quick Check:

   Loop variable capture causes bugs [OK]

Hint: Loop variables captured by reference cause bugs [OK]

Common Mistakes:

• Thinking total can't be captured
• Believing lambda syntax is wrong
• Assuming list can't hold lambdas

5.

You want to create a list of lambdas that each print their own index from 0 to 2. Which code correctly captures the index variable?

var actions = new List<Action>();
for (int i = 0; i < 3; i++) {
    // Fill in here
}
actions.ForEach(a => a());

hard

A. actions.Add((int i) => Console.WriteLine(i));

B. actions.Add(() => Console.WriteLine(i));

C. int copy = i; actions.Add(() => Console.WriteLine(copy));

D. actions.Add(() => Console.WriteLine(0));

Solution

1. Step 1: Understand loop variable capture problem

   Directly capturing i causes all lambdas to print the final value of i (3).

2. Step 2: Use a local copy to capture each index

   Declaring copy = i inside the loop creates a new variable for each iteration, which the lambda captures correctly.

3. Final Answer:

   int copy = i; actions.Add(() => Console.WriteLine(copy)); -> Option C

4. Quick Check:

   Use local copy to capture loop variable [OK]

Hint: Copy loop variable inside loop before lambda [OK]

Common Mistakes:

• Capturing loop variable directly causing all lambdas to print same value
• Trying to add lambda with parameter in Add method
• Hardcoding constant instead of variable