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.