Deferred execution behavior in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of deferred execution with LINQ query

What is the output of this C# code snippet that uses deferred execution in LINQ?

C Sharp (C#)

List<int> numbers = new() {1, 2, 3};
var query = numbers.Select(x => x * 2);
numbers.Add(4);
foreach (var num in query)
{
    Console.Write(num + " ");
}

A 2 4 6 8

B 2 4 6

C 1 2 3 4

D Runtime error

💡 Hint

Remember that LINQ queries are not executed until you enumerate them.

The LINQ query uses deferred execution, so when enumerated, it reflects the current state of the source list, which includes the newly added element 4. Thus, the output doubles all four elements.

❓ Predict Output

intermediate

Effect of modifying source collection after query creation

What will be printed by this C# program?

C Sharp (C#)

var list = new List<string> {"a", "b", "c"};
var filtered = list.Where(s => s != "b");
list.Remove("a");
foreach (var item in filtered)
{
    Console.Write(item);
}

A bc

B c

C ab

D Runtime exception

💡 Hint

Consider when the Where filter is applied and the current state of the list.

The Where query is deferred and uses the current list state when enumerated. After removing "a", the list contains "b" and "c". The filter excludes "b", so only "c" is printed.

🔧 Debug

advanced

Why does this deferred execution code print unexpected results?

Consider this code snippet: List nums = new() {1, 2, 3}; var query = nums.Select(x => x + 1); nums = new List {4, 5, 6}; foreach (var n in query) { Console.Write(n + " "); } Why does the output not reflect the new list values?

C Sharp (C#)

List<int> nums = new() {1, 2, 3};
var query = nums.Select(x => x + 1);
nums = new List<int> {4, 5, 6};
foreach (var n in query)
{
    Console.Write(n + " ");
}

A No output

B 5 6 7

C Runtime error

D 2 3 4

💡 Hint

Think about what the query captures and what the variable reassignment does.

The query captures the original list object. Reassigning the variable nums to a new list does not affect the original list referenced by the query. So the query enumerates the original list {1,2,3}, producing 2 3 4.

📝 Syntax

advanced

Which LINQ query causes a runtime error due to deferred execution?

Given the following code, which option will cause an exception when enumerated?

C Sharp (C#)

List<int> numbers = new() {1, 2, 3};
var query = numbers.Select(x => 10 / x);
foreach (var val in query)
{
    Console.Write(val + " ");
}

A var query = numbers.Select(x => 10 / (x - 1));

B var query = numbers.Select(x => 10 / x);

C var query = numbers.Select(x => 10 / (x + 1));

D var query = numbers.Select(x => 10 / (x - 2));

💡 Hint

Check for division by zero possibilities during enumeration.

Option A causes division by zero when x is 1, because 1 - 1 = 0, causing a runtime DivideByZeroException during deferred execution.

🚀 Application

expert

Predict the final count after deferred execution with side effects

What is the value of count printed by this C# program?

C Sharp (C#)

List<int> data = new() {1, 2, 3, 4};
int count = 0;
var query = data.Where(x => { count++; return x % 2 == 0; });

// No enumeration here
count += 10;

foreach (var item in query)
{
    // Just iterate
}

Console.Write(count);

A 10

B 12

C 14

D Runtime error

💡 Hint

Remember when the lambda inside Where is executed and how many elements it processes.

The lambda runs once per element during enumeration. The count is incremented 10 before enumeration, then 4 times during enumeration (for each element). So final count is 10 + 4 = 14.