Immediate execution methods in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Count() on a filtered list

What is the output of this C# code using LINQ's immediate execution method Count()?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] numbers = {1, 2, 3, 4, 5};
        int count = numbers.Where(n => n > 3).Count();
        Console.WriteLine(count);
    }
}

A 0

B 2

C 3

D 5

💡 Hint

Count() returns how many items match the condition immediately.

The Where clause filters numbers greater than 3: 4 and 5. Count() returns 2 immediately.

❓ Predict Output

intermediate

Result of ToList() on a query

What will be printed by this C# program using ToList() for immediate execution?

C Sharp (C#)

using System;
using System.Linq;
using System.Collections.Generic;

class Program {
    static void Main() {
        var words = new List<string> {"apple", "banana", "cherry"};
        var filtered = words.Where(w => w.Contains('a')).ToList();
        Console.WriteLine(filtered.Count);
    }
}

A 2

B 3

C 1

D 0

💡 Hint

ToList() creates a list immediately with all matching items.

Words containing 'a' are "apple" and "banana", so filtered list has 2 items.

❓ Predict Output

advanced

Output of First() with condition

What does this C# code print when using First() as an immediate execution method?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] nums = {10, 20, 30, 40};
        int first = nums.Where(n => n > 15).First();
        Console.WriteLine(first);
    }
}

A 20

B 10

C 30

D Throws InvalidOperationException

💡 Hint

First() returns the first element that matches the condition immediately.

Numbers greater than 15 are 20, 30, 40. First() returns 20.

❓ Predict Output

advanced

Output of Single() with one matching element

What will this C# program print using Single() as an immediate execution method?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] values = {5, 10, 15};
        int single = values.Single(v => v == 10);
        Console.WriteLine(single);
    }
}

A 15

B 5

C Throws InvalidOperationException

D 10

💡 Hint

Single() returns the only element matching the condition or throws if none or multiple.

Only 10 matches v == 10, so Single() returns 10.

❓ Predict Output

expert

Output of Count() after modifying source collection

What is the output of this C# program that uses Count() immediate execution after modifying the source array?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] data = {1, 2, 3, 4};
        var query = data.Where(x => x > 2);
        data[2] = 10; // change 3 to 10
        int count = query.Count();
        Console.WriteLine(count);
    }
}

A 3

B 1

C 2

D 0

💡 Hint

Count() evaluates the query immediately using the current state of the data.

After changing data[2] to 10, elements > 2 are 10 and 4, so count is 2.

Practice

1. Which of the following methods in C# causes immediate execution of a LINQ query?

easy

A. ToList()

B. Where()

C. Select()

D. OrderBy()

Solution

1. Step 1: Understand deferred vs immediate execution

   Methods like Where(), Select(), and OrderBy() only build the query and do not execute it immediately.

2. Step 2: Identify immediate execution methods

   ToList() forces the query to run and returns a list immediately.

3. Final Answer:

   ToList() -> Option A

4. Quick Check:

   Immediate execution = ToList() [OK]

Hint: Immediate methods like ToList() run queries right away [OK]

Common Mistakes:

• Confusing deferred methods like Where() as immediate
• Thinking Select() executes query immediately
• Assuming OrderBy() runs query instantly

2. Which of the following is the correct syntax to immediately get the count of elements in a LINQ query numbers?

easy

A. int count = numbers.Count;

B. int count = Count(numbers);

C. int count = numbers.Count();

D. int count = numbers.Count[];

Solution

1. Step 1: Recognize Count() as a method

   In LINQ, Count() is a method that must be called with parentheses to execute immediately.

2. Step 2: Check syntax correctness

   numbers.Count() is the correct syntax; Count without parentheses is a property, which does not exist here.

3. Final Answer:

   int count = numbers.Count(); -> Option C

4. Quick Check:

   Count() method needs parentheses [OK]

Hint: Count() always needs parentheses to execute immediately [OK]

Common Mistakes:

• Using Count without parentheses
• Trying to call Count as a standalone function
• Using invalid syntax like Count[]

3. What is the output of the following code?

var numbers = new List<int> {1, 2, 3, 4};
var result = numbers.Where(n => n > 2).Count();
Console.WriteLine(result);

medium

A. 0

B. 3

C. 4

D. 2

Solution

1. Step 1: Filter numbers greater than 2

   The Where method filters numbers: 3 and 4 satisfy the condition, so filtered list is [3,4].

2. Step 2: Count the filtered elements

   Count() returns the number of elements in the filtered list, which is 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Count of numbers > 2 = 2 [OK]

Hint: Count() after Where() counts filtered items immediately [OK]

Common Mistakes:

• Counting original list length instead of filtered
• Confusing Count() with deferred execution
• Assuming Where() returns count directly

4. Identify the error in this code snippet:

var list = new List<string> {"apple", "banana"};
var first = list.First;
Console.WriteLine(first);

medium

A. Missing parentheses after First method

B. List initialization syntax is wrong

C. Console.WriteLine cannot print strings

D. First method does not exist on List

Solution

1. Step 1: Check usage of First method

   First is a method and requires parentheses to execute immediately.

2. Step 2: Identify syntax error

   Code uses list.First without parentheses, which causes a compile-time error.

3. Final Answer:

   Missing parentheses after First method -> Option A

4. Quick Check:

   First() needs () to run immediately [OK]

Hint: Always add () when calling immediate execution methods [OK]

Common Mistakes:

• Forgetting parentheses on First()
• Thinking First is a property
• Misunderstanding List initialization syntax

5. Given a list of integers, how can you immediately get a list of only even numbers using LINQ and immediate execution methods?

hard

A. var evens = numbers.Where(n => n % 2 == 0);

B. var evens = numbers.Where(n => n % 2 == 0).ToList();

C. var evens = numbers.Select(n => n % 2 == 0).ToList();

D. var evens = numbers.ToList().Where(n => n % 2 == 0);

Solution

1. Step 1: Filter even numbers with Where()

   Where(n => n % 2 == 0) filters the list to only even numbers.

2. Step 2: Use ToList() for immediate execution

   Calling ToList() after filtering runs the query immediately and returns a list of even numbers.

3. Final Answer:

   var evens = numbers.Where(n => n % 2 == 0).ToList(); -> Option B

4. Quick Check:

   Filter then ToList() = immediate even list [OK]

Hint: Chain Where() then ToList() for immediate filtered list [OK]

Common Mistakes:

• Not calling ToList() to force immediate execution
• Calling ToList() before Where(), which returns full list
• Using Select() incorrectly to filter