OrderBy and sorting in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this OrderBy example?

Consider the following C# code that sorts a list of integers using OrderBy. What will be printed to the console?

C Sharp (C#)

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

class Program {
    static void Main() {
        List<int> numbers = new() { 5, 3, 8, 1 };
        var sorted = numbers.OrderBy(n => n);
        foreach(var num in sorted) {
            Console.Write(num + " ");
        }
    }
}

A 1 3 5 8

B 8 5 3 1

C 5 3 8 1

D 3 5 1 8

💡 Hint

OrderBy sorts elements in ascending order by default.

OrderBy sorts the list from smallest to largest. The original list is {5,3,8,1}, so sorted ascending is {1,3,5,8}.

❓ Predict Output

intermediate

What is the output when sorting strings by length?

This code sorts a list of strings by their length using OrderBy. What will be printed?

C Sharp (C#)

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

class Program {
    static void Main() {
        List<string> words = new() { "apple", "dog", "banana", "cat" };
        var sorted = words.OrderBy(w => w.Length);
        foreach(var w in sorted) {
            Console.Write(w + " ");
        }
    }
}

A cat dog apple banana

B apple dog banana cat

C banana apple dog cat

D dog cat apple banana

💡 Hint

OrderBy sorts by the key selector, here the string length.

Strings sorted by length ascending: "dog" (3), "cat" (3), "apple" (5), "banana" (6). Since "dog" and "cat" have same length, their original order is preserved.

❓ Predict Output

advanced

What is the output of this OrderByDescending with complex objects?

Given this code that sorts a list of Person objects by Age descending, what will be printed?

C Sharp (C#)

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

record Person(string Name, int Age);

class Program {
    static void Main() {
        List<Person> people = new() {
            new("Alice", 30),
            new("Bob", 25),
            new("Charlie", 35)
        };
        var sorted = people.OrderByDescending(p => p.Age);
        foreach(var p in sorted) {
            Console.Write(p.Name + " ");
        }
    }
}

A Charlie Bob Alice

B Charlie Alice Bob

C Bob Alice Charlie

D Alice Bob Charlie

💡 Hint

OrderByDescending sorts from largest to smallest by the key.

Ages are 30, 25, 35. Descending order is 35 (Charlie), 30 (Alice), 25 (Bob).

❓ Predict Output

advanced

What is the output of this chained OrderBy and ThenBy?

This code sorts a list of tuples first by the first item ascending, then by the second item descending. What is printed?

C Sharp (C#)

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

class Program {
    static void Main() {
        var list = new List<(int, int)>{ (1, 2), (1, 3), (2, 1), (2, 2) };
        var sorted = list.OrderBy(x => x.Item1).ThenByDescending(x => x.Item2);
        foreach(var t in sorted) {
            Console.Write($"({t.Item1},{t.Item2}) ");
        }
    }
}

A (1,3) (1,2) (2,2) (2,1)

B (1,2) (1,3) (2,1) (2,2)

C (2,2) (2,1) (1,3) (1,2)

D (2,1) (2,2) (1,2) (1,3)

💡 Hint

OrderBy sorts by first item ascending, ThenByDescending sorts by second item descending within groups.

Group by first item: 1 and 2. For 1: (1,3) then (1,2). For 2: (2,2) then (2,1).

🧠 Conceptual

expert

Which option causes a runtime error when sorting with OrderBy?

Given a list of objects, which code snippet will cause a runtime exception when using OrderBy in C#?

A list.OrderBy(x => x.GetHashCode())

B list.OrderBy(x => x.PropertyThatExists)

C list.OrderBy(x => x.NonExistentProperty)

D list.OrderBy(x => x.ToString())

💡 Hint

Accessing a property that does not exist will cause an error at runtime.

Option C tries to access a property that does not exist on the objects, causing a runtime exception (usually a RuntimeBinderException or similar). Other options use valid properties or methods.

Practice

1. What does the OrderBy method do in C#?

easy

A. Sorts a collection in ascending order based on a key

B. Deletes elements from a list

C. Reverses the order of elements in a list

D. Filters elements based on a condition

Solution

1. Step 1: Understand the purpose of OrderBy

   The OrderBy method sorts elements in a collection based on a key in ascending order.

2. Step 2: Compare with other options

   Options B, C, and D describe different operations (deletion, reversing, filtering) which OrderBy does not perform.

3. Final Answer:

   Sorts a collection in ascending order based on a key -> Option A

4. Quick Check:

   OrderBy = Sort ascending [OK]

Hint: OrderBy sorts ascending by key, not filtering or deleting [OK]

Common Mistakes:

• Confusing OrderBy with filtering methods like Where
• Thinking OrderBy modifies the original list
• Mixing up OrderBy with reversing or deleting

2. Which of the following is the correct syntax to sort a list of integers named numbers in ascending order using OrderBy?

easy

A. numbers.OrderBy();

B. numbers.OrderBy(n);

C. numbers.OrderBy(n => n);

D. numbers.OrderBy(n => );

Solution

1. Step 1: Check the correct lambda syntax

   OrderBy requires a key selector function like n => n to specify sorting key.

2. Step 2: Validate each option

   numbers.OrderBy(n => n); uses correct lambda syntax. numbers.OrderBy(); misses the key selector. numbers.OrderBy(n); passes a variable, not a lambda. numbers.OrderBy(n => ); has incomplete lambda syntax.

3. Final Answer:

   numbers.OrderBy(n => n); -> Option C

4. Quick Check:

   OrderBy needs a key selector lambda [OK]

Hint: OrderBy always needs a key selector lambda like n => n [OK]

Common Mistakes:

• Omitting the lambda expression inside OrderBy
• Passing a variable instead of a lambda
• Using incomplete or invalid lambda syntax

3. What will be the output of the following code?

var fruits = new List<string> { "banana", "apple", "cherry" };
var sorted = fruits.OrderBy(f => f);
foreach(var fruit in sorted) {
    Console.Write(fruit + " ");
}

medium

A. banana apple cherry

B. apple banana cherry

C. cherry banana apple

D. apple cherry banana

Solution

1. Step 1: Understand the sorting key

   The code sorts the list of fruits alphabetically by their string value.

2. Step 2: Determine the sorted order

   Alphabetically, "apple" comes before "banana", which comes before "cherry".

3. Final Answer:

   apple banana cherry -> Option B

4. Quick Check:

   OrderBy sorts strings alphabetically [OK]

Hint: OrderBy sorts strings alphabetically ascending [OK]

Common Mistakes:

• Assuming original order is preserved
• Confusing OrderBy with OrderByDescending
• Not recognizing alphabetical order

4. The following code throws a compile-time error. What is the mistake?

var numbers = new List<int> { 3, 1, 2 };
var sorted = numbers.OrderBy();

medium

A. numbers must be an array, not a list

B. List<int> cannot be sorted

C. OrderBy should be OrderByDescending

D. OrderBy requires a key selector lambda expression

Solution

1. Step 1: Identify the method signature requirement

   OrderBy requires a key selector function to specify how to sort elements.

2. Step 2: Analyze the error cause

   Calling OrderBy() without any argument causes a compile error because the key selector is missing.

3. Final Answer:

   OrderBy requires a key selector lambda expression -> Option D

4. Quick Check:

   OrderBy needs a lambda key selector [OK]

Hint: Always provide a key selector lambda to OrderBy [OK]

Common Mistakes:

• Calling OrderBy without arguments
• Thinking OrderBy works without a key selector
• Confusing List and array types for sorting

5. Given a list of students with properties Name and Score, how do you sort the list first by Score descending, then by Name ascending using LINQ?

hard

A. students.OrderByDescending(s => s.Score).ThenBy(s => s.Name);

B. students.OrderBy(s => s.Score).OrderBy(s => s.Name);

C. students.OrderBy(s => s.Name).OrderByDescending(s => s.Score);

D. students.OrderByDescending(s => s.Name).ThenByDescending(s => s.Score);

Solution

1. Step 1: Understand multi-level sorting

   To sort by multiple keys, use OrderBy or OrderByDescending for the first key, then ThenBy or ThenByDescending for the next keys.

2. Step 2: Apply correct order and directions

   We want to sort by Score descending first, then by Name ascending, so use OrderByDescending(s => s.Score) followed by ThenBy(s => s.Name).

3. Final Answer:

   students.OrderByDescending(s => s.Score).ThenBy(s => s.Name); -> Option A

4. Quick Check:

   OrderByDescending + ThenBy for multi-level sort [OK]

Hint: Use ThenBy after OrderBy for secondary sorting [OK]

Common Mistakes:

• Using multiple OrderBy calls instead of ThenBy
• Mixing ascending and descending incorrectly
• Sorting by wrong property order