OrderBy and sorting in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the OrderBy method do in C#?

The OrderBy method sorts a collection in ascending order based on a key you specify. It returns a new sorted sequence without changing the original collection.

beginner

How do you sort a list of numbers in descending order using LINQ?

Use OrderByDescending method. For example: numbers.OrderByDescending(n => n) sorts numbers from largest to smallest.

intermediate

What is the difference between OrderBy and ThenBy?

OrderBy sorts the collection by the first key. ThenBy is used to sort by a second key when the first keys are equal, allowing multi-level sorting.

beginner

Can OrderBy modify the original list?

No, OrderBy does not change the original list. It returns a new sorted sequence. To change the original list, you can use List.Sort().

beginner

How do you sort a list of objects by a property using OrderBy?

You provide a lambda expression that selects the property. For example, people.OrderBy(p => p.Age) sorts people by their Age property in ascending order.

Which method sorts a collection in ascending order by a key in C#?

A Reverse

B OrderByDescending

C ThenByDescending

D OrderBy

OrderBy sorts in ascending order. OrderByDescending sorts in descending order.

What does ThenBy do in sorting?

A Sorts by a second key when the first keys are equal

B Sorts the collection by the first key

C Reverses the order of the collection

D Filters the collection

ThenBy is used for secondary sorting after OrderBy.

Does OrderBy change the original list?

A Yes, it modifies the original list

B No, it returns a new sorted sequence

C Only if you call <code>ToList()</code>

D Only for arrays

OrderBy returns a new sorted sequence and does not modify the original collection.

How do you sort a list of strings in descending order?

A list.OrderByDescending(s =&gt; s)

B list.OrderBy(s =&gt; s)

C list.ThenBy(s =&gt; s)

D list.SortDescending()

OrderByDescending sorts in descending order.

Which method would you use to sort a list of objects by their Name property?

A list.Sort()

B list.OrderBy(o =&gt; o.Age)

C list.OrderBy(o =&gt; o.Name)

D list.Reverse()

Use OrderBy with a lambda selecting the Name property.

Explain how to sort a list of objects by multiple properties using LINQ.

Think about how to chain sorting methods.

Describe the difference between OrderBy and List.Sort().

Consider whether the original list changes.

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