LINQ with custom objects in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all names from the list of people.

C Sharp (C#)

var names = people.Select(person => person.[1]);

Drag options to blanks, or click blank then click option'

AGetHashCode()

BAge

CToString()

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting Age instead of Name.

Using methods like ToString() which return a string but are not properties.

The Name property holds the person's name, which we want to select.

2fill in blank

medium

Complete the code to filter people older than 30.

C Sharp (C#)

var olderThan30 = people.Where(person => person.[1] > 30);

Drag options to blanks, or click blank then click option'

AName

BToString()

CAge

DLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using Name property instead of Age.

Trying to use Length which is not a property of Person.

The Age property holds the person's age, which we compare to 30.

3fill in blank

hard

Fix the error in the LINQ query to order people by their age.

C Sharp (C#)

var ordered = people.OrderBy(person => person.[1]);

Drag options to blanks, or click blank then click option'

AAge

BCount

CName

DLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using Name to order when age is required.

Using Count or Length which are not properties of Person.

Ordering by Age sorts people by their age correctly.

4fill in blank

hard

Fill both blanks to create a dictionary with names as keys and ages as values for people older than 25.

C Sharp (C#)

var dict = people.Where(p => p.[1] > 25).ToDictionary(p => p.[2], p => p.Age);

Drag options to blanks, or click blank then click option'

AAge

BName

CToString()

DLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using Name instead of Age in the filter.

Using ToString() or Length which are not suitable here.

Filter by Age > 25, then use Name as dictionary keys.

5fill in blank

hard

Fill all three blanks to select uppercase names of people younger than 40.

C Sharp (C#)

var result = people.Where(p => p.[1] < 40).Select(p => p.[2].[3]());

Drag options to blanks, or click blank then click option'

AAge

BName

CToUpper

DToLower

Attempts:

3 left

💡 Hint

Common Mistakes

Using ToLower instead of ToUpper.

Using Name in the filter instead of Age.

Filter by Age less than 40, select Name, then convert to uppercase with ToUpper().

Practice

1. What does LINQ primarily help you do with custom objects in C#?

easy

A. Create new classes automatically

B. Compile code faster

C. Filter, sort, and select data easily

D. Manage memory manually

Solution

1. Step 1: Understand LINQ's purpose

   LINQ is designed to query collections like lists of objects easily.

2. Step 2: Identify LINQ's main features

   It helps filter, sort, and select data without manual loops.

3. Final Answer:

   Filter, sort, and select data easily -> Option C

4. Quick Check:

   LINQ = Filter, sort, select [OK]

Hint: Remember LINQ is for querying data collections [OK]

Common Mistakes:

• Thinking LINQ creates classes
• Confusing LINQ with compilation
• Assuming LINQ manages memory

2. Which of the following is the correct syntax to select all names from a list of Person objects using LINQ?

easy

A. var names = people.Select(p => p.Name());

B. var names = people.Select(p.Name);

C. var names = people.Select(p => p);

D. var names = people.Select(p => p.Name);

Solution

1. Step 1: Understand Select syntax

   Select expects a lambda expression to pick a property, like p => p.Name.

2. Step 2: Check each option

   var names = people.Select(p => p.Name); uses correct lambda syntax. var names = people.Select(p.Name); misses lambda. var names = people.Select(p => p); selects whole object. var names = people.Select(p => p.Name()); wrongly calls Name as method.

3. Final Answer:

   var names = people.Select(p => p.Name); -> Option D

4. Quick Check:

   Select needs lambda with property [OK]

Hint: Use lambda syntax p => p.Property for Select [OK]

Common Mistakes:

• Omitting lambda arrow =>
• Calling property as method
• Selecting whole object instead of property

3. Given the class Person { public string Name; public int Age; } and list people with three persons: Alice(30), Bob(25), and Carol(35), what does this LINQ query return?

var result = people.Where(p => p.Age > 28).Select(p => p.Name).ToList();

medium

A. ["Alice", "Carol"]

B. ["Bob"]

C. ["Alice", "Bob", "Carol"]

D. Empty list

Solution

1. Step 1: Filter people older than 28

   Alice is 30 (yes), Bob is 25 (no), Carol is 35 (yes).

2. Step 2: Select their names

   Names selected are "Alice" and "Carol".

3. Final Answer:

   ["Alice", "Carol"] -> Option A

4. Quick Check:

   Age > 28 filters Alice and Carol [OK]

Hint: Filter first, then select property [OK]

Common Mistakes:

• Including Bob who is younger
• Selecting whole object instead of names
• Confusing > with <

4. What is wrong with this LINQ query?

var adults = people.Where(p => p.Age >= 18).Select(p => p.Name);
foreach(var name in adults) Console.WriteLine(name);

medium

A. The query is correct and will print all names of adults

B. The lambda expression syntax is incorrect

C. The query is missing ToList() or ToArray(), so it won't compile

D. The Where clause should be after Select

Solution

1. Step 1: Check LINQ query syntax

   Where and Select are used correctly with proper lambda syntax.

2. Step 2: Check foreach usage

   LINQ returns IEnumerable<string>, which foreach can iterate without ToList().

3. Final Answer:

   The query is correct and will print all names of adults -> Option A

4. Quick Check:

   IEnumerable works with foreach directly [OK]

Hint: IEnumerable can be iterated without ToList() [OK]

Common Mistakes:

• Thinking ToList() is mandatory before foreach
• Misplacing Where and Select order
• Incorrect lambda syntax

5. You have a list of Product objects with properties Name (string) and Price (decimal). How do you create a dictionary with product names as keys and prices as values, but only include products costing more than 50 using LINQ?

hard

A. products.ToDictionary(p => p.Name, p => p.Price).Where(p => p.Value > 50);

B. products.Where(p => p.Price > 50).ToDictionary(p => p.Name, p => p.Price);

C. products.Select(p => new {p.Name, p.Price > 50}).ToDictionary(p => p.Name, p => p.Price);

D. products.Where(p => p.Price > 50).Select(p => p.Name).ToDictionary();

Solution

1. Step 1: Filter products with price > 50

   Use Where to keep only products costing more than 50.

2. Step 2: Convert filtered list to dictionary

   Use ToDictionary with key selector p.Name and value selector p.Price.

3. Final Answer:

   products.Where(p => p.Price > 50).ToDictionary(p => p.Name, p => p.Price); -> Option B

4. Quick Check:

   Filter then ToDictionary with key and value selectors [OK]

Hint: Filter first, then use ToDictionary with key and value [OK]

Common Mistakes:

• Trying to filter after ToDictionary (invalid)
• Selecting anonymous types instead of original objects
• Calling ToDictionary without key/value selectors