Select clause projection in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the 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'

ALength

BAge

CToString()

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Using Age instead of Name will select numbers, not names.

Using ToString() may not return just the name.

The Select clause projects each person to their Name property.

2fill in blank

medium

Complete the code to select the ages from the list of people.

C Sharp (C#)

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

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

AAge

BName

CHeight

DWeight

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting Name instead of Age will give names, not ages.

Selecting Height or Weight will give unrelated data.

The Select clause projects each person to their Age property.

3fill in blank

hard

Fix the error in the code to select the uppercase names.

C Sharp (C#)

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

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

AToUpper

Btolower

Ctoupper

DUpperCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase method names like tolower or toupper causes errors.

UpperCase is not a valid method in C#.

The correct method to convert a string to uppercase in C# is ToUpper().

4fill in blank

hard

Fill both blanks to select the first letter of each person's name and convert it to lowercase.

C Sharp (C#)

var firstLetters = people.Select(person => person.Name[1]0[2].ToString().ToLower());

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

A[

B]

C.

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of brackets causes errors.

Forgetting the closing bracket causes syntax errors.

To get the first letter of a string, use square brackets [] with index 0. Then call ToString() and ToLower() to convert it.

5fill in blank

hard

Fill all three blanks to select a new object with uppercase name and age doubled.

C Sharp (C#)

var result = people.Select(person => new { Name = person.Name.[1](), Age = person.[2] * [3] });

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

AToUpper

BAge

C2

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Using Name instead of Age for multiplication causes errors.

Forgetting parentheses after ToUpper causes errors.

This code creates a new object with the name in uppercase and the age multiplied by 2.

Practice

1. What does the select clause do in a LINQ query in C#?

easy

A. It groups the data by a key.

B. It filters the data based on a condition.

C. It sorts the data in ascending order.

D. It chooses and shapes the data to return from the query.

Solution

1. Step 1: Understand the role of select in LINQ

   The select clause defines what data to return from the query, shaping the output.

2. Step 2: Differentiate from other clauses

   Filtering is done by where, sorting by orderby, grouping by group by. Only select shapes the output.

3. Final Answer:

   It chooses and shapes the data to return from the query. -> Option D

4. Quick Check:

   Select clause = shapes output [OK]

Hint: Select clause shapes output data, not filters or sorts [OK]

Common Mistakes:

• Confusing select with where clause
• Thinking select sorts data
• Mixing select with group by

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

easy

A. var names = from p in people select p.Name;

B. var names = from p in people where p.Name;

C. var names = from p in people orderby p.Name select p;

D. var names = from p in people group p by p.Name;

Solution

1. Step 1: Identify correct select syntax

   The correct syntax to project only the Name property is select p.Name.

2. Step 2: Check other options for errors

   var names = from p in people where p.Name; uses where incorrectly without a condition. var names = from p in people orderby p.Name select p; selects whole object, not just names. var names = from p in people group p by p.Name; groups data, not selects.

3. Final Answer:

   var names = from p in people select p.Name; -> Option A

4. Quick Check:

   Select projects properties correctly [OK]

Hint: Select clause syntax: from x in y select x.Property [OK]

Common Mistakes:

• Using where without condition
• Selecting whole object instead of property
• Confusing group by with select

3. What is the output of this code?

var numbers = new[] {1, 2, 3};
var result = from n in numbers select n * 2;
foreach(var r in result) Console.Write(r + " ");

medium

A. 1 2 3

B. 3 4 5

C. 2 4 6

D. Syntax error

Solution

1. Step 1: Understand the select projection

   The query multiplies each number by 2, so 1 becomes 2, 2 becomes 4, 3 becomes 6.

2. Step 2: Trace the output loop

   The foreach prints each doubled number followed by a space: "2 4 6 "

3. Final Answer:

   2 4 6 -> Option C

4. Quick Check:

   Select doubles numbers = 2 4 6 [OK]

Hint: Select transforms each item; multiply 1,2,3 by 2 = 2,4,6 [OK]

Common Mistakes:

• Thinking select filters instead of transforms
• Expecting original numbers
• Confusing output format

4. Identify the error in this LINQ query:

var people = new List<Person>();
var names = from p in people select p.Name.ToUpper;

medium

A. select clause must come before from clause.

B. Missing parentheses after ToUpper method call.

C. List<Person> cannot be queried with LINQ.

D. Cannot select Name property directly.

Solution

1. Step 1: Check method usage in select

   ToUpper is a method and requires parentheses: ToUpper().

2. Step 2: Validate other options

   Cannot select Name property directly. is wrong because selecting Name is valid. List<Person> cannot be queried with LINQ. is wrong because List supports LINQ. select clause must come before from clause. is wrong because from must come first.

3. Final Answer:

   Missing parentheses after ToUpper method call. -> Option B

4. Quick Check:

   Method calls need () [OK]

Hint: Method calls need parentheses, e.g., ToUpper() [OK]

Common Mistakes:

• Forgetting () after method names
• Misordering LINQ clauses
• Thinking List can't use LINQ

5. Given a list of products with properties Name and Price, how would you use a LINQ select clause to create a new list of anonymous objects containing only Name and a discounted price (10% off)?

hard

A. var discounted = from p in products select new { p.Name, DiscountedPrice = p.Price * 0.9 };

B. var discounted = from p in products select p.Name, p.Price * 0.9;

C. var discounted = from p in products where p.Price > 0.9 select p.Name;

D. var discounted = from p in products select new List { p.Name, p.Price * 0.9 };

Solution

1. Step 1: Understand anonymous object creation in select

   To create new shaped data, use new { } with property assignments inside select.

2. Step 2: Check each option's correctness

   var discounted = from p in products select new { p.Name, DiscountedPrice = p.Price * 0.9 }; correctly creates anonymous objects with Name and discounted price. var discounted = from p in products select p.Name, p.Price * 0.9; syntax is invalid (multiple values without new). var discounted = from p in products where p.Price > 0.9 select p.Name; filters but does not create discounted price. var discounted = from p in products select new List { p.Name, p.Price * 0.9 }; tries to create a List incorrectly.

3. Final Answer:

   var discounted = from p in products select new { p.Name, DiscountedPrice = p.Price * 0.9 }; -> Option A

4. Quick Check:

   Select new anonymous objects with discounted price [OK]

Hint: Use new { } in select to create shaped objects [OK]

Common Mistakes:

• Trying to select multiple values without new
• Using where instead of select for projection
• Incorrect anonymous object syntax