Why LINQ is needed in C Sharp (C#) - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a list of integers.

C Sharp (C#)

List<int> numbers = new List<int> [1];

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

A[]

B()

C{}

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets instead of parentheses.

Using curly braces without initialization.

The correct syntax to create a new list in C# uses parentheses () after the constructor.

2fill in blank

medium

Complete the code to filter numbers greater than 5 using LINQ.

C Sharp (C#)

var filtered = numbers.Where(n => n [1] 5);

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

A<

B==

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than operator instead of greater than.

Using equality operator which filters only numbers equal to 5.

The Where method filters elements. To get numbers greater than 5, use the greater than operator >.

3fill in blank

hard

Fix the error in the LINQ query to select names starting with 'A'.

C Sharp (C#)

var result = names.Where(name => name.[1]("A"));

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

AStartsWith

BstartWith

CstartsWith

DStartwith

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect casing for method name.

Misspelling the method name.

The correct method name is StartsWith with capital S and W.

4fill in blank

hard

Fill both blanks to create a dictionary from a list using LINQ.

C Sharp (C#)

var dict = people.ToDictionary([1] => [2].Id);

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

Aperson

Bp

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the lambda expression.

Using variable names not declared in the lambda.

In the lambda expression, the variable name must be consistent. Here, p is used both times.

5fill in blank

hard

Fill all three blanks to select and order names using LINQ.

C Sharp (C#)

var orderedNames = people.Select([1] => [2].Name).OrderBy([3] => [3]);

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

Aperson

Bp

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the two lambda expressions.

Using variable names that are not declared.

Use p as the variable in Select and OrderBy for clarity and consistency.

Practice

1. Why do programmers use LINQ in C#?

easy

A. To handle network connections automatically

B. To make the program run faster by using low-level code

C. To write simpler and clearer code for working with data

D. To create graphical user interfaces easily

Solution

1. Step 1: Understand LINQ's purpose

   LINQ is designed to help programmers write queries on data in a simple and readable way.

2. Step 2: Compare options with LINQ's goal

   Only To write simpler and clearer code for working with data matches LINQ's goal of simplifying data handling. Other options describe unrelated tasks.

3. Final Answer:

   To write simpler and clearer code for working with data -> Option C

4. Quick Check:

   LINQ simplifies data queries [OK]

Hint: Think: LINQ makes data questions easy to read [OK]

Common Mistakes:

• Confusing LINQ with UI or network tools
• Thinking LINQ improves program speed directly
• Assuming LINQ is for graphics or connections

2. Which of the following is the correct way to start a LINQ query in C#?

easy

A. foreach item in collection { select item; }

B. select item from collection;

C. query collection select item;

D. from item in collection select item;

Solution

1. Step 1: Recall LINQ query syntax

   LINQ queries start with the keyword 'from', followed by a variable and the data source.

2. Step 2: Check each option's syntax

   Only from item in collection select item; uses the correct 'from ... in ... select ...' pattern. Others have wrong order or keywords.

3. Final Answer:

   from item in collection select item; -> Option D

4. Quick Check:

   LINQ starts with 'from' keyword [OK]

Hint: LINQ queries always start with 'from' keyword [OK]

Common Mistakes:

• Swapping 'select' and 'from' keywords
• Using foreach instead of LINQ syntax
• Writing incomplete or invalid query statements

3. What will be the output of this LINQ query?

int[] numbers = {1, 2, 3, 4, 5};
var result = from n in numbers where n > 3 select n;
foreach(var num in result) Console.Write(num + " ");

medium

A. 4 5

B. 1 2 3 4 5

C. 3 4 5

D. No output

Solution

1. Step 1: Understand the query condition

   The query selects numbers greater than 3 from the array {1,2,3,4,5}.

2. Step 2: Identify numbers > 3

   Numbers 4 and 5 satisfy the condition n > 3.

3. Final Answer:

   4 5 -> Option A

4. Quick Check:

   Filter numbers > 3 = 4 5 [OK]

Hint: Look for the 'where' condition filtering data [OK]

Common Mistakes:

• Including numbers equal to 3
• Printing all numbers ignoring the condition
• Assuming no output if condition is misunderstood

4. Identify the error in this LINQ query:

var result = from x in numbers where x => 5 select x;

medium

A. Missing semicolon at the end

B. The '=>' operator is incorrect for comparison

C. 'from' keyword is misspelled

D. 'select' keyword should be 'choose'

Solution

1. Step 1: Check the where clause syntax

   The where clause uses 'x => 5' which is incorrect for comparison; it should be 'x >= 5'.

2. Step 2: Verify other parts of the query

   The semicolon is present, 'from' and 'select' keywords are correct.

3. Final Answer:

   The '=>' operator is incorrect for comparison -> Option B

4. Quick Check:

   Use '>=' for comparison, not '=>' [OK]

Hint: Remember: '>=' is comparison, '=>' is lambda arrow [OK]

Common Mistakes:

• Confusing lambda '=>' with comparison '>='
• Thinking 'choose' replaces 'select'
• Ignoring syntax errors in where clause

5. You have a list of students with their scores. You want to get a dictionary of students who scored above 70, with their names as keys and scores as values. Which LINQ query correctly does this?

var students = new List<(string Name, int Score)>
{
    ("Alice", 85), ("Bob", 65), ("Charlie", 90)
};
var result = ???;

hard

A. students.Where(s => s.Score > 70).ToDictionary(s => s.Name, s => s.Score);

B. students.Select(s => s.Score > 70).ToDictionary(s => s.Name, s => s.Score);

C. students.ToDictionary(s => s.Name, s => s.Score > 70);

D. students.Where(s => s.Score > 70).Select(s => s.Name, s => s.Score);

Solution

1. Step 1: Filter students with score > 70

   Use Where to select only students scoring above 70.

2. Step 2: Convert filtered list to dictionary

   Use ToDictionary with key as Name and value as Score.

3. Final Answer:

   students.Where(s => s.Score > 70).ToDictionary(s => s.Name, s => s.Score); -> Option A

4. Quick Check:

   Filter then ToDictionary with correct keys and values [OK]

Hint: Filter first, then convert to dictionary with keys and values [OK]

Common Mistakes:

• Using Select incorrectly instead of Where
• Trying to create dictionary without filtering
• Passing wrong arguments to ToDictionary