LINQ query syntax in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all numbers from the list.

C Sharp (C#)

var numbers = new List<int> {1, 2, 3, 4, 5};
var query = from num in numbers select [1];

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

Anum

Bnumbers

Cnumber

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using the collection name instead of the iteration variable.

Using a variable name not defined in the query.

In LINQ query syntax, you select the variable defined in the from clause, here 'num'.

2fill in blank

medium

Complete the code to filter numbers greater than 3.

C Sharp (C#)

var numbers = new List<int> {1, 2, 3, 4, 5};
var query = from num in numbers where num [1] 3 select num;

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

A<=

B==

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>'.

Using '==' which only selects numbers equal to 3.

To filter numbers greater than 3, use the '>' operator in the where clause.

3fill in blank

hard

Fix the error in the LINQ query to order numbers ascending.

C Sharp (C#)

var numbers = new List<int> {5, 3, 1, 4, 2};
var query = from num in numbers orderby num [1] select num;

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

Adescending

Basc

Cascending

Dascend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'descending', which orders from highest to lowest.

Using invalid shorthands like 'asc' or 'ascend'.

In C# LINQ query syntax, use 'ascending' after 'orderby' to explicitly order ascending (this is the default behavior).

4fill in blank

hard

Fill both blanks to select names starting with 'A' and order them descending.

C Sharp (C#)

var names = new List<string> {"Alice", "Bob", "Anna", "Mike"};
var query = from name in names where name.StartsWith([1]) orderby name [2] select name;

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

A"A"

B"a"

Cascending

Ddescending

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'a' which misses uppercase names.

Using 'ascending' instead of 'descending' for order.

To select names starting with uppercase 'A', use "A" and order descending with 'descending'.

5fill in blank

hard

Fill all three blanks to select even numbers, order ascending, and select their squares.

C Sharp (C#)

var numbers = new List<int> {1, 2, 3, 4, 5};
var query = from num in numbers where num [1] 2 == 0 orderby num [2] select num * [3];

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

A%

Bascending

Cnum

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operator like '+' or '-' for even check.

Using 'descending' instead of 'ascending'.

Using incorrect value like '2' instead of 'num' for squaring.

Use '%' to check even numbers (num % 2 == 0), 'ascending' to order ascending, and 'num' to select squares (num * num).

Practice

1. What does the LINQ query syntax in C# primarily help you do with collections?

easy

A. Write low-level assembly code

B. Create new classes and objects

C. Manage memory allocation manually

D. Filter, sort, group, and select data in a readable way

Solution

1. Step 1: Understand LINQ purpose

   LINQ query syntax is designed to work with collections to filter, sort, group, and select data.

2. Step 2: Compare options

   Options B, C, and D describe unrelated programming tasks, not LINQ's purpose.

3. Final Answer:

   Filter, sort, group, and select data in a readable way -> Option D

4. Quick Check:

   LINQ = Data querying [OK]

Hint: LINQ reads like English to query collections [OK]

Common Mistakes:

• Confusing LINQ with object creation
• Thinking LINQ manages memory
• Assuming LINQ writes low-level code

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

easy

A. var result = select item from collection;

B. var result = collection from item select;

C. var result = from item in collection select item;

D. var result = item select from collection;

Solution

1. Step 1: Recall LINQ query syntax structure

   The correct LINQ query starts with 'from', then the range variable, then 'in' and the collection, followed by 'select'.

2. Step 2: Match options to syntax

   Only var result = from item in collection select item; follows this correct order: 'from item in collection select item;'. Others have wrong order or keywords.

3. Final Answer:

   var result = from item in collection select item; -> Option C

4. Quick Check:

   from ... in ... select ... [OK]

Hint: LINQ starts with 'from' then 'in' then 'select' [OK]

Common Mistakes:

• Swapping 'from' and 'select' keywords
• Omitting 'in' keyword
• Incorrect order of clauses

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

int[] numbers = {1, 2, 3, 4, 5};
var evens = from n in numbers
            where n % 2 == 0
            select n;
foreach(var num in evens) {
    Console.Write(num + " ");
}

medium

A. 1 3 5

B. 2 4

C. 1 2 3 4 5

D. No output

Solution

1. Step 1: Analyze the LINQ query filtering condition

   The query selects numbers where n % 2 == 0, meaning even numbers only.

2. Step 2: Identify even numbers in the array

   From {1,2,3,4,5}, even numbers are 2 and 4.

3. Final Answer:

   2 4 -> Option B

4. Quick Check:

   Filter evens = 2 4 [OK]

Hint: Look for 'where' condition filtering evens [OK]

Common Mistakes:

• Selecting odd numbers by mistake
• Ignoring the 'where' clause
• Assuming all numbers are selected

4. Identify the error in the following LINQ query syntax:

var result = from x collection
             where x > 10
             select x;

medium

A. Missing 'in' keyword between 'x' and 'collection'

B. Incorrect 'where' clause syntax

C. 'select' keyword should come before 'where'

D. Variable 'x' is not declared

Solution

1. Step 1: Check the 'from' clause syntax

   The 'from' clause must have 'from variable in collection'. Here 'in' is missing.

2. Step 2: Verify other clauses

   The 'where' and 'select' clauses are correctly placed and syntactically valid.

3. Final Answer:

   Missing 'in' keyword between 'x' and 'collection' -> Option A

4. Quick Check:

   'from x in collection' needed [OK]

Hint: Check 'from' clause for 'in' keyword [OK]

Common Mistakes:

• Omitting 'in' keyword in 'from' clause
• Misordering 'where' and 'select'
• Assuming variable declaration needed

5. Given a list of strings words, which LINQ query syntax correctly selects words starting with 'a' and orders them alphabetically?

hard

A. var query = from w in words where w.StartsWith("a") orderby w select w;

B. var query = from w in words orderby w where w.StartsWith("a") select w;

C. var query = from w in words select w where w.StartsWith("a") orderby w;

D. var query = from w in words select w orderby w where w.StartsWith("a");

Solution

1. Step 1: Understand LINQ clause order

   The correct order is 'from', then 'where', then 'orderby', then 'select'. var query = from w in words where w.StartsWith("a") orderby w select w; follows this order.

2. Step 2: Verify filtering and ordering

   var query = from w in words where w.StartsWith("a") orderby w select w; filters words starting with 'a' and orders them alphabetically before selecting.

3. Final Answer:

   var query = from w in words where w.StartsWith("a") orderby w select w; -> Option A

4. Quick Check:

   where before orderby in LINQ [OK]

Hint: Remember clause order: from, where, orderby, select [OK]

Common Mistakes:

• Placing 'orderby' before 'where'
• Misplacing 'select' clause
• Using wrong method syntax inside query