LINQ query syntax in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this LINQ query?

Consider the following C# code using LINQ query syntax. What will be printed?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] numbers = { 1, 2, 3, 4, 5 };
        var query = from n in numbers
                    where n % 2 == 0
                    select n * n;
        foreach (var item in query) {
            Console.Write(item + " ");
        }
    }
}

A 2 4

B 1 9 25

C 4 16

D 1 4 9 16 25

💡 Hint

Look at the where clause filtering even numbers, then the select projects squares.

The query filters numbers to only even ones (2 and 4), then selects their squares (4 and 16). So output is "4 16 ".

🧠 Conceptual

intermediate

Which LINQ query syntax correctly selects names starting with 'J'?

Given a list of names, which LINQ query syntax correctly selects only those starting with the letter 'J'?

C Sharp (C#)

string[] names = { "John", "Alice", "James", "Bob" };

A var result = from name in names select name where name.StartsWith("J");

B var result = from name in names where name.StartsWith("J") select name;

C var result = from name in names select name if name.StartsWith("J");

D var result = from name in names where name.Contains("J") select name;

💡 Hint

Remember the where clause comes before select in query syntax.

Option B correctly uses where to filter names starting with 'J' before selecting them. Option B and C have syntax errors. Option B filters names containing 'J' anywhere, not just starting with 'J'.

🔧 Debug

advanced

What error does this LINQ query produce?

Examine the following LINQ query code. What error will it cause when compiled?

C Sharp (C#)

int[] nums = {1, 2, 3};
var q = from n in nums
        where n > 1
        select n > 2 ? n : ;

A SyntaxError: incomplete ternary operator

B TypeError: cannot convert bool to int

C Runtime NullReferenceException

D No error, outputs 3

💡 Hint

Look carefully at the ternary operator syntax in the select clause.

The ternary operator is incomplete because the 'else' part after ':' is missing, causing a syntax error.

❓ Predict Output

advanced

What is the output of this LINQ query with ordering?

What will be printed by this C# program using LINQ query syntax with ordering?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        string[] fruits = { "apple", "banana", "cherry", "date" };
        var query = from f in fruits
                    orderby f.Length descending, f
                    select f;
        foreach (var fruit in query) {
            Console.Write(fruit + " ");
        }
    }
}

A date apple banana cherry

B banana cherry date apple

C cherry banana apple date

D banana cherry apple date

💡 Hint

Order by length descending, then alphabetically ascending.

Fruits ordered by length descending: banana(6), cherry(6), apple(5), date(4). For ties (banana, cherry), order alphabetically ascending: banana before cherry.

🧠 Conceptual

expert

How many items are in the resulting collection?

Given this LINQ query, how many items will the result collection contain?

C Sharp (C#)

int[] values = { 2, 4, 6, 8, 10 };
var result = from v in values
             where v % 4 == 0
             select v / 2;

A 2

B 3

C 4

D 5

💡 Hint

Check which numbers in the array are divisible by 4.

Only 4 and 8 are divisible by 4, so the query selects 2 items (4/2=2 and 8/2=4).

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