LINQ query syntax in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does LINQ stand for in C#?

LINQ stands for Language Integrated Query. It allows querying data in a readable and concise way directly in C# code.

beginner

What is the basic structure of a LINQ query using query syntax?

A LINQ query using query syntax typically has this structure:

from item in collection
where condition
select item

It reads like a sentence: from a collection, pick items that meet a condition, then select them.

beginner

How do you filter elements in a LINQ query syntax?

You use the where keyword to filter elements. For example:

from num in numbers
where num > 5
select num

This selects numbers greater than 5.

intermediate

Can you use multiple conditions in a LINQ query's where clause?

Yes, you can combine conditions using logical operators like && (and) or || (or).
Example:

from item in collection
where item.Age > 18 && item.IsActive
select item

beginner

What keyword do you use in LINQ query syntax to order results?

You use the orderby keyword to sort results.
Example:

from num in numbers
orderby num descending
select num

This orders numbers from highest to lowest.

Which keyword starts a LINQ query in query syntax?

A from

B select

C where

D orderby

A LINQ query starts with the from keyword to specify the data source.

How do you select only items where a number is greater than 10?

A where num < 10

B select num > 10

C orderby num > 10

D where num > 10

The where clause filters items. Use where num > 10 to select numbers greater than 10.

Which keyword is used to sort results in LINQ query syntax?

A sortby

B orderby

C arrange

D filter

The orderby keyword sorts the results in ascending or descending order.

What does the select keyword do in a LINQ query?

A Starts the query

B Filters data

C Specifies the output of the query

D Sorts the data

select defines what the query returns, like which fields or objects.

Can you use multiple where clauses in one LINQ query?

A Yes, multiple <code>where</code> clauses can be chained

B No, only one <code>where</code> is allowed

C Yes, but only one is effective

D Only if combined with <code>orderby</code>

You can chain multiple where clauses to filter data step by step.

Explain the basic structure of a LINQ query using query syntax and how it works.

Think about how you pick items from a list based on conditions.

Describe how to filter and sort data using LINQ query syntax with examples.

Filtering is like choosing only what you want; sorting is arranging them.

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