LINQ query syntax in C Sharp (C#) - Step-by-Step Execution

Concept Flow - LINQ query syntax

Start with data source

↓

Write query expression

↓

Use from, where, select clauses

↓

Query is not executed yet

↓

Execute query by foreach or ToList()

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Get filtered and projected results

LINQ query syntax starts with a data source, then you write a query using from, where, and select clauses. The query runs when you iterate or convert it.

Execution Sample

C Sharp (C#)

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.WriteLine(num);

This code selects even numbers from an array and prints them.

Execution Table

Step	Action	Evaluation	Result
1	Start with numbers array	numbers = {1,2,3,4,5}	Data source ready
2	Define query: from n in numbers	n takes each number	Query prepared but not run
3	Apply where n % 2 == 0	Check each n: 1%2=1 (false), 2%2=0 (true), 3%2=1 (false), 4%2=0 (true), 5%2=1 (false)	Filter applied
4	Select n	Select numbers passing where	Selected: 2,4
5	Execute query with foreach	Iterate over selected numbers	Output: 2 4
6	End	No more numbers	Query complete

💡 All numbers checked; query executed and results printed

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
numbers	{1,2,3,4,5}	{1,2,3,4,5}	{1,2,3,4,5}	{1,2,3,4,5}
n	undefined	1,2,3,4,5 (each in turn)	2,4 (filtered)	2,4 (selected)
evens	undefined	query defined	query defined	2,4 (after execution)

Key Moments - 3 Insights

Why doesn't the query run immediately after defining it?

The query is only defined at first (see Step 2 in execution_table). It runs later when we iterate over it (Step 5). This is called deferred execution.

What does the 'where' clause do in the query?

The 'where' clause filters the data. It checks each number and keeps only those where the condition is true (Step 3 shows how numbers are tested).

How does the 'select' clause affect the output?

The 'select' clause chooses what to output from each filtered item. Here it selects the number itself (Step 4), so output is the filtered numbers.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'n' when the where condition is true for the first time?

A 1

B 2

C 3

D 4

💡 Hint

Check Step 3 in execution_table where each 'n' is tested against the condition.

At which step does the query actually run and produce output?

A Step 2

B Step 3

C Step 5

D Step 4

💡 Hint

Look for the step where foreach iterates and prints results in execution_table.

If we remove the where clause, what would be the output after execution?

A All numbers 1 to 5

B Only odd numbers

C Only even numbers

D No output

💡 Hint

Without filtering (where), select outputs all numbers from the source.

Concept Snapshot

LINQ query syntax:
- Start with 'from' to pick data source
- Use 'where' to filter items
- Use 'select' to choose output
- Query runs when iterated (deferred execution)
- Example: from n in numbers where n%2==0 select n

Full Transcript

This visual trace shows how LINQ query syntax works in C#. We start with a data source array of numbers. We write a query using 'from' to pick each number, 'where' to filter even numbers, and 'select' to choose them. The query is defined but not run until we loop over it with foreach. Step by step, each number is checked against the condition. Only numbers passing the filter are selected. Finally, the query runs and prints the even numbers 2 and 4. Variables like 'n' change as each number is processed. This shows deferred execution and how query clauses work together.