LINQ with custom objects in C Sharp (C#) - Step-by-Step Execution

Concept Flow - LINQ with custom objects

Create list of objects

↓

Write LINQ query

↓

Evaluate query: filter/map

↓

Iterate results

↓

Output filtered/mapped objects

Start with a list of custom objects, write a LINQ query to filter or select, then iterate and output the results.

Execution Sample

C Sharp (C#)

var people = new List<Person> {
  new Person("Alice", 30),
  new Person("Bob", 20),
  new Person("Charlie", 25)
};
var adults = people.Where(p => p.Age >= 21);
foreach(var adult in adults) Console.WriteLine(adult.Name);

This code filters a list of Person objects to find those aged 21 or older and prints their names.

Execution Table

Step	Action	Evaluation	Result
1	Create list 'people'	List contains 3 Person objects	people = [Alice(30), Bob(20), Charlie(25)]
2	Apply Where filter p.Age >= 21	Check Alice: 30 >= 21	True, include Alice
3	Apply Where filter p.Age >= 21	Check Bob: 20 >= 21	False, exclude Bob
4	Apply Where filter p.Age >= 21	Check Charlie: 25 >= 21	True, include Charlie
5	Iterate filtered results	First: Alice	Output: Alice
6	Iterate filtered results	Second: Charlie	Output: Charlie
7	No more items	Iteration ends	Loop exits

💡 All people checked; only those 21 or older included; iteration ends after last item.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 7
people	empty	[Alice(30), Bob(20), Charlie(25)]	[Alice(30), Bob(20), Charlie(25)]	[Alice(30), Bob(20), Charlie(25)]
adults	null	deferred query	[Alice(30), Charlie(25)] (filtered)	[Alice(30), Charlie(25)] (filtered)

Key Moments - 2 Insights

Why does 'adults' not immediately contain filtered results after Step 2?

LINQ uses deferred execution, so 'adults' holds the query but does not run it until iteration (see Steps 5-7).

Why is Bob excluded from the output even though he is in the original list?

Because Bob's age (20) does not satisfy the filter condition p.Age >= 21 (see Steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the result of evaluating Bob's age in Step 3?

A True, include Bob

B False, exclude Bob

C Error, Bob missing age

D True, but exclude Bob

💡 Hint

Check Step 3 in the execution_table where Bob's age is compared to 21.

At which step does the program start outputting names?

A Step 5

B Step 4

C Step 2

D Step 7

💡 Hint

Look at the iteration steps in the execution_table where output happens.

If we change the filter to p.Age >= 20, how would the filtered list change?

A Only Alice included

B Alice and Charlie included

C Alice, Bob, and Charlie included

D Only Bob included

💡 Hint

Refer to variable_tracker and consider ages of all people compared to 20.

Concept Snapshot

LINQ with custom objects:
- Create a list of objects
- Use LINQ methods like Where to filter
- LINQ queries use deferred execution
- Iterate query results to get filtered objects
- Access object properties in lambda conditions

Full Transcript

This example shows how to use LINQ with a list of custom Person objects. First, we create a list with three people: Alice (30), Bob (20), and Charlie (25). Then, we write a LINQ query using Where to select only those with Age 21 or older. The query is not run immediately but waits until we iterate over it. When we loop through the filtered results, we print the names of Alice and Charlie because Bob does not meet the age condition. This demonstrates deferred execution and filtering with LINQ on custom objects.