Custom LINQ extension methods in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Custom LINQ extension methods

Define static class

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Write extension method

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Use 'this' keyword on first parameter

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Call method on IEnumerable object

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Method executes on each element

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Return new IEnumerable or result

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Use result in code

You create a static class with a static method that extends IEnumerable<T> using 'this'. Then you call it like a normal LINQ method on collections.

Execution Sample

C Sharp (C#)

public static class MyLinqExtensions {
  public static IEnumerable<int> EvenNumbers(this IEnumerable<int> source) {
    foreach(var num in source) if(num % 2 == 0) yield return num;
  }
}

var nums = new List<int>{1,2,3,4};
var evens = nums.EvenNumbers();

This code defines an extension method EvenNumbers that filters even integers from a list.

Execution Table

Step	Action	Input	Condition	Output/Result
1	Call EvenNumbers on nums	[1,2,3,4]	N/A	Returns IEnumerable<int> (deferred)
2	Start foreach in EvenNumbers	1	1 % 2 == 0? False	No yield
3	Next foreach	2	2 % 2 == 0? True	yield return 2
4	Next foreach	3	3 % 2 == 0? False	No yield
5	Next foreach	4	4 % 2 == 0? True	yield return 4
6	End foreach	N/A	No more elements	Iteration ends
7	Use evens in code	IEnumerable<int>	N/A	Sequence: 2, 4

💡 All elements processed; method yields only even numbers.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
num	N/A	1	2	3	4	N/A	N/A
evens (deferred)	N/A	IEnumerable<int>	IEnumerable<int>	IEnumerable<int>	IEnumerable<int>	IEnumerable<int>	[2,4]

Key Moments - 3 Insights

Why does EvenNumbers return IEnumerable<int> but not a list immediately?

Because the method uses 'yield return', it creates a deferred sequence. The actual filtering happens when you iterate over the result, as shown in steps 2-6.

How does the 'this' keyword make EvenNumbers an extension method?

The 'this' keyword before the first parameter tells C# this method extends IEnumerable<int>. So you can call it like nums.EvenNumbers(), as in step 1.

What happens if the input list is empty?

The foreach loop in EvenNumbers won't run, so no elements are yielded. The output will be an empty IEnumerable, similar to step 6 but immediately.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'num' at Step 4?

A 2

B 4

C 3

D 1

💡 Hint

Check the 'num' variable column in variable_tracker after Step 4.

At which step does the method yield the first even number?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Output/Result' column in execution_table for when 'yield return' happens.

If the input list was empty, how would the execution table change?

A Method would yield all numbers anyway

B Steps 2-5 would be skipped; method ends immediately

C Method would throw an error

D Method would yield one element

💡 Hint

Refer to key_moments about empty input behavior.

Concept Snapshot

Custom LINQ extension methods:
- Define a static class
- Write static method with 'this' on first param
- Method extends IEnumerable<T>
- Use 'yield return' for deferred execution
- Call like normal LINQ method on collections

Full Transcript

This visual trace shows how to create and use a custom LINQ extension method in C#. We define a static class with a static method called EvenNumbers that extends IEnumerable<int> using the 'this' keyword. The method filters even numbers using a foreach loop and 'yield return' to produce a deferred sequence. When called on a list of numbers, the method does not immediately return a list but an IEnumerable that yields even numbers as you iterate. The execution table walks through each step of the foreach loop, showing which numbers are checked and which are yielded. The variable tracker shows how the 'num' variable changes each iteration and how the 'evens' variable holds the deferred sequence until iteration. Key moments clarify why 'yield return' causes deferred execution and how the 'this' keyword enables extension method syntax. The quiz tests understanding of variable values at steps, when yields happen, and behavior with empty input. This helps beginners see exactly how custom LINQ extension methods run step-by-step.