Custom LINQ extension methods in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a LINQ extension method in C#?

A LINQ extension method is a special method that adds new query capabilities to existing types, allowing you to write queries in a fluent style using dot notation.

beginner

How do you define a custom LINQ extension method?

You define a custom LINQ extension method as a static method inside a static class, where the first parameter uses the 'this' keyword to specify the type it extends.

beginner

Why use custom LINQ extension methods?

Custom LINQ extension methods let you create reusable, readable, and chainable query operations tailored to your needs, making your code cleaner and easier to understand.

intermediate

What is the role of 'IEnumerable<T>' in custom LINQ extension methods?

Most custom LINQ extension methods extend 'IEnumerable<T>' because it represents a sequence of elements that can be queried, allowing your method to work with any collection that supports enumeration.

beginner

Show a simple example of a custom LINQ extension method that returns only even numbers from a sequence.

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

What keyword is used to define an extension method's first parameter?

A this

B static

C public

D override

The 'this' keyword before the first parameter tells the compiler this is an extension method for that type.

Where must extension methods be declared?

A Inside any class

B Inside a sealed class

C Inside an interface

D Inside a static class

Extension methods must be inside a static class to be recognized by the compiler.

Which interface do most LINQ extension methods extend?

A IList<T>

B IEnumerable<T>

C IDisposable

D IComparable

LINQ methods typically extend IEnumerable to work with any enumerable collection.

What does the 'yield return' statement do in an extension method?

A Ends the method immediately

B Returns all elements at once

C Returns elements one at a time lazily

D Throws an exception

'yield return' allows the method to return elements one by one as they are requested.

Why create custom LINQ extension methods?

A To add reusable query logic

B To replace built-in LINQ methods

C To make code slower

D To avoid using IEnumerable

Custom LINQ extension methods help add reusable and readable query logic tailored to your needs.

Explain how to create a custom LINQ extension method and why it is useful.

Think about how you add new features to existing types without changing them.

Describe the role of 'yield return' in a custom LINQ extension method.

Imagine handing out items one by one instead of all at once.

Practice

1. What is a key requirement for creating a custom LINQ extension method in C#?

easy

A. It must be a static method inside a static class with this on the first parameter.

B. It must be an instance method inside any class.

C. It must return void and take no parameters.

D. It must be declared inside the Main method.

Solution

1. Step 1: Understand extension method structure

   Extension methods must be static methods inside static classes.

2. Step 2: Identify the special parameter usage

   The first parameter must have the this keyword to specify the type it extends.

3. Final Answer:

   It must be a static method inside a static class with this on the first parameter. -> Option A

4. Quick Check:

   Extension method = static + static class + this param [OK]

Hint: Remember: static method + static class + this param [OK]

Common Mistakes:

• Making the method non-static
• Forgetting the static class container
• Omitting the 'this' keyword on the first parameter

2. Which of the following is the correct syntax to declare a custom LINQ extension method named IsEven for IEnumerable<int>?

easy

A. public static bool IsEven(this IEnumerable<int> source) { ... }

B. public bool IsEven(IEnumerable<int> source) { ... }

C. static bool IsEven(IEnumerable<int> source) { ... }

D. public static bool IsEven(IEnumerable<int> source) { ... }

Solution

1. Step 1: Check method modifiers and parameters

   The method must be public static and have this before the first parameter to be an extension method.

2. Step 2: Verify the parameter type and method signature

   public static bool IsEven(this IEnumerable<int> source) { ... } correctly uses this IEnumerable<int> source and is static and public.

3. Final Answer:

   public static bool IsEven(this IEnumerable<int> source) { ... } -> Option A

4. Quick Check:

   Extension method syntax = public static + this param [OK]

Hint: Look for 'public static' and 'this' on first parameter [OK]

Common Mistakes:

• Missing 'this' keyword on the first parameter
• Not making the method static
• Incorrect access modifier

3. Given this custom LINQ extension method, what will be the output?

public static IEnumerable<int> GetPositive(this IEnumerable<int> source) {
    foreach (var num in source) {
        if (num > 0) yield return num;
    }
}

var numbers = new List<int> { -1, 0, 3, 5, -2 };
var result = numbers.GetPositive().ToList();
Console.WriteLine(string.Join(",", result));

medium

A. -1,0,3,5,-2

B. 0,3,5

C. 3,5,-2

D. 3,5

Solution

1. Step 1: Understand the extension method logic

   The method yields only numbers greater than zero from the source list.

2. Step 2: Apply the method to the list

   From the list [-1, 0, 3, 5, -2], only 3 and 5 are greater than zero and yielded.

3. Final Answer:

   3,5 -> Option D

4. Quick Check:

   Filter positive numbers = 3,5 [OK]

Hint: Focus on the 'if (num > 0)' filter condition [OK]

Common Mistakes:

• Including zero as positive
• Returning all numbers without filtering
• Confusing yield return with return

4. Identify the error in this custom LINQ extension method:

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

medium

A. Cannot use 'yield return' inside a foreach loop.

B. Method should return void instead of IEnumerable<int>.

C. Missing 'this' keyword on the first parameter to make it an extension method.

D. The modulo operator '%' is not valid in C#.

Solution

1. Step 1: Check extension method signature

   The method lacks the this keyword on the first parameter, so it is not an extension method.

2. Step 2: Verify other syntax elements

   Using yield return inside a foreach is valid, and '%' is a valid operator in C#.

3. Final Answer:

   Missing 'this' keyword on the first parameter to make it an extension method. -> Option C

4. Quick Check:

   Extension method requires 'this' on first parameter [OK]

Hint: Extension methods always need 'this' on first parameter [OK]

Common Mistakes:

• Forgetting 'this' keyword
• Thinking 'yield return' is invalid in loops
• Misunderstanding the modulo operator

5. You want to create a custom LINQ extension method ToDictionaryByLength that converts an IEnumerable<string> into a Dictionary<int, List<string>>, grouping strings by their length. Which implementation correctly achieves this?

hard

A. public static Dictionary ToDictionaryByLength(IEnumerable<string> source) {
return source.ToDictionary(s => s.Length, s => new List<string> { s });
}

B. public static Dictionary ToDictionaryByLength(this IEnumerable<string> source) {
return source.GroupBy(s => s.Length)
.ToDictionary(g => g.Key, g => g.ToList());
}

C. public static Dictionary ToDictionaryByLength(this IEnumerable<string> source) {
var dict = new Dictionary();
foreach (var s in source) {
dict[s.Length] = new List<string> { s };
}
return dict;
}

D. public static Dictionary ToDictionaryByLength(this IEnumerable<string> source) {
return source.ToDictionary(s => s.Length, s => s);
}

Solution

1. Step 1: Understand grouping by length

   We need to group strings by their length, so GroupBy with key s.Length is correct.

2. Step 2: Convert groups to dictionary

   Use ToDictionary with group key and list of strings as value.

3. Step 3: Check other options

   public static Dictionary ToDictionaryByLength(IEnumerable<string> source) { return source.ToDictionary(s => s.Length, s => new List<string> { s }); } does not group; it tries to create dictionary with duplicate keys causing error. public static Dictionary ToDictionaryByLength(this IEnumerable<string> source) { var dict = new Dictionary(); foreach (var s in source) { dict[s.Length] = new List<string> { s }; } return dict; } overwrites values instead of adding to list. public static Dictionary ToDictionaryByLength(this IEnumerable<string> source) { return source.ToDictionary(s => s.Length, s => s); } tries to map to string instead of list.

4. Final Answer:

   public static Dictionary<int, List<string>> ToDictionaryByLength(this IEnumerable<string> source) { return source.GroupBy(s => s.Length) .ToDictionary(g => g.Key, g => g.ToList()); } -> Option B

5. Quick Check:

   GroupBy + ToDictionary with list = correct grouping [OK]

Hint: Use GroupBy then ToDictionary with list conversion [OK]

Common Mistakes:

• Not grouping before ToDictionary
• Overwriting dictionary entries instead of adding to list
• Returning wrong value types in dictionary