I originally posted an extended version of this on my blog. It's part of a series about LINQ and other C# features.
Today a friend asked me about LINQ. She was studying for a technical interview. So, dear Alice: This is what LINQ is and these are the most common LINQ methods with examples in C#.
Language-Integrated Query (LINQ) is the declarative way of working with collections in C#. The most common LINQ methods are: Where
, Select
, Any
GroupBy
and FirstOrDefault
. Apart from extensions methods on the IEnumerable
type, LINQ has a query syntax, a SQL-like syntax.
1. LINQ is declarative
It means you write your code stating the results you want, instead of doing every step to get those results.
With LINQ, you write code to "filter a collection based on a condition". Instead of writing code to "grab an element, check if it satisfies a condition, then move to the next element, check again...", etc.
LINQ is a better alternative to query collections using for
, foreach
or any other loop. Because, with LINQ you can write more expressive and compact code.
Let's find our favorite movies
Let's start with the collection of movies we have watched. We have a Movie
class with a name, release year and rating. Let's find our favorite movies, the ones with rating greater than 4.5.
Here's a console program to print our favorite movies. Let's use records and top-level statements from recent C# versions.
var movies = new List<Movie>
{
new Movie("Titanic", 1998, 4.5f),
new Movie("The Fifth Element", 1997, 4.6f),
new Movie("Terminator 2", 1991, 4.7f),
new Movie("Avatar", 2009, 5),
new Movie("Platoon", 1986, 4),
new Movie("My Neighbor Totoro", 1988, 5)
};
var favorites = new List<Movie>();
foreach (var movie in movies) // π
{
if (movie.Rating > 4.5) // π
{
favorites.Add(movie);
}
}
foreach (var favorite in favorites)
{
Console.WriteLine($"{favorite.Name}: [{favorite.Rating}]");
}
// Output:
// The Fifth Element: [4.6]
// Terminator 2: [4.7]
// Avatar: [5]
// My Neighbor Totoro: [5]
record Movie(string Name, int ReleaseYear, float Rating);
Notice the foreach
loop and the comparison using an if
statement to look for ratings greater than 4.5.
Change the example to use your own movies and see which ones are your favorites!
Let's filter a collection using Where
LINQ methods are extension methods on the IEnumerable
type.
This type represents objects we can loop through, like, arrays, lists, dictionaries, among others.
To work with LINQ, you need to be comfortable with delegates and lambda expressions. A delegate is a reference to a method. And, a lambda function is a method with only the parameters and the body.
Now, to the actual example.
If we want to filter our list of movies to keep only the ones with rating greater than 4.5, the LINQ method to filter collections is Where
.
The LINQ Where
method returns a new collection with all the elements that meet a condition.
Let's replace the foreach
statement from our first example with the Where
method. And, let's use the condition inside the if
statement as the filter condition for the Where
method.
Our example looks like this:
var movies = new List<Movie>
{
new Movie("Titanic", 1998, 4.5f),
new Movie("The Fifth Element", 1995, 4.6f),
new Movie("Terminator 2", 1999, 4.7f),
new Movie("Avatar", 2010, 5),
new Movie("Platoon", 1986, 4),
new Movie("My Neighbor Totoro", 1988, 5)
};
var favorites = movies.Where(movie => movie.Rating > 4.5);
// ππ
foreach (var favorite in favorites)
{
Console.WriteLine($"{favorite.Name}: [{favorite.Rating}]");
}
record Movie(string Name, int ReleaseYear, float Rating);
We replaced the foreach
and if
statements with a single line of code:
var favorites = movies.Where(movie => movie.Rating > 4.5);
More compact, isn't it? Also, we turned the condition inside the if
statement into a lambda function.
Instead of lambda functions, you could use private methods too.
LINQ methods don't change the original collection.
They return a result without modifying the original collection. From our example, when we used the Where
method, it returned a new collection. It didn't remove any elements from the original movies
list.
2. Most common LINQ methods
So far, we have seen only one LINQ method, Where
. Of course, LINQ has more methods.
These are the most common LINQ methods:
1. Select
With Select
, you can transform every element of a collection. It applies a function on every element.
Let's find only the names of our favorite movies.
var favorites = movies.Where(movie => movie.Rating > 4.5)
.Select(movie => movie.Name);
// ππ
foreach (var name in favorites)
{
Console.WriteLine(name);
}
// Output:
// The Fifth Element
// Terminator 2
// Avatar
// My Neighbor Totoro
This time we have nested two LINQ methods. The result from Where
will be the input of Select
.
For more readability, we often align the nested LINQ methods vertically by the (.) dot.
2. Any
The Any
method check if a collection is empty. Also, it checks if a collection has at least one element matching a condition. It doesn't return a new collection. It returns either true
or false
.
Let's see if we have watched movies with a low rating.
var hasAnyMovies = movies.Any();
// true
var hasBadMovies = movies.Any(movie => movie.Rating < 2);
// false
3. GroupBy
GroupBy
groups the elements of a collection based on a key. It returns a collection of "groups" or "buckets" organized by a key.
Let's group our movies by rating.
var groupedByRating = movies.GroupBy(movie => movie.Rating); // π
foreach (var group in groupedByRating)
{
Console.WriteLine($"Rating: {group.Key}");
foreach (var movie in group)
{
Console.WriteLine($"{movie.Name}");
}
Console.WriteLine();
}
// Output:
// Rating: 4.5
// Titanic
//
// Rating: 4.6
// The Fifth Element
//
// Rating: 4.7
// Terminator 2
//
// Rating: 5
// Avatar
// My Neighbor Totoro
//
// Rating: 4
// Platoon
Also, GroupBy
allows you to transform each group.
This time, let's count the movies with the same rating.
var groupedByRating = movies.GroupBy(movie => movie.Rating,
(rating, movies) => new // π
{
Rating = rating,
Count = movies.Count()
});
foreach (var group in groupedByRating)
{
Console.WriteLine($"{group.Rating}: [{group.Count}]");
}
// Output:
// 4.5: [1]
// 4.6: [1]
// 4.7: [1]
// 5: [2]
// 4: [1]
Notice the second parameter of the GroupBy
. It's a Func
with the key and the elements of each group.
We also used an anonymous object new { Rating=..., Count=... }
. It's like a regular object, but we didn't specify a name.
4. First and FirstOrDefault
First
and FirstOrDefault
return the first element in a collection. First
throws an exception if the collection is empty. Unlike First
, FirstOrDefault
returns a default value if the collection is empty.
Also, First
and FirstOrDefault
return the first element matching a condition.
Let's find the oldest movie we have watched.
var oldest = movies.OrderBy(movie => movie.ReleaseYear)
.First(); // π
// Platoon
This time, we used the OrderBy
to sort the movies collection by release year. To then, grab the first one. Two examples for the price of one!
In the same spirit of First
and FirstOrDefault
, you have Last
and LastOrDefault
. But, they return the last element instead of the first one.
3. Cheatsheet
There are more LINQ methods than the ones we've seen so far. These are some of them.
Method | Function |
---|---|
Where |
Filter a collection |
Select |
Transform every element of a collection |
Any |
Check if a collection is empty |
Count |
Count all elements of a collection |
Distinct |
Find the unique elements of a collection |
GroupBy |
Group the elements of a collection based on a key |
OrderBy |
Sort a collection based on a key |
First |
Find the first element of a collection. Throw if the collection is empty |
FirstOrDefault |
Same as First , but it returns a default value if it's empty |
Last |
Find the last element of a collection. Throw if the collection is empty |
LastOrDefault |
It returns a default value if it's empty, instead |
Single |
Find only one element in a collection matching a condition. Throw, otherwise |
SingleOrDefault |
It returns a default value if there isn't one matching element, instead |
Take |
Pick the first n consecutive elements of a collection |
TakeWhile |
Pick the first consecutive elements that satisfy a condition |
Skip |
Return a collection without the first n consecutive elements |
SkipWhile |
Return a collection without the first consecutive elements that satisfy a condition |
Sum |
Sum the elements of a collection |
Min , Max
|
Find the smallest and largest element of a collection |
ToDictionary |
Convert a collection into a dictionary |
VoilΓ ! That's it, Alice. That's all you need to know to start working with LINQ in your code in less than 15 minutes. With LINQ you can write more compact and expressive code.
The next time you need to write logic using loops, give LINQ a try!
Want to write more expressive code for collections? Join my Udemy course, Getting Started with LINQ, and master everything you need to work productively with LINQ β all in less than two hours!
Happy LINQ time!
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