LINQ Distinct

Summary: in this tutorial, you’ll learn how to use the LINQ Distinct() method to return distinct elements of a sequence.

Introduction to the LINQ Distinct() method

LINQ Distinct() is an extension method that returns unique elements from a sequence:

IEnumerable<TSource> Distinct<TSource>(
   this IEnumerable<TSource> source
)Code language: C# (cs)

In this syntax:

TSource is the type of element of the input sequence.
source is the input sequence.

The Distinct() method returns an IEnumerable<T> object that contains distinct elements of the source sequence. Note that the result is an unordered sequence.

The Distinct() method uses a default equality comparer to identify unique elements in the sequence. If you want to use a custom equality comparer, you can use the following overload of the Distinct() method:

IEnumerable<TSource> Distinct<TSource>(
    this IEnumerable<TSource> source,
    IEqualityComparer<TSource>? comparer
);Code language: C# (cs)

In this syntax, you pass an equality comparer to the Distinct() method to compare two elements to identify the distinction.

LINQ Distinct() method examples

Let’s take some examples of using the LINQ Distinct() method.

1) Using LINQ Distinct() method to get a sequence of unique numbers

The following program demonstrates how to use the LINQ Distinct() method to get a sequence of unique numbers from an array of integers.

using static System.Console;

int[] scores = { 1, 2, 2, 3, 4, 4, 5 };

var uniqueScores = scores.Distinct();

foreach (var score in uniqueScores)
{
    Write($"{score} ");
}Code language: C# (cs)

Output:

1 2 3 4 5Code language: C# (cs)

How it works.

First, declare and initialize the scores array with seven integers:

int[] scores = { 1, 2, 2, 3, 4, 4, 5 };Code language: C# (cs)

Second, use Distinct() method to return a new sequence that contains only unique numbers from the scores array. In this case, it returns {1,2,3,4,5}:

var uniqueScores = scores.Distinct();Code language: C# (cs)

Third, write each number in the uniqueScores to the console using a foreach loop and Write() method:

foreach (var score in uniqueScores)
{
    Write($"{score} ");
}Code language: C# (cs)

2) Using LINQ Distinct() method to get a sequence of the unique strings

The following program demonstrates how to use the Distinct() method to return unique string values from an array of the most popular oceans:

using static System.Console;

string[] oceans = { 
    "Pacific", 
    "Atlantic", 
    "Indian", 
    "Southern", 
    "Arctic", 
    "Atlantic", // duplicate 
    "Pacific"   // duplicate
};

var uniqueOceans = oceans.Distinct();

foreach (string ocean in uniqueOceans)
{
    WriteLine(ocean);
}Code language: C# (cs)

Output:

Pacific
Atlantic
Indian
Southern
ArcticCode language: C# (cs)

How it works.

First, declare an array of strings called oceans that contains the most popular oceans. The array contains the duplicate values of “Atlantic” and “Pacific”.

Second, use the Distinct() method to return a new sequence that contains only unique string values from the oceans array.

Third, write each string value in the oceans sequence to the console using a foreach loop and WriteLine method.

3) Using LINQ Distinct() method to return a sequence of unique objects

The following program demonstrates how to use the Distinct() method to retrieve unique products from a list of products:

using static System.Console;

class Product
{
    public int? Id  { get; set; }
    public string? Name { get; set; }
    public decimal? Price { get; set; }

    public override string ToString()
    {
        return $"{Id} {Name} {Price:C}";
    }


}

// Define a custom ProductComparer class that compares
// Product objects based on their id
class ProductComparer : IEqualityComparer<Product>
{
    public bool Equals(Product? x, Product? y)
    {
        // Check for null values
        if (x == null || y == null)
            return false;

        // Check if the two Product objects are the same reference
        if (ReferenceEquals(x, y))
            return true;

        // Compare the Id of the two Product objects
        // to determine if they're the same
        return x.Id == y.Id;
    }

    public int GetHashCode(Product? obj)
    {
        if (obj == null || obj.Id == null)
            return 0;

        return obj.Id.GetHashCode();
    }
}
class Program
{
    static void Main(string[] args)
    {
        // Create a List of Product objects
        List<Product> products = new() {
            new Product { Id =1, Name = "A", Price=9.99M },
            new Product { Id =2, Name = "B", Price=15.99M },
            new Product { Id =3, Name = "C", Price=5.99M },
            new Product { Id =1, Name = "A", Price=9.99M },
            new Product { Id =2, Name = "B", Price=15.99M },
        };

        var uniqueProducts = products.Distinct(new ProductComparer());

        foreach (var product in uniqueProducts)
        {
            WriteLine(product);
        }
        
    }
}Code language: C# (cs)

Output:

1 A $9.99
2 B $15.99
3 C $5.99Code language: C# (cs)

How it works.

First, define a Product class that has three properties Id, Name, and Price.

Next, define the ProductComparer that compares two products and returns true if two products have the same id.

Then, create a new list of products with five Product objects. Notice that the list contains duplicate Product objects.

After that, retrieve unique Product objects from the list using the Distinct() method. The Distinct() method uses a ProductComparer object to identify the unique products.

Finally, write each product to the console using a foreach loop and the Writeline method.

Summary

Use the LINQ Distinct() method to retrieve unique elements of a sequence.

Was this tutorial helpful ?