Distinct and Contains

Learn to deduplicate with Distinct and check membership with Contains.

Deduplication with Distinct

The Distinct method removes duplicate elements from a collection; each value appears only once. new[] { 1, 2, 2, 3, 3, 3 }.Distinct() yields { 1, 2, 3 }. Equality is checked with the default Equals.

When you need a custom equality comparer, the Distinct(comparer) overload is available. The result is again an IEnumerable<T>; order is preserved: the first occurrence of each value is kept.

Membership with Contains

The Contains method asks whether the collection includes a specific value. numbers.Contains(7) checks whether 7 is in the array and returns bool. This is shorter and clearer than numbers.Any(n => n == 7).

Contains uses Equals by default for reference types. For custom equality there is a Contains(value, comparer) overload. Lists do a linear search with Contains; for large data, membership on a HashSet<T> is faster.

Exercise Print distinct numbers

Remove duplicates from the numbers array with Distinct and print each in a foreach loop.

Show example solution

Dedupe with numbers.Distinct(), loop with foreach, print with Console.WriteLine(n).
Example solution:
using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int[] numbers = { 1, 2, 2, 3, 3, 3 };
        foreach (var n in numbers.Distinct())
        {
            Console.WriteLine(n);
        }
    }
}

Test yourself

Match the Distinct result

{1,2,2,3}

{1,2,3}
{a,a,b}

{a,b}
{5,5,5}

{5}
{x,y}

{x,y}

Complete the Contains call

bool has = numbers.____(7);

Show answer

bool has = numbers.Contains(7);

What does Distinct return?
```
new[] { 4, 4, 5, 5 }.Distinct()
```
- int (a number)
- bool
- IEnumerable<int>
- List<int>
Show answer

List<int> — Correct. Distinct returns IEnumerable<int>; it is a deferred query. Convert with ToList if needed.

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