C# - How to check if an item exists in a list

In this article we’ll present and compare two functions which offer us a possibility to check whether an item exists in a given list. These functions are Exists and Contains, both available in System.Collections.Generic namespace. Provided examples are for two different lists: one containing integers and other one containing objects of custom created class.

List of integers

For the purpose of this article, let’s create an example list containing few integers.

var items = new List<int>() { 1, 2, 3, 5, 8, 13, 21 };

1	var items = new List<int>() { 1, 2, 3, 5, 8, 13, 21 };

Exists method

Exists function receives a predicate as an argument and returns bool representing the result of given formula. Predicate can be either simple comparison of two objects or really complicated equation. Returned value is true when list contains at least one item which meets defined criteria. No matter if one item is matched or multiple, the result will be the same.

public bool Exists (Predicate<T> match);

1	public bool Exists (Predicate<T> match);

var result1 = items.Exists(x => x == 8); // result1 = true
var result2 = items.Exists(x => x == 4); // result2 = false
var result3 = items.Exists(x => x > 5 && x < 20); // result3 = true

var result1 = items.Exists(x => x == 8); // result1 = true

var result2 = items.Exists(x => x == 4); // result2 = false

var result3 = items.Exists(x => x > 5 && x < 20); // result3 = true

Contains method

Contains behavior is similar to Exists, however instead of a predicate, it receives an argument which is actual value to be checked. The result type is the same, which is a bool indicating whether given item is in the list or not.

public bool Contains (T item);

1	public bool Contains (T item);

var result4 = items.Contains(5); // result4 = true
var result5 = items.Contains(10); // result5 = false

1 2	var result4 = items.Contains(5); // result4 = true var result5 = items.Contains(10); // result5 = false

List of custom objects

Now, let’s create a class which will be used to present examples of usage Exists and Contains with custom defined objects.

public class Person
{
    public string Forename { get; set; }
    public string Surname { get; set; }
}

public class Person

{

public string Forename { get; set; }

public string Surname { get; set; }

}

Having a class, we can create a list of Person objects and populate it with few entities.

var items = new List<Person>()
{
    new Person() { Forename = "John", Surname = "Smith" },
    new Person() { Forename = "Laura", Surname = "Jones" },
    new Person() { Forename = "Tom", Surname= "Wilson" }
};

var items = new List<Person>()

{

new Person() { Forename = "John", Surname = "Smith" },

new Person() { Forename = "Laura", Surname = "Jones" },

new Person() { Forename = "Tom", Surname= "Wilson" }

};

Exists method

The usage of Exists function is very similar as in list with integers. It’s required to provide a predicate which will be executed against items list to verify whether any item is matched. Again, predicate might be simple or extended formula.

var r1 = items.Exists(x => x.Forename == "Laura");
// r1 = true
var r2 = items.Exists(x => x.Forename == "John" && x.Surname == "Wilson");
// r2 = false
var r3 = items.Exists(x => x.Surname == "Smith" || x.Surname == "Wilson");
// r3 = true

var r1 = items.Exists(x => x.Forename == "Laura");

// r1 = true

var r2 = items.Exists(x => x.Forename == "John" && x.Surname == "Wilson");

// r2 = false

var r3 = items.Exists(x => x.Surname == "Smith" || x.Surname == "Wilson");

// r3 = true

Contains method

Contains method expects a Person object to be passed as a parameter.

Below we’ve created an object p1 where forename is Laura and surname is Jones. Even though such person exists on a list, we still get result r4 = false. It’s because the object we passed is not exactly the same instance as the one added to the list.

Another object, p2 is assigned as a second item from the list. This time Contains method returns true, because that object’s instance actually exists on the list.

var p1 = new Person()
{
    Forename = "Laura",
    Surname = "Jones"
};
var r4 = items.Contains(p1); // r4 = false

var p2 = items[1];
var r5 = items.Contains(p2); // r5 = true

var p1 = new Person()

{

Forename = "Laura",

Surname = "Jones"

};

var r4 = items.Contains(p1); // r4 = false

var p2 = items[1];

var r5 = items.Contains(p2); // r5 = true

If we’d like to change the way how objects are compared, we can easily do it by implementing IEquatable interface and defining our custom Equal function.

public class Person : IEquatable<Person>
{
    public string Forename { get; set; }
    public string Surname { get; set; }

    public bool Equals(Person? other)
    {
        if (other == null)
        {
            return false;
        }

        if (Forename == other.Forename && Surname == other.Surname)
        {
            return true;
        }

        return false;
    }
}

public class Person : IEquatable<Person>

{

public string Forename { get; set; }

public string Surname { get; set; }

public bool Equals(Person? other)

{

if (other == null)

{

return false;

}

if (Forename == other.Forename && Surname == other.Surname)

{

return true;

}