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Kenichiro Nakamura
Kenichiro Nakamura

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C#: Compare two List items

Linq has useful method to compare two lists.

Intersect

By using Intersect, we can check which elements in source list are also contained in compare list.

var source = new List<string>() { "a", "b", "c" };
var compare = new List<string>() { "b", "c", "d" };
var result = source.Intersect(compare);
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The result is below.

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Except

By using Except, we can check which elements in source list are not contained in compare list.

var source = new List<string>() { "a", "b", "c" };
var compare = new List<string>() { "b", "c", "d" };
var result = source.Except(compare);
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Complex type

We can also use complex type to compare list, by specifying which property to compare. If we use Person model for example, we cannot simply use Intersect nor Except. Following code result has no value.

var source = new List<Person>() { new Person("Ken", "Nakamura"), new Person("Nozomi", "Nakamura") };
var compare = new List<Person>() { new Person("Ken", "Nakamura"), new Person("Keiko", "Nakamura") };
var result = source.Intersect(compare);
Console.ReadLine();

public class Person
{
    public Person(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
    }
    public string FirstName { get; set; } = string.Empty;
    public string LastName { get; set; } = string.Empty;
}
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Use Comparer

Both method takes comparer as second argument. Let's compare by LastName.

public class PersonComparer : IEqualityComparer<Person>
{
    public bool Equals(Person x, Person y)
    {
        return x.LastName == y.LastName;
    }

    public int GetHashCode(Person x)
    {
        return x.LastName.GetHashCode();
    }
}
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Pass this class instance as second argument of Intersect.

var result = source.Intersect(compare, new PersonComparer());
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Use IntersectBy method

If we simply want to compare elements by key, then we can use IntersectBy instead of creating comparer. Following code generates same output as using comparer.

var source = new List<Person>() { new Person("Ken", "Nakamura"), new Person("Nozomi", "Nakamura") };
var compare = new List<Person>() { new Person("Ken", "Nakamura"), new Person("Keiko", "Nakamura") };
var result = source.IntersectBy(compare.Select(x => x.LastName), x => x.LastName);
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Use IEquatable

Other way to achieve the same is to have IEquatable for class itself.

public class Person: IEquatable<Person>
{
    public Person(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
    }

    public string FirstName { get; set; } = string.Empty;
    public string LastName { get; set; } = string.Empty;

    public bool Equals(Person? other) => this.LastName == other?.LastName;
    public override int GetHashCode() => (LastName).GetHashCode();
}
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Then we can simply call Intersect.

var result = source.Intersect(compare);
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Summary

There are multiple ways to compare two lists which depends on our implementation.

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