Kenichiro Nakamura Posted on Jun 30
Linq has useful method to compare two lists. IntersectBy 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); The result is below. ExceptBy 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); Complex typeWe 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; } Use ComparerBoth 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(); } } Pass this class instance as second argument of Intersect. var result = source.Intersect(compare, new PersonComparer()); Use IntersectBy methodIf 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); Use IEquatableOther 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(); } Then we can simply call Intersect. var result = source.Intersect(compare); SummaryThere are multiple ways to compare two lists which depends on our implementation.
Shinji Nakamatsu - Jul 26
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