IEquatable<T>.Equals Method
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Indicates whether the current object is equal to another object of the same type.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- other
- Type: T
An object to compare with this object.
Return Value
Type: System.Booleantrue if the current object is equal to the other parameter; otherwise, false.
The implementation of the Equals method is intended to perform a test for equality with another object of type T, the same type as the current object. The Equals method is called in the following circumstances:
When the Equals method is called and other represents a strongly-typed object of type T. (Otherwise, if other is of type Object, the base Object.Equals(Object) method is called. Of the two methods, IEquatable<T>.Equals offers slightly better performance.
When the search methods of some generic collection objects are called. These types and their methods include the following:
Some of the generic overloads of the BinarySearch method.
The search methods of the List<T> class, including List<T>.Contains(T), List<T>.IndexOf, List<T>.LastIndexOf, and List<T>.Remove.
The search methods of the Dictionary<TKey, TValue> class, including ContainsKey and Remove.
In other words, to handle the possibility that objects of a class will be stored in an array or a generic collection object, it is a good idea to implement IEquatable<T> so that the object can be easily identified and manipulated.
When implementing the Equals method, define equality appropriately for the type specified by the generic type argument. For example, if the type argument is Int32, define equality appropriately for the comparison of two 32-bit signed integers.
The following example shows the partial implementation of a Person class that implements IEquatable<T> and has two properties, LastName and SSN. The Equals method returns True if the SSN property of two Person objects is identical; otherwise, it returns False.
using System; using System.Collections.Generic; using System.Text.RegularExpressions; public class Person : IEquatable<Person> { private string uniqueSsn; private string lName; public Person(string lastName, string ssn) { this.SSN = ssn; this.LastName = lastName; } public string SSN { get { return this.uniqueSsn; } set { if (Regex.IsMatch(value, @"\d{9}")) uniqueSsn = String.Format("{0}-(1}-{2}", value.Substring(0, 3), value.Substring(3, 2), value.Substring(5, 4)); else if (Regex.IsMatch(value, @"\d{3}-\d{2}-\d{4}")) uniqueSsn = value; else throw new FormatException("The social security number has an invalid format."); } } public string LastName { get { return this.lName; } set { if (String.IsNullOrEmpty(value)) throw new NullReferenceException("The last name cannot be null or empty."); else this.lName = value; } } public bool Equals(Person other) { if (other == null) return false; if (this.uniqueSsn == other.SSN) return true; else return false; } public override bool Equals(Object obj) { if (obj == null) return base.Equals(obj); Person personObj = obj as Person; if (personObj == null) return false; else return Equals(personObj); } public override int GetHashCode() { return this.SSN.GetHashCode(); } public static bool operator == (Person person1, Person person2) { if ((object)person1 == null || ((object)person2) == null) return Object.Equals(person1, person2); return person1.Equals(person2); } public static bool operator != (Person person1, Person person2) { if (person1 == null || person2 == null) return ! Object.Equals(person1, person2); return ! (person1.Equals(person2)); } }
Person objects can then be stored in a List<T> object and can be identified by the Contains method, as the following example shows.
public class Example { public static void Demo(System.Windows.Controls.TextBlock outputBlock) { // Create a Person object for each job applicant. Person applicant1 = new Person("Jones", "099-29-4999"); Person applicant2 = new Person("Jones", "199-29-3999"); Person applicant3 = new Person("Jones", "299-49-6999"); // Add applicants to a List object. List<Person> applicants = new List<Person>(); applicants.Add(applicant1); applicants.Add(applicant2); applicants.Add(applicant3); // Create a Person object for the final candidate. Person candidate = new Person("Jones", "199-29-3999"); if (applicants.Contains(candidate)) outputBlock.Text += String.Format("Found {0} (SSN {1}).", candidate.LastName, candidate.SSN) + "\n"; else outputBlock.Text += String.Format("Applicant {0} not found.", candidate.SSN) + "\n"; // Call the shared inherited Equals(Object, Object) method. // It will in turn call the IEquatable(Of T).Equals implementation. outputBlock.Text += String.Format("{0}({1}) already on file: {2}.", applicant2.LastName, applicant2.SSN, Person.Equals(applicant2, candidate)) + "\n"; } } // The example displays the following output: // Found Jones (SSN 199-29-3999). // Jones(199-29-3999) already on file: True.