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IEquatable<T>.Equals-Methode: (T)

 

Veröffentlicht: Oktober 2016

Gibt an, ob das aktuelle Objekt gleich einem anderen Objekt des gleichen Typs ist.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

bool Equals(
	T other
)

Parameter

other
Type: T

Ein Objekt, das mit diesem Objekt verglichen werden soll.

Rückgabewert

Type: System.Boolean

true, wenn das aktuelle Objekt gleich dem other-Parameter ist, andernfalls false.

The implementation of the M:System.IEquatable`1.Equals(`0) method is intended to perform a test for equality with another object of type T, the same type as the current object. The M:System.IEquatable`1.Equals(`0) method is called in the following circumstances:

  • When the Equals method is called and the other parameter represents a strongly-typed object of type T. (If other is of type T:System.Object, the base M:System.Object.Equals(System.Object) method is called. Of the two methods, M:System.IEquatable`1.Equals(`0) offers slightly better performance.)

  • When the search methods of a number of generic collection objects are called. Some of these types and their methods include the following:

    • Some of the generic overloads of the M:System.Array.BinarySearch(System.Array,System.Object) method.

    • The search methods of the T:System.Collections.Generic.List`1 class, including M:System.Collections.Generic.List`1.Contains(`0), M:System.Collections.Generic.List`1.IndexOf(`0), M:System.Collections.Generic.List`1.LastIndexOf(`0), and M:System.Collections.Generic.List`1.Remove(`0).

    • The search methods of the T:System.Collections.Generic.Dictionary`2 class, including M:System.Collections.Generic.Dictionary`2.ContainsKey(`0) and M:System.Collections.Generic.IDictionary`2.Remove(`0).

    • The search methods of the generic T:System.Collections.Generic.LinkedList`1 class, including M:System.Collections.Generic.LinkedList`1.Contains(`0) and M:System.Collections.Generic.LinkedList`1.Remove(System.Collections.Generic.LinkedListNode{`0}).

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 T:System.IEquatable`1 so that the object can be easily identified and manipulated.

When implementing the M:System.IEquatable`1.Equals(`0) method, define equality appropriately for the type specified by the generic type argument. For example, if the type argument is T:System.Int32, define equality appropriately for the comparison of two 32-bit signed integers.

Hinweise für Implementierer:

If you implement M:System.IEquatable`1.Equals(`0), you should also override the base class implementations of M:System.Object.Equals(System.Object) and M:System.Object.GetHashCode so that their behavior is consistent with that of the M:System.IEquatable`1.Equals(`0) method. If you do override M:System.Object.Equals(System.Object), your overridden implementation is also called in calls to the static Equals(System.Object, System.Object) method on your class. In addition, you should overload the op_Equality and op_Inequality operators. This ensures that all tests for equality return consistent results, which the example illustrates.

The following example shows the partial implementation of a Person class that implements T:System.IEquatable`1 and has two properties, LastName and SSN. The M:System.IEquatable`1.Equals(`0) 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)
   {
      if (Regex.IsMatch(ssn, @"\d{9}"))
         uniqueSsn = String.Format("{0}-(1}-{2}", ssn.Substring(0, 3),
                                                  ssn.Substring(3, 2),
                                                  ssn.Substring(5, 4));
      else if (Regex.IsMatch(ssn, @"\d{3}-\d{2}-\d{4}"))
         uniqueSsn = ssn;
      else
         throw new FormatException("The social security number has an invalid format.");

      uniqueSsn = ssn;
      this.LastName = lastName;
   }

   public string SSN
   {
      get { return this.uniqueSsn; }
   }

   public string LastName
   {
      get { return this.lName; }
      set {
         if (String.IsNullOrEmpty(value))
            throw new ArgumentException("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.uniqueSsn)
         return true;
      else
         return false;
   }

   public override bool Equals(Object obj)
   {
      if (obj == null)
         return false;

      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 (((object)person1) == null || ((object)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 TestIEquatable
{
   public static void Main()
   {
      // 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))
          Console.WriteLine("Found {0} (SSN {1}).", 
                             candidate.LastName, candidate.SSN);
      else
         Console.WriteLine("Applicant {0} not found.", candidate.SSN);

      // Call the shared inherited Equals(Object, Object) method.
      // It will in turn call the IEquatable(Of T).Equals implementation.
      Console.WriteLine("{0}({1}) already on file: {2}.",  
                        applicant2.LastName, 
                        applicant2.SSN, 
                        Person.Equals(applicant2, candidate)); 
   }
}
// The example displays the following output:
//       Found Jones (SSN 199-29-3999).
//       Jones(199-29-3999) already on file: True.

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