Predicate<T> Predicate<T> Predicate<T> Predicate<T> Delegate

Définition

Représente la méthode qui définit un ensemble de critères et détermine si l'objet spécifié répond à ces critères.Represents the method that defines a set of criteria and determines whether the specified object meets those criteria.

generic <typename T>
public delegate bool Predicate(T obj);
public delegate bool Predicate<in T>(T obj);
type Predicate<'T> = delegate of 'T -> bool
Public Delegate Function Predicate(Of In T)(obj As T) As Boolean 

Paramètres de type

T

Type de l'objet à comparer.The type of the object to compare.

Paramètres

obj

Objet à comparer par rapport aux critères définis dans la méthode représentée par ce délégué.The object to compare against the criteria defined within the method represented by this delegate.

Valeur renvoyée

System.Boolean

true si obj correspond aux critères définis dans la méthode représentée par ce délégué ; sinon, false.true if obj meets the criteria defined within the method represented by this delegate; otherwise, false.

Héritage
Predicate<T>Predicate<T>Predicate<T>Predicate<T>

Exemples

Le code suivant exemple utilise un Predicate<T> délégué avec le Array.Find méthode pour rechercher un tableau de Point structures.The following code example uses a Predicate<T> delegate with the Array.Find method to search an array of Point structures. L’exemple définit explicitement une Predicate<T> délégué nommé predicate et lui attribue une méthode nommée FindPoints qui retourne true si le produit de la Point.X et Point.Y champs est supérieur à 100 000.The example explicitly defines a Predicate<T> delegate named predicate and assigns it a method named FindPoints that returns true if the product of the Point.X and Point.Y fields is greater than 100,000. Notez qu’il est habituel d’utiliser une expression lambda au lieu de définir explicitement un délégué du type Predicate<T>, comme l’illustre le deuxième exemple.Note that it is customary to use a lambda expression rather than to explicitly define a delegate of type Predicate<T>, as the second example illustrates.

using System;
using System.Drawing;

public class Example
{
   public static void Main()
   {
      // Create an array of Point structures.
      Point[] points = { new Point(100, 200), 
                         new Point(150, 250), new Point(250, 375), 
                         new Point(275, 395), new Point(295, 450) };

      // Define the Predicate<T> delegate.
      Predicate<Point> predicate = FindPoints;
      
      // Find the first Point structure for which X times Y  
      // is greater than 100000. 
      Point first = Array.Find(points, predicate);

      // Display the first structure found.
      Console.WriteLine("Found: X = {0}, Y = {1}", first.X, first.Y);
   }

   private static bool FindPoints(Point obj)
   {
      return obj.X * obj.Y > 100000;
   }
}
// The example displays the following output:
//        Found: X = 275, Y = 395
Imports System.Drawing

Public Class Example
   Public Shared Sub Main()

      ' Create an array of Point structures. 
      Dim points() As Point = { new Point(100, 200), new Point(150, 250), 
                                new Point(250, 375), new Point(275, 395), 
                                new Point(295, 450) }
      
      ' Define the Predicate(Of T) delegate.
      Dim predicate As Predicate(Of Point) = AddressOf Example.FindPoints
      
      ' Find the first Point structure for which X times Y  
      ' is greater than 100000. 
      Dim first As Point = Array.Find(points, predicate)

      ' Display the first structure found.
      Console.WriteLine("Found: X = {0}, Y = {1}", first.X, first.Y)
   End Sub 
   
   Private Shared Function FindPoints(obj As Point) As Boolean
      Return obj.X * obj.Y > 100000
   End Function

End Class 
' The example displays the following output:
'       Found: X = 275, Y = 395

L’exemple suivant est identique à l’exemple précédent, sauf qu’elle utilise une expression lambda pour représenter le Predicate<T> déléguer.The following example is identical to the previous example, except that it uses a lambda expression to represent the Predicate<T> delegate. Chaque élément de la points tableau est passé à l’expression lambda jusqu'à ce que l’expression recherche un élément qui répond aux critères de recherche.Each element of the points array is passed to the lambda expression until the expression finds an element that meets the search criteria. Dans ce cas, l’expression lambda retourne true si le produit des champs X et Y est supérieur à 100 000.In this case, the lambda expression returns true if the product of the X and Y fields is greater than 100,000.

using System;
using System.Drawing;

public class Example
{
   public static void Main()
   {
      // Create an array of Point structures.
      Point[] points = { new Point(100, 200), 
                         new Point(150, 250), new Point(250, 375), 
                         new Point(275, 395), new Point(295, 450) };

      // Find the first Point structure for which X times Y  
      // is greater than 100000. 
      Point first = Array.Find(points, x => x.X * x.Y > 100000 );

      // Display the first structure found.
      Console.WriteLine("Found: X = {0}, Y = {1}", first.X, first.Y);
   }
}
// The example displays the following output:
//        Found: X = 275, Y = 395
Imports System.Drawing

Public Class Example
   Public Shared Sub Main()

      ' Create an array of Point structures. 
      Dim points() As Point = { new Point(100, 200), new Point(150, 250), 
                                new Point(250, 375), new Point(275, 395), 
                                new Point(295, 450) }

      ' Find the first Point structure for which X times Y  
      ' is greater than 100000. 
      Dim first As Point = Array.Find(points, 
                                 Function(x) x.X * x.Y > 100000 )

      ' Display the first structure found.
      Console.WriteLine("Found: X = {0}, Y = {1}", first.X, first.Y)
   End Sub 
End Class 
' The example displays the following output:
'       Found: X = 275, Y = 395

Remarques

Ce délégué est utilisé par plusieurs méthodes de la Array et List<T> classes pour rechercher des éléments dans la collection.This delegate is used by several methods of the Array and List<T> classes to search for elements in the collection.

En règle générale, le Predicate<T> délégué est représenté par une expression lambda.Typically, the Predicate<T> delegate is represented by a lambda expression. Étant donné que les variables ayant une étendue locale sont disponibles pour l’expression lambda, il est facile de tester une condition qui n’est pas précisément connue au moment de la compilation.Because locally scoped variables are available to the lambda expression, it is easy to test for a condition that is not precisely known at compile time. Cela est simulé dans l’exemple suivant, qui définit un HockeyTeam classe qui contient des informations sur une équipe ligue nationale de Hockey et l’année dans lequel il a été créé.This is simulated in the following example, which defines a HockeyTeam class that contains information about a National Hockey League team and the year in which it was founded. L’exemple définit un tableau d’entiers qui représentent des années et affecte de manière aléatoire un élément du tableau à foundedBeforeYear, qui est une variable de portée est définie localement à l’exemple Main (méthode).The example defines an array of integer values that represent years, and randomly assigns one element of the array to foundedBeforeYear, which is a variable that is locally scoped to the example's Main method. Étant donné que les variables ayant une étendue locale sont disponibles pour une expression lambda, l’expression lambda passée à la List<T>.FindAll méthode est en mesure de retourner un HockeyTeam objet pour chaque équipe a fondé sur ou avant cette année.Because locally scoped variables are available to a lambda expression, the lambda expression passed to the List<T>.FindAll method is able to return a HockeyTeam object for each team founded on or before that year.

using System;
using System.Collections.Generic;

public class HockeyTeam
{
   private string _name;
   private int _founded;
   
   public HockeyTeam(string name, int year)
   {
      _name = name;
      _founded = year;
   }

   public string Name {
      get { return _name; }
   }

   public int Founded {
      get { return _founded; }
   }
}

public class Example
{
   public static void Main()
   {
      Random rnd = new Random();
      List<HockeyTeam> teams = new List<HockeyTeam>();
      teams.AddRange( new HockeyTeam[] { new HockeyTeam("Detroit Red Wings", 1926), 
                                         new HockeyTeam("Chicago Blackhawks", 1926),
                                         new HockeyTeam("San Jose Sharks", 1991),
                                         new HockeyTeam("Montreal Canadiens", 1909),
                                         new HockeyTeam("St. Louis Blues", 1967) } );
      int[] years = { 1920, 1930, 1980, 2000 };
      int foundedBeforeYear = years[rnd.Next(0, years.Length)];
      Console.WriteLine("Teams founded before {0}:", foundedBeforeYear);
      foreach (var team in teams.FindAll( x => x.Founded <= foundedBeforeYear))
         Console.WriteLine("{0}: {1}", team.Name, team.Founded);
   }
}
// The example displays output similar to the following:
//       Teams founded before 1930:
//       Detroit Red Wings: 1926
//       Chicago Blackhawks: 1926
//       Montreal Canadiens: 1909
Imports System.Collections.Generic

Public Class HockeyTeam
   Private _name As String
   Private _founded As Integer
   
   Public Sub New(name As String, year As Integer)
      _name = name
      _founded = year
   End Sub

   Public ReadOnly Property Name As String
      Get
         Return _name
      End Get
   End Property

   Public ReadOnly Property Founded As Integer
      Get 
         Return _founded
      End Get   
   End Property
End Class

Module Example
   Public Sub Main()
      Dim rnd As New Random()
      Dim teams As New List(Of HockeyTeam)()
      teams.AddRange( { new HockeyTeam("Detroit Red Wings", 1926), 
                        new HockeyTeam("Chicago Blackhawks", 1926),
                        new HockeyTeam("San Jose Sharks", 1991),
                        new HockeyTeam("Montreal Canadiens", 1909),
                        new HockeyTeam("St. Louis Blues", 1967) } )
      Dim years() As Integer = { 1920, 1930, 1980, 2000 }
      Dim foundedBeforeYear As Integer = years(rnd.Next(0, years.Length))
      Console.WriteLine("Teams founded before {0}:", foundedBeforeYear)
      For Each team in teams.FindAll( Function(x) x.Founded <= foundedBeforeYear )
         Console.WriteLine("{0}: {1}", team.Name, team.Founded)
      Next   
   End Sub
End Module
' The example displays output similar to the following:
'       Teams founded before 1930:
'       Detroit Red Wings: 1926
'       Chicago Blackhawks: 1926
'       Montreal Canadiens: 1909

S’applique à

Voir aussi