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

Definizione

Rappresenta il metodo che definisce un insieme di criteri e determina se l'oggetto specificato soddisfa tali criteri.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 

Parametri di tipo

T

Tipo di oggetto da confrontare.The type of the object to compare.

Parametri

obj

Oggetto da confrontare in base ai criteri definiti all'interno del metodo rappresentato da questo delegato.The object to compare against the criteria defined within the method represented by this delegate.

Valore restituito

System.Boolean

true se obj soddisfa i criteri definiti all'interno del metodo rappresentato da questo delegato; in caso contrario false.true if obj meets the criteria defined within the method represented by this delegate; otherwise, false.

Ereditarietà
Predicate<T>Predicate<T>Predicate<T>Predicate<T>

Esempi

Nell'esempio di codice seguente viene Predicate<T> usato un delegato Array.Find con il metodo per eseguire la Point ricerca in una matrice di strutture.The following code example uses a Predicate<T> delegate with the Array.Find method to search an array of Point structures. Nell'esempio viene definito in modo esplicito un delegato Predicate<T> denominato predicate e gli viene assegnato un metodo denominato FindPoints che restituisce true se il prodotto dei campi Point.X e Point.Y è maggiore di 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. Si noti che solitamente si utilizza un'espressione lambda anziché definire in modo esplicito un delegato di tipo Predicate<T>, come illustrato nel secondo esempio.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'esempio seguente è identico a quello precedente, con la differenza che viene utilizzata un'espressione lambda per rappresentare il delegato Predicate<T>.The following example is identical to the previous example, except that it uses a lambda expression to represent the Predicate<T> delegate. Ogni elemento della points matrice viene passato all'espressione lambda fino a quando l'espressione non trova un elemento che soddisfa i criteri di ricerca.Each element of the points array is passed to the lambda expression until the expression finds an element that meets the search criteria. In questo caso, l'espressione lambda restituisce true se il prodotto dei campi X e Y è maggiore di 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

Commenti

Questo delegato viene usato da diversi metodi delle Array classi e List<T> per cercare gli elementi nella raccolta.This delegate is used by several methods of the Array and List<T> classes to search for elements in the collection.

In genere, Predicate<T> il delegato è rappresentato da un'espressione lambda.Typically, the Predicate<T> delegate is represented by a lambda expression. Poiché le variabili con ambito locale sono disponibili per l'espressione lambda, è facile verificare la presenza di una condizione che non è nota in fase di compilazione.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. Questa operazione viene simulata nell'esempio seguente, che definisce una HockeyTeam classe che contiene informazioni su una squadra nazionale di Hockey League e sull'anno in cui è stata creata.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. Nell'esempio viene definita una matrice di valori integer che rappresentano gli anni e viene assegnato in modo casuale un elemento della matrice foundedBeforeYeara, ovvero una variabile con ambito locale al Main metodo dell'esempio.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. Poiché le variabili con ambito locale sono disponibili per un'espressione lambda, l'espressione lambda passata al List<T>.FindAll metodo può restituire un HockeyTeam oggetto per ogni team creato in o prima dell'anno.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

Metodi di estensione

GetMethodInfo(Delegate) GetMethodInfo(Delegate) GetMethodInfo(Delegate) GetMethodInfo(Delegate)

Ottiene un oggetto che rappresenta il metodo rappresentato dal delegato specificato.Gets an object that represents the method represented by the specified delegate.

Si applica a

Vedi anche