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

Definição

Representa o método que define um conjunto de critérios e determina se o objeto especificado atende a esses critérios. 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 
Parâmetros de tipo
T

Especifica o tipo do objeto a ser comparado. The type of the object to compare.

Parâmetros
obj

O objeto a ser comparado com os critérios definidos dentro do método representado por esse delegado. The object to compare against the criteria defined within the method represented by this delegate.

Valor Retornado
System.Boolean

true se obj atender aos critérios definidos dentro do método representado por esse delegado; caso contrário, false. true if obj meets the criteria defined within the method represented by this delegate; otherwise, false.

Herança
Predicate<T>Predicate<T>Predicate<T>Predicate<T>

Exemplos

O seguinte exemplo de código usa um Predicate<T> delegado com o Array.Find método para pesquisar uma matriz de Point estruturas.The following code example uses a Predicate<T> delegate with the Array.Find method to search an array of Point structures. O exemplo define explicitamente uma Predicate<T> delegado chamado predicate e atribui a ele um método chamado FindPoints que retorna true se o produto dos Point.X e Point.Y campos é maior que 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. Observe que é comum usar uma expressão lambda em vez de definir explicitamente um delegado do tipo Predicate<T>, conforme ilustra o segundo exemplo.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

O exemplo a seguir é idêntico ao exemplo anterior, exceto que ele usa uma expressão lambda para representar o Predicate<T> delegar.The following example is identical to the previous example, except that it uses a lambda expression to represent the Predicate<T> delegate. Cada elemento da points matriz é passada para a expressão lambda, até que a expressão localiza um elemento que atende aos critérios de pesquisa.Each element of the points array is passed to the lambda expression until the expression finds an element that meets the search criteria. Nesse caso, a expressão lambda retorna true se o produto dos campos X e Y for maior que 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

Comentários

Esse delegado é usado por vários métodos para o Array e List<T> classes para procurar por elementos na coleção.This delegate is used by several methods of the Array and List<T> classes to search for elements in the collection.

Normalmente, o Predicate<T> delegado é representado por uma expressão lambda.Typically, the Predicate<T> delegate is represented by a lambda expression. Como as variáveis com escopo local estão disponíveis para a expressão lambda, é fácil de testar uma condição que não é conhecido em tempo de compilação com precisão.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. Isso é simulado no exemplo a seguir, que define um HockeyTeam classe que contém informações sobre uma equipe liga de hóquei nacional e o ano no qual ele foi fundado.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. O exemplo define uma matriz de valores inteiros que representam anos e atribui aleatoriamente um elemento da matriz a ser foundedBeforeYear, que é uma variável que tem o escopo para o exemplo localmente Main método.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. Como variáveis com escopo local estão disponíveis para uma expressão lambda, expressão lambda passada para o List<T>.FindAll método é capaz de retornar um HockeyTeam objeto para cada equipe Fundada em ou antes desse ano.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

Aplica-se a

Veja também