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

定义

表示定义一组条件并确定指定对象是否符合这些条件的方法。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 

类型参数

T

要比较的对象的类型。The type of the object to compare.

参数

obj

要按照由此委托表示的方法中定义的条件进行比较的对象。The object to compare against the criteria defined within the method represented by this delegate.

返回值

System.Boolean

如果 obj 符合由此委托表示的方法中定义的条件,则为 true;否则为 falsetrue if obj meets the criteria defined within the method represented by this delegate; otherwise, false.

继承
Predicate<T>Predicate<T>Predicate<T>Predicate<T>

示例

下面的代码示例使用Predicate<T>委托,其Array.Find方法搜索的数组Point结构。The following code example uses a Predicate<T> delegate with the Array.Find method to search an array of Point structures. 该示例定义了显式Predicate<T>委托名为predicate并将其分配一个名为方法FindPoints返回true如果的产品Point.XPoint.Y字段大于 100000。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. 请注意,它是使用 lambda 表达式而不是显式定义类型的委托惯用Predicate<T>,如第二个示例所示。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

下面的示例等同于上一示例中,只不过它使用 lambda 表达式来表示Predicate<T>委托。The following example is identical to the previous example, except that it uses a lambda expression to represent the Predicate<T> delegate. 每个元素points数组传递给 lambda 表达式,该表达式查找符合搜索条件的元素之前。Each element of the points array is passed to the lambda expression until the expression finds an element that meets the search criteria. 在这种情况下,lambda 表达式返回true是否大于 100,000 的 X 和 Y 字段。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

注解

通过多种方法来使用此委托ArrayList<T>类来搜索集合中的元素。This delegate is used by several methods of the Array and List<T> classes to search for elements in the collection.

通常情况下,Predicate<T>委托表示的 lambda 表达式。Typically, the Predicate<T> delegate is represented by a lambda expression. 由于本地作用域内的变量适用于 lambda 表达式,因此很容易地测试准确地说不在编译时已知的条件。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. 这在以下示例中,它定义模拟HockeyTeam类,其中包含有关美国国家 Hockey 联盟团队并在其中它成立的年份的信息。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. 该示例定义了一个数组的整数值表示年中,并随机分配到的数组中的一个元素foundedBeforeYear,这是一个变量,本地作用域为该示例的Main方法。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. 由于本地作用域内的变量适用于 lambda 表达式,因此 lambda 表达式传递给List<T>.FindAll方法是无法返回HockeyTeam成立了当天或之前该年度的每个团队的对象。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

扩展方法

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

获取指示指定委托表示的方法的对象。Gets an object that represents the method represented by the specified delegate.

适用于

另请参阅