Predicate<T> 委托

参考

定义

命名空间:: System

程序集:: System.Runtime.dll

程序集:: mscorlib.dll

程序集:: netstandard.dll

重要

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表示一种方法，该方法定义一组条件并确定指定对象是否符合这些条件。

generic <typename T>
public delegate bool Predicate(T obj);

public delegate bool Predicate<in T>(T obj);

public delegate bool Predicate<T>(T obj);

type Predicate<'T> = delegate of 'T -> bool

Public Delegate Function Predicate(Of In T)(obj As T) As Boolean

Public Delegate Function Predicate(Of T)(obj As T) As Boolean

类型参数

T

要比较的对象的类型。

这是逆变类型参数。即，可以使用指定的类型，也可以使用派生程度较低的任何类型。有关协变和逆变的详细信息，请参阅泛型中的协变和逆变。

参数

obj: T

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。

返回值

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 false。

示例

下面的代码示例使用 Predicate<T> 具有 Array.Find 方法的 Point 委托来搜索结构数组。该示例显式定义一个Predicate<T>命名predicate的委托，并为其分配一个命名FindPoints的方法，该方法在字段Point.Y的Point.X乘积大于 100，000 时返回true。请注意，通常使用 lambda 表达式而不是显式定义类型的 Predicate<T>委托，如第二个示例所示。

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

open System
open System.Drawing

let findPoints (obj: Point) =
    obj.X * obj.Y > 100000

// Create an array of Point structures.
let points = 
    [| Point(100, 200)
       Point(150, 250) 
       Point(250, 375)
       Point(275, 395) 
       Point(295, 450) |]

// Define the Predicate<T> delegate.
let predicate = Predicate<Point> findPoints

// Find the first Point structure for which X times Y
// is greater than 100000.
let first = Array.Find(points, predicate)

// Display the first structure found.
printfn $"Found: X = {first.X}, Y = {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) }
      
      ' 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

以下示例与上一个示例相同，只不过它使用 lambda 表达式来表示 Predicate<T> 委托。数组的每个元素 points 将传递到 lambda 表达式，直到表达式找到满足搜索条件的元素。在这种情况下，lambda 表达式返回 true X 和 Y 字段的乘积大于 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

open System
open System.Drawing

// Create an array of Point structures.
let points = 
    [| Point(100, 200)
       Point(150, 250)
       Point(250, 375)
       Point(275, 395)
       Point(295, 450) |]

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

// Display the first structure found.
printfn $"Found: X = {first.X}, Y = {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

此委托由多个方法和类用于Array List<T>搜索集合中的元素。

通常， Predicate<T> 委托由 lambda 表达式表示。由于本地范围的变量对 lambda 表达式可用，因此可以轻松测试编译时无法准确知道的条件。以下示例中模拟了这一点，该类定义一个 HockeyTeam 类，其中包含有关国家曲棍球联赛球队的信息以及创建该团队的年份。该示例定义表示年份的整数值的数组，并随机将数组的一个元素分配给该数组，该数组 foundedBeforeYear是一个变量，该变量在本地限定为示例 Main 的方法。由于本地范围的变量可用于 lambda 表达式，因此传递给 List<T>.FindAll 该方法的 lambda 表达式能够返回 HockeyTeam 在该年份或之前创建的每个团队的对象。

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

open System

type HockeyTeam =
    { Name: string
      Founded: int }

let rnd = Random()
let teams = ResizeArray()
teams.AddRange 
    [| { Name = "Detroit Red Wings"; Founded = 1926 }
       { Name = "Chicago Blackhawks"; Founded = 1926 }
       { Name = "San Jose Sharks"; Founded = 1991 }
       { Name = "Montreal Canadiens"; Founded = 1909 }
       { Name = "St. Louis Blues"; Founded = 1967 }|]

let years = [| 1920; 1930; 1980; 2000 |]
let foundedBeforeYear = years[rnd.Next(0, years.Length)]
printfn $"Teams founded before {foundedBeforeYear}:"
for team in teams.FindAll(fun x -> x.Founded <= foundedBeforeYear) do
    printfn $"{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)

获取指示指定委托表示的方法的对象。

适用于

另请参阅

Find<T>(T[], Predicate<T>)

通过

Predicate<T> 委托

定义

类型参数

参数

返回值

示例

注解

扩展方法

适用于

另请参阅

其他资源