Predicate<t> Delegate (System) | Microsoft Learn</t>

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Type Parameters

The type of the object to compare.
This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics .
Parameters

true if obj meets the criteria defined within the method represented by this delegate; otherwise, false .
Examples

The following code example uses a Predicate<T> delegate with the Array.Find method to search an array of Point structures. 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. 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
   
    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
     
      The following example is identical to the previous example, except that it uses a lambda expression to represent the
      
       Predicate<T>
      
      delegate.  Each element of the
      
       points
      
      array is passed to the lambda expression until the expression finds an element that meets the search criteria. 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
      
       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
        
         Remarks
        
        
         This delegate is used by several methods of the
         
          Array
         
         and
         
          List<T>
         
         classes to search for elements in the collection.
        
        
         Typically, the
         
          Predicate<T>
         
         delegate is represented by a lambda expression. 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. 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. 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. 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
         
          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
         Return _name
      End Get
   End Property
   Public ReadOnly Property Founded As Integer
         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)
   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
           
            Extension Methods