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Predicate(Of T) Delegate

Represents the method that defines a set of criteria and determines whether the specified object meets those criteria.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
Public Delegate Function Predicate(Of In T) ( _
	obj As T _
) As Boolean

Type Parameters

In T

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

obj
Type: T
The object to compare against the criteria defined within the method represented by this delegate.

Return Value

Type: System.Boolean
true if obj meets the criteria defined within the method represented by this delegate; otherwise, false.

This delegate is used by several methods of the Array and List(Of T) classes to search for elements in the collection.

The following code example uses a Predicate(Of T) delegate with the Array.Find(Of T) method to search an array of Point structures. The method the delegate represents, ProductGT10, returns true if the product of the X and Y fields is greater than 100,000. The Find(Of T) method calls the delegate for each element of the array, stopping at the first point that meets the test condition.

NoteNote

Visual Basic and C# users do not need to create the delegate explicitly, or to specify the type argument of the generic method. The compilers determine the necessary types from the method arguments you supply.


Imports System
Imports System.Drawing

Public Class Example

    Public Shared Sub Main()

        ' Create an array of five 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) }

        ' To find the first Point structure for which X times Y 
        ' is greater than 100000, pass the array and a delegate 
        ' that represents the ProductGT10 method to the Shared
        ' Find method of the Array class.
        Dim first As Point = Array.Find(points, _
            AddressOf ProductGT10)

        ' Note that you do not need to create the delegate 
        ' explicitly, or to specify the type parameter of the 
        ' generic method, because the compiler has enough
        ' context to determine that information for you.

        ' Display the first structure found.
        Console.WriteLine("Found: X = {0}, Y = {1}", _
            first.X, first.Y)
    End Sub

    ' This method implements the test condition for the Find
    ' method.
    Private Shared Function ProductGT10(ByVal p As Point) As Boolean
        If p.X * p.Y > 100000 Then
            Return True
        Else
            Return False
        End If
    End Function
End Class

' This code example produces the following output:
'
'Found: X = 275, Y = 395


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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