List<T>.Exists(Predicate<T>) Método

public:
 bool Exists(Predicate<T> ^ match);

public bool Exists (Predicate<T> match);

member this.Exists : Predicate<'T> -> bool

Public Function Exists (match As Predicate(Of T)) As Boolean

Parámetros

Devoluciones

Es true si List<T> contiene uno o varios elementos que cumplen las condiciones definidas por el predicado especificado; en caso contrario, es false.

Excepciones

Ejemplos

En el ejemplo siguiente se muestran los Contains métodos y Exists en un List<T> objeto que contiene un objeto de negocio simple que implementa Equals.

using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify a part
// but the part name can change.
public class Part : IEquatable<Part>
{
    public string PartName { get; set; }
    public int PartId { get; set; }

    public override string ToString()
    {
        return "ID: " + PartId + "   Name: " + PartName;
    }
    public override bool Equals(object obj)
    {
        if (obj == null) return false;
        Part objAsPart = obj as Part;
        if (objAsPart == null) return false;
        else return Equals(objAsPart);
    }
    public override int GetHashCode()
    {
        return PartId;
    }
    public bool Equals(Part other)
    {
        if (other == null) return false;
        return (this.PartId.Equals(other.PartId));
    }
    // Should also override == and != operators.
}
public class Example
{
    public static void Main()
    {
        // Create a list of parts.
        List<Part> parts = new List<Part>();

        // Add parts to the list.
        parts.Add(new Part() { PartName = "crank arm", PartId = 1234 });
        parts.Add(new Part() { PartName = "chain ring", PartId = 1334 });
        parts.Add(new Part() { PartName = "regular seat", PartId = 1434 });
        parts.Add(new Part() { PartName = "banana seat", PartId = 1444 });
        parts.Add(new Part() { PartName = "cassette", PartId = 1534 });
        parts.Add(new Part() { PartName = "shift lever", PartId = 1634 }); ;

        // Write out the parts in the list. This will call the overridden ToString method
        // in the Part class.
        Console.WriteLine();
        foreach (Part aPart in parts)
        {
            Console.WriteLine(aPart);
        }

        // Check the list for part #1734. This calls the IEquatable.Equals method
        // of the Part class, which checks the PartId for equality.
        Console.WriteLine("\nContains: Part with Id=1734: {0}",
            parts.Contains(new Part { PartId = 1734, PartName = "" }));

        // Find items where name contains "seat".
        Console.WriteLine("\nFind: Part where name contains \"seat\": {0}",
            parts.Find(x => x.PartName.Contains("seat")));

        // Check if an item with Id 1444 exists.
        Console.WriteLine("\nExists: Part with Id=1444: {0}",
            parts.Exists(x => x.PartId == 1444));

        /*This code example produces the following output:

        ID: 1234   Name: crank arm
        ID: 1334   Name: chain ring
        ID: 1434   Name: regular seat
        ID: 1444   Name: banana seat
        ID: 1534   Name: cassette
        ID: 1634   Name: shift lever

        Contains: Part with Id=1734: False

        Find: Part where name contains "seat": ID: 1434   Name: regular seat

        Exists: Part with Id=1444: True
         */
    }
}

Imports System.Collections.Generic

' Simple business object. A PartId is used to identify a part 
' but the part name can change. 
Public Class Part
    Implements IEquatable(Of Part)
    Public Property PartName() As String
        Get
            Return m_PartName
        End Get
        Set(value As String)
            m_PartName = Value
        End Set
    End Property
    Private m_PartName As String
    Public Property PartId() As Integer
        Get
            Return m_PartId
        End Get
        Set(value As Integer)
            m_PartId = Value
        End Set
    End Property
    Private m_PartId As Integer

    Public Overrides Function ToString() As String
        Return Convert.ToString("ID: " & PartId & "   Name: ") & PartName
    End Function
    Public Overrides Function Equals(obj As Object) As Boolean
        If obj Is Nothing Then
            Return False
        End If
        Dim objAsPart As Part = TryCast(obj, Part)
        If objAsPart Is Nothing Then
            Return False
        Else
            Return Equals(objAsPart)
        End If
    End Function
    Public Overrides Function GetHashCode() As Integer
        Return PartId
    End Function
    Public Overloads Function Equals(other As Part) As Boolean _
        Implements IEquatable(Of Part).Equals
        If other Is Nothing Then
            Return False
        End If
        Return (Me.PartId.Equals(other.PartId))
    End Function
    ' Should also override == and != operators.
End Class
Public Class Example
    Public Shared Sub Main()
        ' Create a list of parts.
        Dim parts As New List(Of Part)()

        ' Add parts to the list.
        parts.Add(New Part() With { _
             .PartName = "crank arm", _
             .PartId = 1234 _
        })
        parts.Add(New Part() With { _
             .PartName = "chain ring", _
             .PartId = 1334 _
        })
        parts.Add(New Part() With { _
             .PartName = "regular seat", _
             .PartId = 1434 _
        })
        parts.Add(New Part() With { _
             .PartName = "banana seat", _
             .PartId = 1444 _
        })
        parts.Add(New Part() With { _
             .PartName = "cassette", _
             .PartId = 1534 _
        })
        parts.Add(New Part() With { _
             .PartName = "shift lever", _
             .PartId = 1634 _
        })



        ' Write out the parts in the list. This will call the overridden ToString method
        ' in the Part class.
        Console.WriteLine()
        For Each aPart As Part In parts
            Console.WriteLine(aPart)
        Next


        ' Check the list for part #1734. This calls the IEquatable.Equals method
        ' of the Part class, which checks the PartId for equality.
        Console.WriteLine(vbLf & "Contains: Part with Id=1734: {0}",
                          parts.Contains(New Part() With { _
             .PartId = 1734, _
             .PartName = "" _
        }))

        ' Find items where name contains "seat".
        Console.WriteLine(vbLf & "Find: Part where name contains ""seat"": {0}",
                          parts.Find(Function(x) x.PartName.Contains("seat")))

        ' Check if an item with Id 1444 exists.
        Console.WriteLine(vbLf & "Exists: Part with Id=1444: {0}",
                          parts.Exists(Function(x) x.PartId = 1444))

        'This code example produces the following output:
        '        
        '        ID: 1234   Name: crank arm
        '        ID: 1334   Name: chain ring
        '        ID: 1434   Name: regular seat
        '        ID: 1444   Name: banana seat
        '        ID: 1534   Name: cassette
        '        ID: 1634   Name: shift lever
        '
        '        Contains: Part with Id=1734: False
        '
        '        Find: Part where name contains "seat": ID: 1434   Name: regular seat
        '
        '        Exists: Part with Id=1444: True 
        '         

    End Sub
End Class

En el ejemplo siguiente se muestra el Exists método y otros métodos que usan el Predicate<T> delegado genérico.

Se crea una List<T> de cadenas, que contiene 8 nombres de dinosaurios, dos de los cuales (en las posiciones 1 y 5) terminan con "diccionario de sinónimos". En el ejemplo también se define un método de predicado de búsqueda denominado EndsWithSaurus, que acepta un parámetro de cadena y devuelve un valor booleano que indica si la cadena de entrada termina en "diccionario de sinónimos".

Los Findmétodos , FindLasty FindAll se usan para buscar en la lista con el método de predicado de búsqueda y, a continuación, el RemoveAll método se usa para quitar todas las entradas que terminan con "diccionario de sinónimos".

Por último, se llama al Exists método . Recorre la lista desde el principio, pasando cada elemento a su vez al EndsWithSaurus método . La búsqueda se detiene y el método devuelve true si el EndsWithSaurus método devuelve true para cualquier elemento. El Exists método devuelve false porque se han quitado todos estos elementos.

using namespace System;
using namespace System::Collections::Generic;

// Search predicate returns true if a string ends in "saurus".
bool EndsWithSaurus(String^ s)
{
    return s->ToLower()->EndsWith("saurus");
};

void main()
{
    List<String^>^ dinosaurs = gcnew List<String^>();

    dinosaurs->Add("Compsognathus");
    dinosaurs->Add("Amargasaurus");
    dinosaurs->Add("Oviraptor");
    dinosaurs->Add("Velociraptor");
    dinosaurs->Add("Deinonychus");
    dinosaurs->Add("Dilophosaurus");
    dinosaurs->Add("Gallimimus");
    dinosaurs->Add("Triceratops");

    Console::WriteLine();
    for each(String^ dinosaur in dinosaurs )
    {
        Console::WriteLine(dinosaur);
    }

    Console::WriteLine("\nTrueForAll(EndsWithSaurus): {0}",
        dinosaurs->TrueForAll(gcnew Predicate<String^>(EndsWithSaurus)));

    Console::WriteLine("\nFind(EndsWithSaurus): {0}", 
        dinosaurs->Find(gcnew Predicate<String^>(EndsWithSaurus)));

    Console::WriteLine("\nFindLast(EndsWithSaurus): {0}",
        dinosaurs->FindLast(gcnew Predicate<String^>(EndsWithSaurus)));

    Console::WriteLine("\nFindAll(EndsWithSaurus):");
    List<String^>^ sublist = 
        dinosaurs->FindAll(gcnew Predicate<String^>(EndsWithSaurus));

    for each(String^ dinosaur in sublist)
    {
        Console::WriteLine(dinosaur);
    }

    Console::WriteLine(
        "\n{0} elements removed by RemoveAll(EndsWithSaurus).", 
        dinosaurs->RemoveAll(gcnew Predicate<String^>(EndsWithSaurus)));

    Console::WriteLine("\nList now contains:");
    for each(String^ dinosaur in dinosaurs)
    {
        Console::WriteLine(dinosaur);
    }

    Console::WriteLine("\nExists(EndsWithSaurus): {0}", 
        dinosaurs->Exists(gcnew Predicate<String^>(EndsWithSaurus)));
}

/* This code example produces the following output:

Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops

TrueForAll(EndsWithSaurus): False

Find(EndsWithSaurus): Amargasaurus

FindLast(EndsWithSaurus): Dilophosaurus

FindAll(EndsWithSaurus):
Amargasaurus
Dilophosaurus

2 elements removed by RemoveAll(EndsWithSaurus).

List now contains:
Compsognathus
Oviraptor
Velociraptor
Deinonychus
Gallimimus
Triceratops

Exists(EndsWithSaurus): False
 */

using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        List<string> dinosaurs = new List<string>();

        dinosaurs.Add("Compsognathus");
        dinosaurs.Add("Amargasaurus");
        dinosaurs.Add("Oviraptor");
        dinosaurs.Add("Velociraptor");
        dinosaurs.Add("Deinonychus");
        dinosaurs.Add("Dilophosaurus");
        dinosaurs.Add("Gallimimus");
        dinosaurs.Add("Triceratops");

        Console.WriteLine();
        foreach(string dinosaur in dinosaurs)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine("\nTrueForAll(EndsWithSaurus): {0}",
            dinosaurs.TrueForAll(EndsWithSaurus));

        Console.WriteLine("\nFind(EndsWithSaurus): {0}",
            dinosaurs.Find(EndsWithSaurus));

        Console.WriteLine("\nFindLast(EndsWithSaurus): {0}",
            dinosaurs.FindLast(EndsWithSaurus));

        Console.WriteLine("\nFindAll(EndsWithSaurus):");
        List<string> sublist = dinosaurs.FindAll(EndsWithSaurus);

        foreach(string dinosaur in sublist)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine(
            "\n{0} elements removed by RemoveAll(EndsWithSaurus).",
            dinosaurs.RemoveAll(EndsWithSaurus));

        Console.WriteLine("\nList now contains:");
        foreach(string dinosaur in dinosaurs)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine("\nExists(EndsWithSaurus): {0}",
            dinosaurs.Exists(EndsWithSaurus));
    }

    // Search predicate returns true if a string ends in "saurus".
    private static bool EndsWithSaurus(String s)
    {
        return s.ToLower().EndsWith("saurus");
    }
}

/* This code example produces the following output:

Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops

TrueForAll(EndsWithSaurus): False

Find(EndsWithSaurus): Amargasaurus

FindLast(EndsWithSaurus): Dilophosaurus

FindAll(EndsWithSaurus):
Amargasaurus
Dilophosaurus

2 elements removed by RemoveAll(EndsWithSaurus).

List now contains:
Compsognathus
Oviraptor
Velociraptor
Deinonychus
Gallimimus
Triceratops

Exists(EndsWithSaurus): False
 */

Imports System.Collections.Generic

Public Class Example

    Public Shared Sub Main()

        Dim dinosaurs As New List(Of String)

        dinosaurs.Add("Compsognathus")
        dinosaurs.Add("Amargasaurus")
        dinosaurs.Add("Oviraptor")
        dinosaurs.Add("Velociraptor")
        dinosaurs.Add("Deinonychus")
        dinosaurs.Add("Dilophosaurus")
        dinosaurs.Add("Gallimimus")
        dinosaurs.Add("Triceratops")

        Console.WriteLine()
        For Each dinosaur As String In dinosaurs
            Console.WriteLine(dinosaur)
        Next

        Console.WriteLine(vbLf & _
            "TrueForAll(AddressOf EndsWithSaurus: {0}", _
            dinosaurs.TrueForAll(AddressOf EndsWithSaurus))

        Console.WriteLine(vbLf & _
            "Find(AddressOf EndsWithSaurus): {0}", _
            dinosaurs.Find(AddressOf EndsWithSaurus))

        Console.WriteLine(vbLf & _
            "FindLast(AddressOf EndsWithSaurus): {0}", _
            dinosaurs.FindLast(AddressOf EndsWithSaurus))

        Console.WriteLine(vbLf & _
            "FindAll(AddressOf EndsWithSaurus):")
        Dim sublist As List(Of String) = _
            dinosaurs.FindAll(AddressOf EndsWithSaurus)

        For Each dinosaur As String In sublist
            Console.WriteLine(dinosaur)
        Next

        Console.WriteLine(vbLf & _
            "{0} elements removed by RemoveAll(AddressOf EndsWithSaurus).", _
            dinosaurs.RemoveAll(AddressOf EndsWithSaurus))

        Console.WriteLine(vbLf & "List now contains:")
        For Each dinosaur As String In dinosaurs
            Console.WriteLine(dinosaur)
        Next

        Console.WriteLine(vbLf & _
            "Exists(AddressOf EndsWithSaurus): {0}", _
            dinosaurs.Exists(AddressOf EndsWithSaurus))
        
    End Sub

    ' Search predicate returns true if a string ends in "saurus".
    Private Shared Function EndsWithSaurus(ByVal s As String) _
        As Boolean

        Return s.ToLower().EndsWith("saurus")
    End Function
End Class

' This code example produces the following output:
'
'Compsognathus
'Amargasaurus
'Oviraptor
'Velociraptor
'Deinonychus
'Dilophosaurus
'Gallimimus
'Triceratops
'
'TrueForAll(AddressOf EndsWithSaurus: False
'
'Find(AddressOf EndsWithSaurus): Amargasaurus
'
'FindLast(AddressOf EndsWithSaurus): Dilophosaurus
'
'FindAll(AddressOf EndsWithSaurus):
'Amargasaurus
'Dilophosaurus
'
'2 elements removed by RemoveAll(AddressOf EndsWithSaurus).
'
'List now contains:
'Compsognathus
'Oviraptor
'Velociraptor
'Deinonychus
'Gallimimus
'Triceratops
'
'Exists(AddressOf EndsWithSaurus): False

Comentarios

Predicate<T> es un delegado de un método que devuelve true si el objeto pasado a él coincide con las condiciones definidas en el delegado. Los elementos del actual List<T> se pasan individualmente al Predicate<T> delegado y el procesamiento se detiene cuando se encuentra una coincidencia.

Este método realiza una búsqueda lineal; por lo tanto, este método es una operación de O(n), donde n es Count.

Se aplica a

Consulte también

• Find(Predicate<T>)
• FindLast(Predicate<T>)
• FindAll(Predicate<T>)
• FindIndex
• FindLastIndex
• BinarySearch
• IndexOf
• LastIndexOf
• TrueForAll(Predicate<T>)
• Predicate<T>