List<T>.Exists(Predicate<T>) Metodo
Definizione
Importante
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Determina se List<T> contiene gli elementi che corrispondono alle condizioni definite dal predicato specificato.
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
Parametri
- match
- Predicate<T>
Delegato Predicate<T> che definisce le condizioni degli elementi da cercare.
Restituisce
true
se List<T> contiene uno o più elementi che corrispondono alle condizioni definite dal predicato specificato; in caso contrario, false
.
Eccezioni
match
è null
.
Esempio
Nell'esempio seguente vengono illustrati i metodi e Exists in un List<T> oggetto contenente un oggetto business semplice che implementa Equals.Contains
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
Nell'esempio seguente viene illustrato il Exists metodo e diversi altri metodi che usano il Predicate<T> delegato generico.
Viene creato un List<T> di stringhe contenente 8 nomi di dinosauro, due dei quali (in corrispondenza delle posizioni 1 e 5) terminano con "saurus". L'esempio definisce anche un metodo di predicato di ricerca denominato EndsWithSaurus
, che accetta un parametro stringa e restituisce un valore booleano che indica se la stringa di input termina con "saurus".
I Findmetodi , FindLaste FindAll vengono usati per eseguire ricerche nell'elenco con il metodo del predicato di ricerca e quindi il RemoveAll metodo viene usato per rimuovere tutte le voci che terminano con "saurus".
Infine, viene chiamato il Exists metodo . Attraversa l'elenco dall'inizio, passando ogni elemento a sua volta al EndsWithSaurus
metodo . La ricerca viene arrestata e il metodo restituisce true
se il EndsWithSaurus
metodo restituisce true
per qualsiasi elemento. Il Exists metodo restituisce false
perché tutti questi elementi sono stati rimossi.
Nota
In C# e Visual Basic non è necessario creare in modo esplicito il Predicate<string>
delegato (Predicate(Of String)
in Visual Basic). Questi linguaggi deducono il delegato corretto dal contesto e lo creano automaticamente.
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
Commenti
Predicate<T> è un delegato a un metodo che restituisce true
se l'oggetto passato corrisponde alle condizioni definite nel delegato. Gli elementi dell'oggetto corrente List<T> vengono passati singolarmente al delegato e l'elaborazione Predicate<T> viene arrestata quando viene trovata una corrispondenza.
Questo metodo esegue una ricerca lineare; pertanto, questo metodo è un'operazione O(n), dove n è Count.