# List<T>.Exists(Predicate<T>)List<T>.Exists(Predicate<T>)List<T>.Exists(Predicate<T>)List<T>.Exists(Predicate<T>) Method

## 定義

``````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``

#### 參數

match
Predicate<T> Predicate<T> Predicate<T> Predicate<T>

#### 例外狀況

`match``null``match` is `null`.

## 範例

Contains下列範例示範EqualsExists的和方法, 其中包含可執行檔簡單商務物件。 List<T>The following example demonstrates the Contains and Exists methods on a List<T> that contains a simple business object that implements 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.
.PartName = "crank arm", _
.PartId = 1234 _
})
.PartName = "chain ring", _
.PartId = 1334 _
})
.PartName = "regular seat", _
.PartId = 1434 _
})
.PartName = "banana seat", _
.PartId = 1444 _
})
.PartName = "cassette", _
.PartId = 1534 _
})
.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
``````

List<T>會建立字串的, 其中包含8個恐龍名稱, 其中兩個 (位於位置1和 5) 以 "saurus" 結尾。A List<T> of strings is created, containing 8 dinosaur names, two of which (at positions 1 and 5) end with "saurus". 此範例也會定義名為`EndsWithSaurus`的搜尋述詞方法, 它會接受字串參數, 並傳回布林值, 指出輸入字串是否以 "saurus" 結尾。The example also defines a search predicate method named `EndsWithSaurus`, which accepts a string parameter and returns a Boolean value indicating whether the input string ends in "saurus".

、和方法是用來搜尋具有搜尋述詞方法的清單, 然後RemoveAll使用方法來移除所有以 "saurus" 結尾的專案。 FindAll FindLast FindThe Find, FindLast, and FindAll methods are used to search the list with the search predicate method, and then the RemoveAll method is used to remove all entries ending with "saurus".

``````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^>();

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>();

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
Imports System.Collections.Generic

Public Class Example

Public Shared Sub Main()

Dim dinosaurs As New List(Of String)

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

Console.WriteLine(vbLf & _

Console.WriteLine(vbLf & _

Console.WriteLine(vbLf & _

Console.WriteLine(vbLf & _
Dim sublist As List(Of String) = _

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

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

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

Console.WriteLine(vbLf & _

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
'
'
'
'
'Amargasaurus
'Dilophosaurus
'
'2 elements removed by RemoveAll(AddressOf EndsWithSaurus).
'
'List now contains:
'Compsognathus
'Oviraptor
'Velociraptor
'Deinonychus
'Gallimimus
'Triceratops
'