List<T>.Contains(T) Method
Definition
Important
Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.
Determines whether an element is in the List<T>.
public:
virtual bool Contains(T item);public bool Contains (T item);abstract member Contains : 'T -> bool
override this.Contains : 'T -> boolPublic Function Contains (item As T) As BooleanParameters
- item
- T
The object to locate in the List<T>. The value can be null for reference types.
Returns
true if item is found in the List<T>; otherwise, false.
Implements
Examples
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.
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
The following example contains a list of complex objects of type Cube. The Cube class implements the IEquatable<T>.Equals method so that two cubes are considered equal if their dimensions are the same. In this example, the Contains method returns true, because a cube that has the specified dimensions is already in the collection.
using System;
using System.Collections.Generic;
class Program
{
static void Main(string[] args)
{
List<Cube> cubes = new List<Cube>();
cubes.Add(new Cube(8, 8, 4));
cubes.Add(new Cube(8, 4, 8));
cubes.Add(new Cube(8, 6, 4));
if (cubes.Contains(new Cube(8, 6, 4))) {
Console.WriteLine("An equal cube is already in the collection.");
}
else {
Console.WriteLine("Cube can be added.");
}
//Outputs "An equal cube is already in the collection."
}
}
public class Cube : IEquatable<Cube>
{
public Cube(int h, int l, int w)
{
this.Height = h;
this.Length = l;
this.Width = w;
}
public int Height { get; set; }
public int Length { get; set; }
public int Width { get; set; }
public bool Equals(Cube other)
{
if (this.Height == other.Height && this.Length == other.Length
&& this.Width == other.Width) {
return true;
}
else {
return false;
}
}
}
Imports System.Collections.Generic
Class Program
Public Shared Sub Main(ByVal args As String())
Dim cubes As New List(Of Cube)()
cubes.Add(New Cube(8, 8, 4))
cubes.Add(New Cube(8, 4, 8))
cubes.Add(New Cube(8, 6, 4))
If cubes.Contains(New Cube(8, 6, 4)) Then
Console.WriteLine("An equal cube is already in the collection.")
Else
Console.WriteLine("Cube can be added.")
End If
'Outputs "An equal cube is already in the collection."
End Sub
End Class
Public Class Cube
Implements IEquatable(Of Cube)
Public Sub New(ByVal h As Integer, ByVal l As Integer, ByVal w As Integer)
Me.Height = h
Me.Length = l
Me.Width = w
End Sub
Private _Height As Integer
Public Property Height() As Integer
Get
Return _Height
End Get
Set(ByVal value As Integer)
_Height = value
End Set
End Property
Private _Length As Integer
Public Property Length() As Integer
Get
Return _Length
End Get
Set(ByVal value As Integer)
_Length = value
End Set
End Property
Private _Width As Integer
Public Property Width() As Integer
Get
Return _Width
End Get
Set(ByVal value As Integer)
_Width = value
End Set
End Property
Public Overloads Function Equals(ByVal other As Cube) _
As Boolean Implements IEquatable(Of Cube).Equals
If Me.Height = other.Height And Me.Length = other.Length _
And Me.Width = other.Width Then
Return True
Else
Return False
End If
End Function
End Class
Remarks
This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable<T>.Equals method for T (the type of values in the list).
This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.
Applies to
See also
ทํางานร่วมกับเราใน GitHub
แหล่งที่มาสำหรับเนื้อหานี้สามารถพบได้บน GitHub ซึ่งคุณยังสามารถสร้างและตรวจสอบปัญหาและคำขอดึงข้อมูลได้ สำหรับข้อมูลเพิ่มเติม ให้ดูคู่มือผู้สนับสนุนของเรา
