Comparisons and sorts within collections
The System.Collections classes perform comparisons in almost all the processes involved in managing collections, whether searching for the element to remove or returning the value of a key-and-value pair.
Collections typically utilize an equality comparer and/or an ordering comparer. Two constructs are used for comparisons.
Check for equality
Methods such as Contains, IndexOf, LastIndexOf, and Remove use an equality comparer for the collection elements. If the collection is generic, then items are compared for equality according to the following guidelines:
If type T implements the IEquatable<T> generic interface, then the equality comparer is the Equals method of that interface.
If type T does not implement IEquatable<T>, Object.Equals is used.
In addition, some constructor overloads for dictionary collections accept an IEqualityComparer<T> implementation, which is used to compare keys for equality. For an example, see the Dictionary<TKey,TValue> constructor.
Determine sort order
Methods such as BinarySearch and Sort use an ordering comparer for the collection elements. The comparisons can be between elements of the collection, or between an element and a specified value. For comparing objects, there is the concept of a default comparer and an explicit comparer.
The default comparer relies on at least one of the objects being compared to implement the IComparable interface. It is a good practice to implement IComparable on all classes which are used as values in a list collection or as keys in a dictionary collection. For a generic collection, equality comparison is determined according to the following:
If type T implements the System.IComparable<T> generic interface, then the default comparer is the IComparable<T>.CompareTo(T) method of that interface
If type T implements the non-generic System.IComparable interface, then the default comparer is the IComparable.CompareTo(Object) method of that interface.
If type T doesn't implement either interface, then there is no default comparer, and a comparer or comparison delegate must be provided explicitly.
To provide explicit comparisons, some methods accept an IComparer implementation as a parameter. For example, the List<T>.Sort method accepts an System.Collections.Generic.IComparer<T> implementation.
The current culture setting of the system can affect the comparisons and sorts within a collection. By default, the comparisons and sorts in the Collections classes are culture-sensitive. To ignore the culture setting and therefore obtain consistent comparison and sorting results, use the InvariantCulture with member overloads that accept a CultureInfo. For more information, see Perform culture-insensitive string operations in collections and Perform culture-insensitive string operations in arrays.
Equality and sort example
The following code demonstrates an implementation of IEquatable<T> and IComparable<T> on a simple business object. In addition, when the object is stored in a list and sorted, you will see that calling the Sort() method results in the use of the default comparer for the Part type, and the Sort(Comparison<T>) method implemented by using an anonymous method.
using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify the
// type of part but the part name can change.
public class Part : IEquatable<Part>, IComparable<Part>
{
public string PartName { get; set; }
public int PartId { get; set; }
public override string ToString() =>
$"ID: {PartId} Name: {PartName}";
public override bool Equals(object obj) =>
(obj is Part part)
? Equals(part)
: false;
public int SortByNameAscending(string name1, string name2) =>
name1?.CompareTo(name2) ?? 1;
// Default comparer for Part type.
// A null value means that this object is greater.
public int CompareTo(Part comparePart) =>
comparePart == null ? 1 : PartId.CompareTo(comparePart.PartId);
public override int GetHashCode() => PartId;
public bool Equals(Part other) =>
other is null ? false : PartId.Equals(other.PartId);
// Should also override == and != operators.
}
public class Example
{
public static void Main()
{
// Create a list of parts.
var parts = new List<Part>
{
// Add parts to the list.
new Part { PartName = "regular seat", PartId = 1434 },
new Part { PartName = "crank arm", PartId = 1234 },
new Part { PartName = "shift lever", PartId = 1634 },
// Name intentionally left null.
new Part { PartId = 1334 },
new Part { PartName = "banana seat", PartId = 1444 },
new Part { PartName = "cassette", PartId = 1534 }
};
// Write out the parts in the list. This will call the overridden
// ToString method in the Part class.
Console.WriteLine("\nBefore sort:");
parts.ForEach(Console.WriteLine);
// Call Sort on the list. This will use the
// default comparer, which is the Compare method
// implemented on Part.
parts.Sort();
Console.WriteLine("\nAfter sort by part number:");
parts.ForEach(Console.WriteLine);
// This shows calling the Sort(Comparison<T> comparison) overload using
// a lambda expression as the Comparison<T> delegate.
// This method treats null as the lesser of two values.
parts.Sort((Part x, Part y) =>
x.PartName == null && y.PartName == null
? 0
: x.PartName == null
? -1
: y.PartName == null
? 1
: x.PartName.CompareTo(y.PartName));
Console.WriteLine("\nAfter sort by name:");
parts.ForEach(Console.WriteLine);
/*
Before sort:
ID: 1434 Name: regular seat
ID: 1234 Name: crank arm
ID: 1634 Name: shift lever
ID: 1334 Name:
ID: 1444 Name: banana seat
ID: 1534 Name: cassette
After sort by part number:
ID: 1234 Name: crank arm
ID: 1334 Name:
ID: 1434 Name: regular seat
ID: 1444 Name: banana seat
ID: 1534 Name: cassette
ID: 1634 Name: shift lever
After sort by name:
ID: 1334 Name:
ID: 1444 Name: banana seat
ID: 1534 Name: cassette
ID: 1234 Name: crank arm
ID: 1434 Name: regular seat
ID: 1634 Name: shift lever
*/
}
}
Imports System.Collections.Generic
' Simple business object. A PartId is used to identify the type of part
' but the part name can change.
Public Class Part
Implements IEquatable(Of Part)
Implements IComparable(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 "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 Function SortByNameAscending(name1 As String, name2 As String) As Integer
Return name1.CompareTo(name2)
End Function
' Default comparer for Part.
Public Function CompareTo(comparePart As Part) As Integer _
Implements IComparable(Of ListSortVB.Part).CompareTo
' A null value means that this object is greater.
If comparePart Is Nothing Then
Return 1
Else
Return Me.PartId.CompareTo(comparePart.PartId)
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 ListSortVB.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 = "regular seat", _
.PartId = 1434 _
})
parts.Add(New Part() With { _
.PartName = "crank arm", _
.PartId = 1234 _
})
parts.Add(New Part() With { _
.PartName = "shift lever", _
.PartId = 1634 _
})
' Name intentionally left null.
parts.Add(New Part() With { _
.PartId = 1334 _
})
parts.Add(New Part() With { _
.PartName = "banana seat", _
.PartId = 1444 _
})
parts.Add(New Part() With { _
.PartName = "cassette", _
.PartId = 1534 _
})
' Write out the parts in the list. This will call the overridden
' ToString method in the Part class.
Console.WriteLine(vbLf & "Before sort:")
For Each aPart As Part In parts
Console.WriteLine(aPart)
Next
' Call Sort on the list. This will use the
' default comparer, which is the Compare method
' implemented on Part.
parts.Sort()
Console.WriteLine(vbLf & "After sort by part number:")
For Each aPart As Part In parts
Console.WriteLine(aPart)
Next
' This shows calling the Sort(Comparison(T) overload using
' an anonymous delegate method.
' This method treats null as the lesser of two values.
parts.Sort(Function(x As Part, y As Part)
If x.PartName Is Nothing AndAlso y.PartName Is Nothing Then
Return 0
ElseIf x.PartName Is Nothing Then
Return -1
ElseIf y.PartName Is Nothing Then
Return 1
Else
Return x.PartName.CompareTo(y.PartName)
End If
End Function)
Console.WriteLine(vbLf & "After sort by name:")
For Each aPart As Part In parts
Console.WriteLine(aPart)
Next
'
'
' Before sort:
' ID: 1434 Name: regular seat
' ID: 1234 Name: crank arm
' ID: 1634 Name: shift lever
' ID: 1334 Name:
' ID: 1444 Name: banana seat
' ID: 1534 Name: cassette
'
' After sort by part number:
' ID: 1234 Name: crank arm
' ID: 1334 Name:
' ID: 1434 Name: regular seat
' ID: 1444 Name: banana seat
' ID: 1534 Name: cassette
' ID: 1634 Name: shift lever
'
' After sort by name:
' ID: 1334 Name:
' ID: 1444 Name: banana seat
' ID: 1534 Name: cassette
' ID: 1234 Name: crank arm
' ID: 1434 Name: regular seat
' ID: 1634 Name: shift lever
End Sub
End Class
