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comparisons-and-sorts-within-collections.md

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Comparisons and Sorts Within Collections
Do comparisons & sorts using the System.Collections classes in .NET, which help in finding an element to remove or returning the value of a key-and-value pair.
04/30/2020
csharp
vb
sorting data, collections
IComparable.CompareTo method
Collections classes
Equals method
collections [.NET], comparisons
5e4d3b45-97f0-423c-a65f-c492ed40e73b

Comparisons and sorts within collections

The xref: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, xref:System.Collections.IList.IndexOf%2A, xref:System.Collections.Generic.List%601.LastIndexOf%2A, 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 xref:System.IEquatable%601 generic interface, then the equality comparer is the xref:System.IEquatable%601.Equals%2A method of that interface.

  • If type T does not implement xref:System.IEquatable%601, xref:System.Object.Equals%2A?displayProperty=nameWithType is used.

In addition, some constructor overloads for dictionary collections accept an xref:System.Collections.Generic.IEqualityComparer%601 implementation, which is used to compare keys for equality. For an example, see the xref:System.Collections.Generic.Dictionary%602.%23ctor%2A 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 xref:System.IComparable%601?displayProperty=nameWithType generic interface, then the default comparer is the xref:System.IComparable%601.CompareTo%28%600%29?displayProperty=nameWithType method of that interface

  • If type T implements the non-generic xref:System.IComparable?displayProperty=nameWithType interface, then the default comparer is the xref:System.IComparable.CompareTo%28System.Object%29?displayProperty=nameWithType 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 xref:System.Collections.Generic.List%601.Sort%2A?displayProperty=nameWithType method accepts an xref:System.Collections.Generic.IComparer%601?displayProperty=nameWithType 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 xref:System.Globalization.CultureInfo.InvariantCulture%2A with member overloads that accept a xref:System.Globalization.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 xref:System.IEquatable%601 and xref:System.IComparable%601 on a simple business object. In addition, when the object is stored in a list and sorted, you will see that calling the xref:System.Collections.Generic.List%601.Sort method results in the use of the default comparer for the Part type, and the xref:System.Collections.Generic.List%601.Sort%28System.Comparison%7B%600%7D%29 method implemented by using an anonymous method.

[!code-csharpSystem.Collections.Generic.List.Sort#1] [!code-vbSystem.Collections.Generic.List.Sort#1]

See also

  • xref:System.Collections.IComparer
  • xref:System.IEquatable%601
  • xref:System.Collections.Generic.IComparer%601
  • xref:System.IComparable
  • xref:System.IComparable%601