SortedSet

The C# SortedSet is an ordered set collection. It helps when we have many elements and want to store them in a sorted order, eliminating all duplicates.

This class is part of the System.Collections.Generic namespace. Its performance tends to be worse than a hash table like Dictionary—it does not support hashing.

First example

This program creates a new, empty SortedSet instance. Then it adds 3 elements to the set with the Add method. Finally it performs operations on the set.

using System;
using System.Collections.Generic;

// Create sorted set.
SortedSet<string> set = new SortedSet<string>();

// Add 3 elements.
set.Add("carrot");
set.Add("bird");
set.Add("apple");

// Use contains method.
Console.WriteLine(set.Contains("bird"));

// Print all element in sorted order.
foreach (string val in set)
{
    Console.WriteLine(val);
}
True
apple
bird
carrot

Copy list

Often you may want to create a SortedSet instance from the elements in another collection, such as an array or List. You can do this with the SortedSet constructor.

using System;
using System.Collections.Generic;

// Create list with elements.
var list = new List<string>();
list.Add("x");
list.Add("y");
list.Add("perls");
list.Add("perls");

// Created sorted set from list.
SortedSet<string> set = new SortedSet<string>(list);

// Display contents.
foreach (string val in set)
{
    Console.WriteLine(val);
}
perls
x
y

RemoveWhere

Sometimes you may need to remove all elements from your SortedSet that match a certain condition. You can invoke the RemoveWhere method.

using System;
using System.Collections.Generic;

// Create sorted set.
SortedSet<string> set = new SortedSet<string>();
set.Add("sam");
set.Add("sally");
set.Add("sandra");
set.Add("steve");
set.Add("mark");
set.Add("mark");

// Remove all elements where first letter is "s".
set.RemoveWhere(element => element.StartsWith("s"));

// Display.
foreach (string val in set)
{
    Console.WriteLine(val);
}
mark

Add method

We call the Add method to put additional elements into the set. The Add method returns a boolean value that tells us whether or not a new element was added.

using System;
using System.Collections.Generic;

// Test add method.
SortedSet<string> set = new SortedSet<string>();
bool a = set.Add("sam");
bool b = set.Add("sam");
bool c = set.Add("sam");
bool d = set.Add("mike");

Console.WriteLine("{0} {1} {2} {3}", a, b, c, d);
True False False True

Count, clear

As with other collections, you can use the Count property and the Clear method on the SortedSet type. The Count property can only be read, not assigned to.

using System;
using System.Collections.Generic;

SortedSet<string> set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("a"); // Not successful.

Console.WriteLine(set.Count);
set.Clear();
Console.WriteLine(set.Count);
2
0

UnionWith

This returns the union of two collections. If the set contains A, B and C, and the second collection contains C and D, the union will be equal to A, B, C and D.

using System;
using System.Collections.Generic;

SortedSet<string> set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("x");

List<string> list = new List<string>();
list.Add("a");
list.Add("y");

// Union the two collections.
set.UnionWith(list);

// Enumerate.
foreach (string val in set)
{
    Console.WriteLine(val);
}
a
x
y
z

SymmetricExceptWith

This returns all elements that are found in only one collection. If both contain "A," SymmetricExceptWith will remove "A" from the SortedSet instance.

using System;
using System.Collections.Generic;

SortedSet<string> set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("x");

List<string> list = new List<string>();
list.Add("a");
list.Add("y");

// Determine symmetric set.
set.SymmetricExceptWith(list);

// Display elements.
foreach (string val in set)
{
    Console.WriteLine(val);
}
x
y
z

ExceptWith

This removes all elements found in a collection from the SortedSet. The resulting SortedSet will contain all its elements except those that were found in the other collection.

using System;
using System.Collections.Generic;

SortedSet<string> set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("x");

List<string> list = new List<string>();
list.Add("a");
list.Add("x");

// Call ExceptWith to remove elements.
set.ExceptWith(list);

// Display elements.
foreach (string val in set)
{
    Console.WriteLine(val);
}
z

Overlaps

This tells us whether a collection has any elements in common with the SortedSet. Even if only one element is found in common, the result is True. Otherwise the result is False.

using System;
using System.Collections.Generic;

SortedSet<string> set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("x");

List<string> list = new List<string>();
list.Add("a");
list.Add("y");

HashSet<string> hash = new HashSet<string>();
hash.Add("v");
hash.Add("u");

bool a = set.Overlaps(list); // Set and list overlap.
bool b = set.Overlaps(hash); // Set and hash do not overlap.

Console.WriteLine(a);
Console.WriteLine(b);
True
False

IntersectWith

This changes the set instance so that it contains only the elements that were present in both collections. The element count is reduced or stays the same.

using System;
using System.Collections.Generic;

var set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("x");

var list = new List<string>();
list.Add("a");
list.Add("x");

// Compute intersection.
set.IntersectWith(list);

// Enumerate.
foreach (string val in set)
{
    Console.WriteLine(val);
}
a
x

Min, max

The SortedSet contains elements that are stored in ascending sorted order. So it is trivial for the set to compute its lowest and highest values.

using System;
using System.Collections.Generic;

// Three-element set.
var set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("x");

// Write Min and Max.
Console.WriteLine(set.Min);
Console.WriteLine(set.Max);
a
z

Subsets, supersets

SortedSet has methods to compute subsets and supersets. These are IsSubsetOf, IsSupersetOf, IsProperSubsetOf and IsProperSupersetOf.

using System;
using System.Collections.Generic;

var set = new SortedSet<string>();
set.Add("a");
set.Add("z");
set.Add("x");

var list1 = new List<string>();
list1.Add("a");
list1.Add("z");
list1.Add("x");

var list2 = new List<string>();
list2.Add("a");
list2.Add("z");
list2.Add("x");
list2.Add("y");

Console.WriteLine("IsProperSubsetOf: {0}", set.IsProperSubsetOf(list1));
Console.WriteLine("IsSubsetOf: {0}", set.IsSubsetOf(list1));

Console.WriteLine("IsProperSubsetOf: {0}", set.IsProperSubsetOf(list2));
Console.WriteLine("IsSubsetOf: {0}", set.IsSubsetOf(list2));

var list3 = new List<string>();
list3.Add("a");
list3.Add("z");

Console.WriteLine("IsProperSupersetOf: {0}", set.IsProperSupersetOf(list3));
Console.WriteLine("IsSupersetOf: {0}", set.IsSupersetOf(list3));
IsProperSubsetOf: False
IsSubsetOf: True
IsProperSubsetOf: True
IsSubsetOf: True
IsProperSupersetOf: True
IsSupersetOf: True

SetEquals

This tells us if 2 collections have the same elements. In my testing, I found that the SetEquals method treats the second collection as though it has no duplicate elements.

using System;
using System.Collections.Generic;

var set = new SortedSet<string>();
set.Add("s");
set.Add("a");
set.Add("m");

var list = new List<string>();
list.Add("s");
list.Add("a");
list.Add("m");

// See if the 2 collections contain the same elements.
bool a = set.SetEquals(list);
Console.WriteLine(a);
True

GetViewBetween

How can you see what elements a SortedSet instance contains between and including 2 values? You can use GetViewBetween with 2 arguments: the minimum and maximum values.

using System;
using System.Collections.Generic;

var set = new SortedSet<int>();
set.Add(5);
set.Add(7);
set.Add(8);
set.Add(9);
set.Add(4);

// Call GetViewBetween method.
SortedSet<int> view = set.GetViewBetween(4, 7);

foreach (int val in view)
{
    Console.WriteLine(val);
}
4
5
7

Add

When you add elements to your SortedSet, they are automatically stored in ascending order: lowest to highest values. With Reverse() we get the elements in the opposite, reverse order.

using System;
using System.Collections.Generic;

var set = new SortedSet<int>();
set.Add(8);
set.Add(7);
set.Add(8);
set.Add(9);
set.Add(3);

foreach (int val in set)
{
    Console.Write(val);
    Console.Write(' ');
}
Console.WriteLine();

// Use Reverse.
foreach (int val in set.Reverse())
{
    Console.Write(val);
    Console.Write(' ');
}
Console.WriteLine();
3 7 8 9
9 8 7 3

Performance

Is SortedSet efficient? This depends of course on your program. A key point is that the SortedSet does not include hashing. To look up a node, a logarithmic search is needed.

Internally, the SortedSet is implemented as a tree with a Root, Left and Right node on every node instance. Every node is allocated on the managed heap. This reduces performance.

A summary

SortedSet can replace confusing, custom types with a simple type. This reduces program complexity. SortedSet is an interesting combination of HashSet and SortedList.