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The non-generic SortedList collection class stores a sorted collection of key-value pairs, where key and value are objects. In SortedList, the key-value pairs are sorted by the keys, where each key is used as an index at which its associated value is stored.

SortedList collection class implements IDictionary, ICollection, IEnumerable, and ICloneable interfaces and it is defined in the System.Collections namespace.

Note: In SortedList, each key value can also be referred by its index position and using this index position of a key we can get, set its associated value. Basically, it can even be said that each key-value pair can be referred by its index position in the SortedList.

Some important features of SortedList

• A null cannot be stored as a key.
• A null can be stored as a value.
• SortedList elements are ordered on the basis of the ordering of the keys.







A commonly used constructor of SortedList




Constructor	Description
SortedList()	This constructor creates an empty instance of the SortedList class by using the default initial capacity, hash code provider, and comparer. The capacity of an SortedList is the number of elements it can hold, as the new elements are added to the SortedList, its capacity is automatically increased.
SortedList(int capacity)	This constructor creates a SortedList that is empty and has a specified initial capacity.








Properties of SortedList:




Properties	Description
Capacity	This property is used to get or set the total number of elements a SortedList can hold.
Count	This property gives total number of elements in a SortedList.
IsFixedSize	This property determines whether a SortedList has a fixed size or not.
IsReadOnly	This property determines whether a SortedList is read-only or not.
IsSynchronzied	This property determines whether a SortedList is thread-safe or not.
Item[Object key]	This property gets or sets a value associated with the specified key in a SortedList. To use this property, you should not type Item, but you only need to specify the specific key within a pair of square brackets [ ]
Keys	This property gets an ICollection containing the keys of a SortedList.
Values	This property gets an ICollection containing the values of a SortedList.








Some commonly used methods in a SortedList.





Methods	Description
int Add(Object key, Object value)	This method adds the specified key-value pair in a SortedList.
bool Contains(Object key)	This method checks if a SortedList contains a specific key.
bool ContainsKey(Object key)	This method checks if a SortedList contains a specific key.
bool ContainsValue(Object key)	This method checks if a SortedList contains a specific value.
void Clear()	This method removes all of the elements from a SortedList.
Object GetKey(int index)	This method gets the key at the specified index of a SortedList object.
Object GetByIndex(int index)	This method gets the value at the specified index of a SortedList object.
int IndexOfKey(Object key)	This method returns the zero-based index of the specified key in a SortedList object
int IndexOfValue(Object value)	This method returns the zero-based index of the first occurrence of the specified value in a SortedList object.
Ilist GetKeyList()	This method gets a list of keys of a SortedList object.
IList GetValueList()	This method gets a list of values of a SortedList object.
IDictionaryEnumerator GetEnumerator()	This method returns to enumerate an entire SortedList.
void Remove(Object key)	This method removes the element with the specified key in a SortedList.
void RemoveAt(int index)	This method removes the element at the specified index in a SortedList.



Note: Please know that passing an invalid/out-of-range index to any method that takes index position, will throw an ArgumentOutOfRangeException.






SortedList example



In the upcoming example, we will create a SortedList to store a sorted key-value pair of objects, where:
• Each key is a String object.
• Each key's associated value is an int, and because C# has a unified type system, so any primitive values like int, char, float, etc are implicitly boxed into Object references.

Next, we will iterate over all the key-value pairs(sorted on the basis of keys) and we will also obtain a value on the basis its key.



//C# Example of SortedList

using System;
using System.Collections;

class SortedListDemo
{
public static void Main(String[] ar)
{
	//Creating a SortedList
	SortedList sl = new SortedList();

	//Adding key-value pairs by the Add() method
	sl.Add("Max", 1000);
	sl.Add("John", 4000);
	sl.Add("Tom", 2000);
	sl.Add("Ana", 6000);
	sl.Add("Rick", 5000);

	//Getting a collection of the keys using property Keys
	ICollection c = sl.Keys;
	
	// Use the keys to obtain the values.
	Console.WriteLine("The sorted key-value pairs of SortedList:");
	foreach(String str in c)
		Console.WriteLine(str + ": " + sl[str]);

	//Using the property Item to retrieve the value of key "Tom"
	//Remember the property Item is not used with its name
	Console.WriteLine("Value at the key, Tom is "+ sl["Tom"]);

	//Using the property Item to retrieve the value of key "Ana"
	Console.WriteLine("Value at the key, Ana is "+ sl["Ana"]);
}
}

Output is :

The sorted key-value pairs of SortedList:
Ana: 6000
John: 4000
Max: 1000
Rick: 5000
Tom: 2000
Value at the key, Tom is 2000
Value at the key, Ana is 6000





Program Analysis



• The method Add() is called to add the values of Key-Value pair by specifying a Key and its Value.
• Next, we have retrieved a collection of the keys stored in the SortedList, by using its property Keys, and using which, we have iterated over each value stored in the SortedList.
• Next we have used the Item[key] property of the SortedList(which is used without specifying the word Item), to obtain a value associated with a particular key in our SortedList.





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Checking if SortedList contains a specific key or a specific value



In this example of SortedList we will perform two operations:
• We will determine if our SortedList contains a specific key by using the methods, Contains(Object), ContainsKey(Object).
• We will determine if our SortedList contains a specific value by using the method, ContainsValue(Object).




//C# Example of SortedList

using System;
using System.Collections;

class SortedListDemo
{
public static void Main(String[] ar)
{
	//Creating a SortedList
	SortedList sl = new SortedList();


	//Adding key-value pairs by the Add() method
	sl.Add("Max", 1000);
	sl.Add("John", 4000);
	sl.Add("Tom", 2000);
	sl.Add("Ana", 6000);
	sl.Add("Rick", 5000);


	//Getting a collection of the keys using property Keys
	ICollection c = sl.Keys;
	

	// Use the keys to obtain the values.
	Console.WriteLine("The contents of SortedList:");
	foreach(String str in c)
		Console.WriteLine(str + ": " + sl[str]);


	//Calling the method Contains() to see if SortedList contains key "Tom"
	Console.WriteLine("Does our SortedList contains the key, Tom: "+ sl.Contains("Tom"));


	//Calling the method ContainsKey() to see if SortedList contains key "Ana"
	Console.WriteLine("Does our SortedList contains the key, Tom: "+ sl.ContainsKey("Ana"));


	//Calling the method ContainsKey() to see if SortedList contains key "Jack"
	Console.WriteLine("Does our SortedList contains the key, Jack: "+ sl.ContainsKey("Jack"));


	//Calling the method ContainsValue() to see if SortedList contains value 4000
	Console.WriteLine("Does our SortedList contains the value, 4000: "+ sl.ContainsValue(4000));


	//Calling the method ContainsValue() to see if SortedList contains value 10000
	Console.WriteLine("Does our SortedList contains the value, 10000: "+ sl.ContainsValue(10000));
}
}





Output is :

The contents of SortedList:
Ana: 6000
John: 4000
Max: 1000
Rick: 5000
Tom: 2000
Does our SortedList contains the key, Tom: True
Does our SortedList contains the key, Tom: True
Does our SortedList contains the key, Jack: False
Does our SortedList contains the value, 4000: True
Does our SortedList contains the value, 10000: False








Accessing each key-value pair by its index position.



As we have said in in the beginning of this article, in SortedList, each key can also be referred by its index position and by using this index position of a key we can get, set its associated value. Basically, it can even be said that each key-value pair can referred by its index position in a SortedList.



//C# Accessing each key-value pair by its index position in a SortedList

using System;
using System.Collections;

class SortedListDemo
{
public static void Main(String[] ar)
{
	//Creating a SortedList
	SortedList sl = new SortedList();


	//Adding key-value pairs by theAdd() method
	//Where, each key is a character and each value is a String.
	sl.Add('B', "Banana");
	sl.Add('M', "Mango");
	sl.Add('G', "Grapes");
	sl.Add('A', "Apple");
	sl.Add('O', "Orange");
	sl.Add('K', "Kiwi");	//Trying to store a null as a key


	//Getting a list of keys
	IList keys = sl.GetKeyList();

	//Getting a list of values
	IList values = sl.GetValueList();


	//Printing all the key-value pairs by their index
	//Starting with index 0
	Console.WriteLine("The key-value pairs of SortedList:");
	for(int i=0;i<sl.Count; i++)
		Console.WriteLine("At index " + i + " index-> "  + keys[i] + ": " + values[i]);


	//Getting a key by its index position 4
	//where, the first key is stored at zero-index position
	Console.WriteLine("The key at index 4: " + sl.GetKey(4));


	//Getting a value associated with a specific index position 4
	Console.WriteLine("The value at index 4: " + sl.GetByIndex(4));


	//Getting the index of the key 'B'(it is char value)
	//where, the first key is stored at zero-index position
	Console.WriteLine("The index of key B: " + sl.IndexOfKey('B'));


	//Getting the index of value "Mango"
	Console.WriteLine("The index of value Mango: " + sl.IndexOfValue("Mango"));	
}
}





Output is :

The key-value pairs of SortedList:
At 0 index-> A: Apple
At 1 index-> B: Banana
At 2 index-> G: Grapes
At 3 index-> K: Kiwi
At 4 index-> M: Mango
At 5 index-> O: Orange
The key at index 4: M
The value at index 4: Mango
The index of key B: 1
The index of value Mango: 4








Removing elements from a SortedList



In this example of SortedList we will call the Remove(Object key) and RemoveAt(int index) method to remove the elements of a SortedList.
• The Remove(Object key) removes an element with the specified key in a SortedList.
• The Remove(int index) method removes an element at the specified index in a SortedList.




//C# Example of SortedList

using System;
using System.Collections;

class SortedListDemo
{
public static void Main(String[] ar)
{
	//Creating a SortedList
	SortedList sl = new SortedList();


	//Adding key-value pairs by theAdd() method
	//Where, each key is a character and each value is a String.
	sl.Add('B', "Banana");
	sl.Add('M', "Mango");
	sl.Add('G', "Grapes");
	sl.Add('A', "Apple");
	sl.Add('O',"Orange");


	//Getting a collection of the keys using property Keys
	ICollection c = sl.Keys;
	

	// Use the keys to obtain the values.
	Console.WriteLine("The contents of SortedList:");
	foreach(char ch in c)
		Console.WriteLine(ch + ": " + sl[ch]);


	//Calling the method Remove() to remove a key 'A' and its associated value
	sl.Remove('A');

	//Calling the method Remove() to remove a key 'M' and its associated value
	sl.Remove('M');


	//Printing all elements of SortedList
	Console.WriteLine("The updated contents of SortedList after removing keys 'A' and 'M' and their values:");
	foreach(char ch in c)
		Console.WriteLine(ch + ": " + sl[ch]);


	//Calling the method RemoveAt() to remove the element at index 3
	sl.RemoveAt(2);


	//Printing all elements of SortedList
	Console.WriteLine("The updated contents of SortedList after removing the element at index 2:");
	foreach(char ch in c)
		Console.WriteLine(ch + ": " + sl[ch]);
}
}





Output is :

The contents of SortedList:
A: Apple
B: Banana
G: Grapes
M: Mango
O: Orange
The updated contents of SortedList after removing keys 'A' and 'M' and their values:
B: Banana
G: Grapes
O: Orange
The updated contents of SortedList after removing the element at index 2:
B: Banana
G: Grapes



As you can see in the just concluded example, we have created a SortedList to store objects in the form of key-value pair of objects, where:
• Each key is a char value, and because C# has a unified type system, so any primitive values like int, char, float, etc are implicitly boxed into an Object references.
• Each value is a String object.






A key in SortedList cannot be null



In this example of SortedList, we will show you how inserting a null value for a key, throws an exception.



//C# Key cannot be null in a SortedList

using System;
using System.Collections;

class SortedListDemo
{
public static void Main(String[] ar)
{
	//Creating a SortedList
	SortedList sl = new SortedList();


	//Adding key-value pairs by theAdd() method
	//Where, each key is a character and each value is a String.
	sl.Add('B', "Banana");
	sl.Add('M', "Mango");
	sl.Add('G', "Grapes");
	sl.Add('A', "Apple");
	sl.Add('O', "Orange");
	sl.Add(null, "no value");	//Trying to store a null as a key
}
}





Output

Unhandled Exception: System.ArgumentNullException: Key cannot be null.
Parameter name: key
   at System.Collections.SortedList.Add(Object key, Object value)
   at SortedListDemo.Main(String[] ar)


As you can see, the program has thrown a ArgumentNullException because a Key cannot be null.






A value in SortedList can be null



In this example of SortedList, we will show you how inserting a null for a value is completely allowed and doesn't throws an exception.



//C# A value can  be null in a SortedList

using System;
using System.Collections;

class SortedListDemo
{
public static void Main(String[] ar)
{
	//Creating a SortedList
	SortedList sl = new SortedList();


	//Adding key-value pairs by theAdd() method
	//Where, each key is a character and each value is a String.
	sl.Add('B', "Banana");
	sl.Add('M', "Mango");
	sl.Add('G', "Grapes");
	sl.Add('A', "Apple");
	sl.Add('O', "Orange");
	sl.Add('N', null);	//Trying to store a null as a key


	//Getting a collection of the keys using property Keys
	ICollection c = sl.Keys;
	

	//Printing the key-value pairs by using the collection of keys to obtain their values.
	Console.WriteLine("The contents of SortedList:");
	foreach(char ch in c)
		Console.WriteLine(ch + ": " + sl[ch]);

}
}





Output

The contents of SortedList:
A: Apple
B: Banana
G: Grapes
M: Mango
N:
O: Orange


As you can see, the program has not thown any exception even when we have specified a null as a value for a key N.





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