C# - SortedList

The SortedList collection stores key-value pairs in the ascending order of key by default. SortedList class implements IDictionary & ICollection interfaces, so elements can be accessed both by key and index.

C# includes two types of SortedList, generic SortedList and non-generic SortedList. Here, we will learn about non-generic SortedList.

The following diagram illustrates the non-generic SortedList hierarchy.

Important Properties and Methods of SortedList

Property	Description
Capacity	Gets or sets the number of elements that the SortedList instance can store.
Count	Gets the number of elements actually contained in the SortedList.
IsFixedSize	Gets a value indicating whether the SortedList has a fixed size.
IsReadOnly	Gets a value indicating whether the SortedList is read-only.
Item	Gets or sets the element at the specified key in the SortedList.
Keys	Get list of keys of SortedList.
Values	Get list of values in SortedList.

Method	Description
Add(object key, object value)	Add key-value pairs into SortedList.
Remove(object key)	Removes element with the specified key.
RemoveAt(int index)	Removes element at the specified index.
Contains(object key)	Checks whether specified key exists in SortedList.
Clear()	Removes all the elements from SortedList.
GetByIndex(int index)	Returns the value by index stored in internal array
GetKey(int index)	Returns the key stored at specified index in internal array
IndexOfKey(object key)	Returns an index of specified key stored in internal array
IndexOfValue(object value)	Returns an index of specified value stored in internal array

Add elements in SortedList

Use the Add() method to add key-value pairs into a SortedList.

Add() method signature: void Add(object key, object value)

Key cannot be null but value can be null. Also, datatype of all keys must be same, so that it can compare otherwise it will throw runtime exception.

Example: Add key-value pairs in SortedList 

                SortedList sortedList1 = new SortedList();
sortedList1.Add(3, "Three");
sortedList1.Add(4, "Four");
sortedList1.Add(1, "One");
sortedList1.Add(5, "Five");
sortedList1.Add(2, "Two");

SortedList sortedList2 = new SortedList();
sortedList2.Add("one", 1);
sortedList2.Add("two", 2);
sortedList2.Add("three", 3);
sortedList2.Add("four", 4);
    
SortedList sortedList3 = new SortedList();
sortedList3.Add(1.5, 100);
sortedList3.Add(3.5, 200);
sortedList3.Add(2.4, 300);
sortedList3.Add(2.3, null);
sortedList3.Add(1.1, null);

            

Note:

Internally, SortedList maintains two object[] array, one for keys and another for values. So when you add key-value pair, it runs a binary search using the key to find an appropriate index to store a key and value in respective arrays. It re-arranges the elements when you remove the elements from it.

SortedList collection sorts the elements everytime you add the elements. So if you debug the above example, you will find keys in ascending order even if they are added randomly, as below:

Please notice that sortedList2 sorts the key in alphabetical order for string key in the above image.

Use Object Initializer Syntax to initialize the SortedList, as shown below.

Example: Add key-value pairs in SortedList 

                SortedList sortedList = new SortedList()
{
    {3, "Three"},
    {4, "Four"},
    {1, "One"},
    {5, "Five"},
    {2, "Two"}
};

            

SortedList key can be of any data type, but you cannot add keys of different data types in the same SortedList. The key type of the first key-value pair remains the same for all other key-value pairs. The following example will throw run time exception because we are trying to add the second item with a string key:

Example: Key of different datatypes throws exception:

                SortedList sortedList = new SortedList();

sortedList.Add(3, "Three");
sortedList.Add("Four", "Four"); // Throw exception: InvalidOperationException
sortedList.Add(1, "One");
sortedList.Add(8, "Five");
sortedList.Add(2, "Two");


            
Try it 

Access SortedList

SortedList can be accessed by index or key. Unlike other collection, SortedList requires key instead of index to access a value for that key.

Example: Access SortedList

                SortedList sortedList = new SortedList()
                            {
                                {"one", 1},
                                {"two", 2},
                                {"three", 3},
                                {"four", "Four"}
                            }

int i = (int) sortedList["one"];
int j = (int) sortedList["two"];
string str = (string) sortedList["four"];

Console.WriteLine(i);
Console.WriteLine(j);
Console.WriteLine(str);

            
Try it 

Output:

1 2 Four

Note:

Non-generic SortedList collection can contain key and value of any data type. So values must be cast to the appropriate data type otherwise it will give compile-time error.

Use for loop to access SortedList as shown below.

Example: Accessing Values using For Loop

                SortedList sortedList = new SortedList()
                            {
                                {3, "Three"},
                                {4, "Four"},
                                {1, "One"},
                                {5, "Five"},
                                {2, "Two"}
                            };
for (int i = 0; i < sortedList.Count; i++)
{
    Console.WriteLine("key: {0}, value: {1}", 
        sortedList.GetKey(i), sortedList.GetByIndex(i));
}

            
Try it 

Output:

key: 1, value: One key: 2, value: Two key: 3, value: Three key: 4, value: Four key: 5, value: Five

The foreach statement can also be used to access the SortedList collection. SortedList includes key-value pairs. So, the type of element would be DictionaryEntry rather than the type of a key or a value.

Example: Access values using foreach

                SortedList sortedList = new SortedList()
                            {
                                {3, "Three"},
                                {4, "Four"},
                                {1, "One"},
                                {5, "Five"},
                                {2, "Two"}
                            };
foreach(DictionaryEntry kvp in sortedList )
    Console.WriteLine("key: {0}, value: {1}", kvp.Key , kvp.Value );

            
Try it 

Remove elements from SortedList

Use the Remove() or RemoveAt() method to remove elements from a SortedList.

Remove() signature: void Remove(object key)

RemoveAt() signature: void RemoveAt(int index)

Example: Remove elements in SortedList

                SortedList sortedList = new SortedList();
sortedList.Add("one", 1);
sortedList.Add("two", 2);
sortedList.Add("three", 3);
sortedList.Add("four", 4);
    
sortedList.Remove("one");//removes element whose key is 'one'
sortedList.RemoveAt(0);//removes element at zero index i.e first element: four

foreach(DictionaryEntry kvp in sortedList )
    Console.WriteLine("key: {0}, value: {1}", kvp.Key , kvp.Value );

            
Try it 

Output:

key: three, value: 3 key: two, value: 2

Check for an existing key in SortedList

The Contains() & ContainsKey() methods determine whether the specified key exists in the SortedList collection or not.

Contains() signature: bool Contains(object key)

ContainsKey() signature: bool ContainsKey(object key)

The ContainsValue() method determines whether the specified value exists in the SortedList or not.

ContainValue() signature: bool ContainValue(object value)

Example: Contains 

                SortedList sortedList = new SortedList();
                            {
                                {3, "Three"},
                                {4, "Four"},
                                {1, "One"},
                                {8, "Eight"},
                                {2, "Two"}
                            };
sortedList.Contains(2); // returns true
sortedList.Contains(4); // returns true
sortedList.Contains(6); // returns false

sortedList.ContainsKey(2); // returns true
sortedList.ContainsKey(6); // returns false

sortedList.ContainsValue("One"); // returns true
sortedList.ContainsValue("Ten"); // returns false

            
Try it 

Visit MSDN for more information on the properties and methods of SortedList.

Points to Remember :

C# has generic and non-generic SortedList.
SortedList stores the key-value pairs in ascending order of the key. Key must be unique and cannot be null whereas value can be null or duplicate.
Non-generic SortedList stores keys and values of any data types. So values needs to be cast to appropriate data type.
Key-value pair can be cast to DictionaryEntry.
Access individual value using indexer. SortedList indexer accepts key to return value associated with it.