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C# | Search in a SortedList object

Last Updated : 11 Jul, 2025

SortedList class is a collection of

(key, value) pairs

which are sorted according to keys. Those pairs can be accessible by key and as well as by index(zero-based indexing). This comes under

System.Collections

namespace.

SortedList.ContainsKey(Object)

method is used to check whether a SortedList object contains a specific key or not.

Properties:

A SortedList element can be accessed by its key or by its index.
A SortedList object internally maintains two arrays to store the elements of the list, i.e, one array for the keys and another array for the associated values.
A key cannot be null, but a value can be.
The capacity of a SortedList object is the number of elements the SortedList can hold.
A SortedList does not allow duplicate keys.
Operations on a SortedList object tend to be slower than operations on a Hashtable object because of the sorting.
Elements in this collection can be accessed using an integer index. Indexes in this collection are zero-based.

Syntax:

public virtual bool ContainsKey (object key);

Here,

key

is the key to locate in the SortedList object.

Return Value:

This method will return

True

if the SortedList object contains an element with the specified key otherwise it returns

False

Exceptions:

ArgumentNullException : If the key is null.
InvalidOperationException : If the comparer throws an exception.

Below given are some examples to understand the implementation in a better way:

Example 1: CSHARP


 // C# code to check if a SortedList
// object contains a specific key
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating an SortedList
        SortedList mySortedList = new SortedList();

        // Adding elements to SortedList
        mySortedList.Add("h", "Hello");
        mySortedList.Add("g", "Geeks");
        mySortedList.Add("f", "For");
        mySortedList.Add("n", "Noida");

        // Checking if a SortedList object
        // contains a specific key
        Console.WriteLine(mySortedList.ContainsKey("g"));
    }
}

Example 2: CSHARP


 // C# code to check if a SortedList
// object contains a specific key
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating an SortedList
        SortedList mySortedList = new SortedList();

        // Adding elements to SortedList
        mySortedList.Add("h", "Hello");
        mySortedList.Add("g", "Geeks");
        mySortedList.Add("f", "For");
        mySortedList.Add("n", "Noida");

        // Checking if a SortedList object
        // contains a specific key
        // It should throw ArgumentNullException
        // as the Key can not be null
        Console.WriteLine(mySortedList.ContainsKey(null));
    }
}

Error:

Unhandled Exception: System.ArgumentNullException: Key cannot be null. Parameter name: key

Note:

This method uses a binary search algorithm, therefore, this method is an O(log n) operation, where n is Count.

Reference:

https://learn.microsoft.com/en-us/dotnet/api/system.collections.sortedlist.containskey?view=netframework-4.7.2

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