C# | Get or Set the value associated with specified key in Hashtable

Hashtable.Item[Object] Property is used to get or set the value associated with the specified key in the Hashtable.

Syntax:

public virtual object this[object key] { get; set; }

Here, key is key of object type whose value is to get or set.

Exceptions:

• ArgumentNullException: If the key is null.
• NotSupportedException: If the property is set and the Hashtable is read-only. Or the property is set, key does not exist in the collection, and the Hashtable has a fixed size.

Note:

• This property returns the value associated with the specific key. If that key is not found, and one is trying to get that, then this property will return null and if trying to set, it will result into the creation of a new element with the specified key.
• Retrieving and setting the value of this property is an O(1) operation.

Below programs illustrate the use of above-discussed property:

Example 1:

// C# code to Gets or sets the value
// associated with the specified key
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a Hashtable
        Hashtable myTable = new Hashtable();

        // Adding elements in Hashtable
        myTable.Add("g", "geeks");
        myTable.Add("c", "c++");
        myTable.Add("d", "data structures");
        myTable.Add("q", "quiz");

        // Get a collection of the keys.
        ICollection c = myTable.Keys;

        // Displaying the contents
        foreach(string str in c)
            Console.WriteLine(str + ": " + myTable[str]);

        // Setting the value associated with key "c"
        myTable["c"] = "C#";

        Console.WriteLine("Updated Values:");

        // Displaying the contents
        foreach(string str in c)
            Console.WriteLine(str + ": " + myTable[str]);
    }
}

Output:

d: data structures
c: c++
q: quiz
g: geeks
Updated Values:
d: data structures
c: C#
q: quiz
g: geeks

Example 2:

// C# code to Gets or sets the value
// associated with the specified key
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a Hashtable
        Hashtable myTable = new Hashtable();

        // Adding elements in Hashtable
        myTable.Add("4", "Even");
        myTable.Add("9", "Odd");
        myTable.Add("5", "Odd and Prime");
        myTable.Add("2", "Even and Prime");

        // Get a collection of the keys.
        ICollection c = myTable.Keys;

        // Displaying the contents
        foreach(string str in c)
            Console.WriteLine(str + ": " + myTable[str]);

        // Setting the value associated
        // with key "56" which is not present
        // will result in the creation of
        // new key and value will be set which
        // is given by the user
        myTable["56"] = "New Value";

        Console.WriteLine("Updated Values:");

        // Displaying the contents
        foreach(string str in c)
            Console.WriteLine(str + ": " + myTable[str]);
    }
}

Output:

5: Odd and Prime
9: Odd
2: Even and Prime
4: Even
Updated Values:
5: Odd and Prime
9: Odd
2: Even and Prime
56: New Value
4: Even

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.hashtable.item?view=netframework-4.7.2