C# Hashtable: Get Value by Key

The Hashtable class in C# is a non-generic collection that stores key-value pairs. To retrieve values efficiently, you can access them directly using the key. This article explains how to retrieve values from a Hashtable in C# using different approaches.

Accessing Values by Key

Using Indexer Syntax

The simplest way to access values in a Hashtable is by using the indexer syntax hashtable[key]. This method will return the value associated with the specified key. However, if the key is not found, this will return null.

using System;
using System.Collections;

public class HashtableIndexerExample
{
    public static void Main()
    {
        // Create a Hashtable and populate it with key-value pairs
        Hashtable employees = new Hashtable
        {
            { "E101", "John Doe" },
            { "E102", "Jane Smith" },
            { "E103", "Mary Johnson" }
        };

        // Retrieve values using indexer syntax
        Console.WriteLine($"Employee E101: {employees["E101"]}");
        Console.WriteLine($"Employee E102: {employees["E102"]}");

        // Handle a missing key gracefully
        object missingEmployee = employees["E200"];
        if (missingEmployee == null)
        {
            Console.WriteLine("Employee E200 not found.");
        }
    }
}

Using ContainsKey Method

To safely check if a key exists before retrieving its value, you can use the ContainsKey method. This approach helps prevent errors when accessing non-existent keys.

using System;
using System.Collections;

public class HashtableContainsKeyExample
{
    public static void Main()
    {
        // Create a Hashtable with some initial data
        Hashtable products = new Hashtable
        {
            { "P001", "Laptop" },
            { "P002", "Smartphone" },
            { "P003", "Tablet" }
        };

        // Safely check if a key exists before accessing its value
        string key = "P002";
        if (products.ContainsKey(key))
        {
            Console.WriteLine($"Product {key}: {products[key]}");
        }
        else
        {
            Console.WriteLine($"Product {key} not found.");
        }

        // Attempt to access a non-existent key
        key = "P999";
        if (!products.ContainsKey(key))
        {
            Console.WriteLine($"Product {key} not found.");
        }
    }
}

Best Practices

• Check for Existence: Use ContainsKey to verify the key before accessing it to avoid potential errors.
• Null Handling: Handle null values gracefully since Hashtable allows storing them.
• Type Safety: When retrieving objects, always cast them to the appropriate data type.

Conclusion

Retrieving values from a Hashtable in C# is straightforward using indexer syntax or the ContainsKey method. By following best practices, you can efficiently and safely access values by key while preventing runtime errors.