System.Collections Namespace

The System.Collections namespace provides the base classes and interfaces for non‑generic collections in .NET. These collections store objects of type object and are primarily maintained for backward compatibility. For new development, the generic equivalents in System.Collections.Generic are recommended.

Key Types

ArrayList – Dynamically sized array of objects.
Hashtable – Key/value map using hashing.
Stack – LIFO collection.
Queue – FIFO collection.
SortedList – Sorted key/value pairs.

Example: Using ArrayList

Program.cs

using System;
using System.Collections;

class Program
{
    static void Main()
    {
        // Create an ArrayList and add heterogeneous items
        ArrayList items = new ArrayList();
        items.Add(10);
        items.Add("hello");
        items.Add(DateTime.Now);

        foreach (object o in items)
        {
            Console.WriteLine($"{o} ({o.GetType()})");
        }

        // Insert at a specific index
        items.Insert(1, 3.14);
        Console.WriteLine("\nAfter Insert:");
        foreach (var o in items) Console.WriteLine(o);
    }
}

When to Use Non‑Generic Collections

Non‑generic collections are useful when you need to store heterogeneous types or when working with legacy code that predates .NET 2.0. However, they incur boxing/unboxing overhead for value types and lack compile‑time type safety.

System.Collections Namespace

Key Types

Example: Using ArrayList

When to Use Non‑Generic Collections

Related Topics