ArrayList and Hashtable (legacy non-generic collections) — C# Language

13. ArrayList and Hashtable (legacy non-generic collections)

BasicMediumLesson 13 of 38

You will meet these in old .NET Framework code and in interviews.

csharp

using System.Collections;

var list = new ArrayList();
list.Add(1);            // boxed to object
list.Add("two");        // anything goes — no type safety
list.Add(3.5);
foreach (object o in list) Console.WriteLine(o);
int first = (int)list[0]!;          // cast needed; wrong cast → InvalidCastException at runtime

var table = new Hashtable();
table["IN"] = "India";
table["TW"] = "Taiwan";
table["IN"] = "Bharat";             // overwrite
Console.WriteLine(table["XX"] == null);   // True — missing key returns null (Dictionary throws)
foreach (DictionaryEntry e in table) Console.WriteLine($"{e.Key} = {e.Value}");

	`ArrayList`	`List<T>`	`Hashtable`	`Dictionary<K,V>`
Type safety	❌ object	✅	❌ object	✅
Boxing of value types	Yes	No	Yes	No
Missing key	—	—	Returns `null`	Throws (use `TryGetValue`)
Thread safety	`ArrayList.Synchronized` wrapper	No (use concurrent collections)	Safe for many readers + one writer	No

Interview tip: "Hashtable vs Dictionary" — Dictionary is generic (type-safe, no boxing, faster); a missing key throws in Dictionary but returns null in Hashtable; both are hash-based with O(1) average lookup. Use ConcurrentDictionary for multi-threaded writes.