List vs ArrayList vs Dictionary vs Hashtable vs Stack vs Queue?

We can use any of these (includes List, ArrayList, Dictionary, Hashtable, Stack, Queue) to hold value or hold reference to other objects as a collection.

But, my question is which one is used when?

Lists

Lists allow duplicate items, can be accessed by index, and support linear traversal.

• ArrayList - An array-based list that doesn't support generic types. It does not enforce type safety and should generally be avoided.

• List - An array list that supports generic types and enforces type-safety. Since it is non-contiguous, it can grow in size without re-allocating memory for the entire list. This is the more commonly used list collection.

Hashes

Hashes are look-ups in which you give each item in a list a "key" which will be used to retrieve it later. Think of a hash like a table index where you can ask questions like "I'm going to find this object by this string value. Duplicate keys are not allowed.

• HashTable - A basic key-value-pair map that functions like an indexed list.

• Dictionary - A hashtable that supports generic types and enforces type-safety.

Queues

Queues control how items in a list are accessed. You typically push/pop records from a queue in a particular direction (from either the front or back). Not used for random access in the middle.

• Stack - A LIFO (last in, first out) list where you push/pop records on top of each other.

• Queue - A FIFO (first in, first out) list where you push records on top and pop them off the bottom.

• List can hold duplicate objects

• ArrayList is just for compatibility with older versions of the framework where IList didn't exist

• Dictionary is used to store pairs of key/value. You cannot have duplicate keys.

• Hashtable is basically a List with no possibility of duplicates (and better performance in some scenarios)

• Stack stores objects in order they were added (through Push()), and when you retrieve an object (through Pop()) it is removed from the stack in a LIFO manner.

• Queue quite similar to a Stack except it is FIFO.

Here are some uses for them.

List: If you just want a list and don't care about any duplicates, i.e list of people, shopping list, list of things to do in life.

Queues: If you want to simulate a queue for example, in a hospital you have a queue and also priority queue (in emergency departments). The triage would determine who is in critical condition and needs to be treated.

Another example is a shopping queue, first person in line is 'usually' the first one to checkout.

Stacks: Used in your internal memory to push and pop values as you pass them to functions/methods.

Another interesting use is, in video game inventory method, where you can pick up an item (push) onto the stack, and drop an item (pop) off the stack.

Hash/Dictionary: These are usually seen used in database, for look up and index.

Depending on what you want to simulate, I do agree with the others, it's handy to read up on data-structures. A book helps but the internet also has a wealth of information.