LIFO collections with Stack of T in .NET C#

LIFO, that is last-in-first-out, collections are represented by the generic Stack of T class in .NET. Stacks are collections where a new element is placed on top of the collection and is removed first when an item is being retrieved. Hence the item that was entered first will get to stay longest in the stack.

Let’s say that you’re throwing a party where you follow a Stack policy as far as guests are concerned. As time goes by you’d like all of them to leave eventually and the first one to go will be the last person who has arrived. This might not be a very fair way to treat your guests but there you go.

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Using linked lists in .NET with C#

Linked lists are very efficient if you want to insert and remove items in a collection frequently. Linked lists do not store their elements in contiguous regions in memory, like e.g. the List of T object. Instead elements in linked lists can be stored anywhere.

The elements are linked by their “next” and “previous” references. The first element will have a “next” pointer to the element that comes after that. The second element in turn will have a reference to the previous and next elements. The first element will of course only have a reference to the “next” element and “previous” will be null, whereas the last element will point to the previous element and “next” will be null. All elements in between will have two references. Such a linked list is also called a doubly linked list.

One big difference between linked lists and “normal” lists is that linked lists have no index accessors. You just cannot extract the element #3 for example without looping through the linked list and stopping at the third element.

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Using the KeyedCollection object in C# .NET

The abstract generic KeyedCollection object can be used to declare which field of your custom object to use as a key in a Dictionary. It provides sort of a short-cut where you’d want to organise your objects in a Dictionary by an attribute of that object.

Let’s take the following object as an example:

public class CloudServer
{
	public string CloudProvider { get; set; }
	public string ImageId { get; set; }
	public string Size { get; set; }
}

The Image IDs are always unique so the ImageId property seems to be a good candidate for a dictionary key.

Here’s an example:

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Getting notified when collection changes with ObservableCollection in C# .NET

Imagine that you’d like to be notified when something is changed in a collection, e.g. an item is added or removed. One possible solution is to use the built-in .NET generic collection type ObservableCollection of T which is located in the System.Collections.ObjectModel namespace. The ObservableCollection object has an event called CollectionChanged. You can hook up an event handler to be notified of the changes.

If you don’t know what events, event handlers and delegates mean then start here.

Let’s see a simple example with a collection of strings:

ObservableCollection<string> names = new ObservableCollection<string>();
names.Add("Adam");
names.Add("Eve");
names.Add("Clive");
names.Add("Anne");
names.Add("John");
names.Add("Peter");
names.Add("Hannah");

Nothing special so far I guess. This looks exactly like a “normal” collection.

The following example demonstrates the CollectionChanged event:

private static void RunObservableCollectionCode()
{
	ObservableCollection<string> names = new ObservableCollection<string>();
	names.CollectionChanged += names_CollectionChanged;
	names.Add("Adam");
	names.Add("Eve");
	names.Remove("Adam");
	names.Add("John");
	names.Add("Peter");
	names.Clear();
}

static void names_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
	Debug.WriteLine("Change type: " + e.Action);
	if (e.NewItems != null)
	{
		Debug.WriteLine("Items added: ");
		foreach (var item in e.NewItems)
		{
			Debug.WriteLine(item);
		}
	}

	if (e.OldItems != null)
	{
		Debug.WriteLine("Items removed: ");
		foreach (var item in e.OldItems)
		{
			Debug.WriteLine(item);
		}
	}
}

The NotifyCollectionChangedEventArgs object resides in the System.Collections.Specialized namespace.

If I run the RunObservableCollectionCode method I get the following output:

Change type: Add
Items added:
Adam
Change type: Add
Items added:
Eve
Change type: Remove
Items removed:
Adam
Change type: Add
Items added:
John
Change type: Add
Items added:
Peter
Change type: Reset

You can then decide how to handle additions and deletions in the event handler.

View all various C# language feature related posts here.

Customise your list by overriding Collection of T with C# .NET

Imagine that you’d like to build a list type of collection where you want to restrict the insertion and/or deletion of items in some way. Let’s say we need an integer list with the following rules:

• The allowed range of integers is between 0 and 10 inclusive
• A user should not be able to remove an item at index 0
• A user should not be able to remove all items at once

One possible solution is to derive from the Collection of T class. The generic Collection of T class in the System.Collections.ObjectModel namespace provides virtual methods that you can override in your custom collection.

The virtual InsertItem and SetItem methods are necessary to control the behaviour of the Collection.Add and the way items can be modified through an indexer:

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Using the HashSet of T object in C# .NET to store unique elements

The generic HashSet of T is at first sight a not very “sexy” collection. It simply stores objects with no order, index or key to look up individual elements.

Here’s a simple HashSet with integers:

HashSet<int> intHashSet = new HashSet<int>();
intHashSet.Add(1);
intHashSet.Add(3);
intHashSet.Add(5);
intHashSet.Add(2);
intHashSet.Add(10);

HashSets can be handy when you want to guarantee uniqueness. The following example will only put the unique integers in the set:

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