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:

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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 a thread-safe dictionary in .NET C# Part 4: thread-safe insertlookups

In the previous post we looked at how the AddOrUpdate method worked. We saw that it was a very neat and thread-safe way to either insert a new key-value pair or update an existing one.

We’ve already seen a thread-safe method that helps you retrieve values by their keys in this post: TryGet. It returns true if the item could be retrieved. TryAdd on the other hand is used to insert a new key-value pair. It also returns true if the item could be inserted successfully. However, what do you do with the returned boolean values? Do you keep trying in some loop until it succeeds?

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Using a thread-safe dictionary in .NET C# Part 3: thread-safe modifications

In the previous post we looked at the 4 Try methods of ConcurrentDictionary that support CRUD operations: retrieval, deletion, update and insertion. We saw some basic examples for their usage and concluded that TryUpdate was not a very good solution to actually update an item due to race conditions.

This is where another method called AddOrUpdate enters the scene.

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Using a thread-safe dictionary in .NET C# Part 2: CRUD operations

In the previous post we briefly introduced the ConcurrentDictionary object. We said that it was the thread-safe counterpart of the standard Dictionary object. The Dictionary object is not suited as a shared resource in multi-threaded scenarios as you can never be sure if another thread has added to or removed an element from the dictionary just milliseconds earlier. A ConcurrentDictionary is a good option to cure the shortcomings of the thread-sensitive Dictionary object but it is also more difficult to use.

We’ll briefly look at the 4 Try methods that enable you to insert, remove, update and lookup elements in the ConcurrentDictionary.

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Implementing an enumerator for a custom object in .NET C#

You can create an enumerator for a custom type by implementing the generic IEnumerable of T interface. Normally you’d do that if you want to create a custom collection that others will be able to iterate over using foreach. However, there’s nothing stopping you from adding an enumerator to any custom type if you feel like it, it’s really simple.

Consider the following Guest class:

public class Guest
{
	public string Name { get; set; }
	public int Age { get; set; }
}

Guests can be invited to a Party:

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FIFO collections with Queue of T in .NET C#

FIFO, that is first-in-first-out, collections are represented by the generic Queue of T class in .NET. Queues are collections where a new element is placed on top of the collection and is removed last when the items are retrieved.

Let’s say that you’re throwing a party where you follow a Queue 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 first person who has arrived. This is probably a most just policy than what we saw in the post on stack collections.

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How to check whether two HashSets are equal in C# .NET

Two HashSet objects in C# are equal if they contain the same values regardless of their order in the collection.

Consider the following integer sets:

HashSet<int> intHashSetOne = new HashSet<int>()
{
	1,2,6,5,7,5
};

HashSet<int> intHashSetTwo = new HashSet<int>()
{
	2,2,8,5,9,4
};

HashSet<int> intHashSetThree = new HashSet<int>()
{
	6,7,5,5,2,1
};

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Creating sorted sets with C# .NET

The SortedSet of T object is the sorted version of the HashSet object. We’ve already seen what a HashSet can do for you in the referenced post. A SortedSet keeps the elements in increasing order.

Consider the following integer set:

SortedSet<int> sortedInts = new SortedSet<int>();
sortedInts.Add(1);
sortedInts.Add(4);
sortedInts.Add(3);
sortedInts.Add(1);
sortedInts.Add(3);
sortedInts.Add(10);
sortedInts.Add(8);
sortedInts.Add(3);
sortedInts.Add(1);
sortedInts.Add(4);
foreach (int i in sortedInts)
{
	Debug.WriteLine(i);
}

This will print…

1
3
4
8
10

Notice that duplicates were rejected to ensure uniqueness just like in the case of HashSets.

That is straightforward for primitive types like integers since .NET “knows” how to compare them. It can decide whether 10 is greater than 5, we don’t need to provide any help.

However what about reference types like your own objects, such as this one?

public class Band
{
	public string Name { get; set; }
	public int YearFormed { get; set; }
	public int NumberOfMembers { get; set; }
	public int NumberOfRecords { get; set; }
}

How can .NET decide on the ordering of your objects? We’ll need to give it a hint by providing an object which implements the generic IComparer of T interface like we saw in this post. We’ll let the Band objects be sorted by their names:

public class BandNameComparer : IComparer<Band>
{
	public int Compare(Band x, Band y)
	{
		return x.Name.CompareTo(y.Name);
	}
}

Let’s see this in action:

SortedSet<Band> bands = new SortedSet<Band>(new BandNameComparer());
bands.Add(new Band() { YearFormed = 1979, Name = "Great band", NumberOfMembers = 4, NumberOfRecords = 10 });
bands.Add(new Band() { YearFormed = 1985, Name = "Best band", NumberOfMembers = 5, NumberOfRecords = 15 });
bands.Add(new Band() { YearFormed = 1985, Name = "Well known band", NumberOfMembers = 5, NumberOfRecords = 15 });
bands.Add(new Band() { YearFormed = 1979, Name = "Famous band", NumberOfMembers = 4, NumberOfRecords = 10 });
bands.Add(new Band() { YearFormed = 1979, Name = "Great band", NumberOfMembers = 4, NumberOfRecords = 10 });
bands.Add(new Band() { YearFormed = 1985, Name = "Best band", NumberOfMembers = 5, NumberOfRecords = 15 });
bands.Add(new Band() { YearFormed = 1985, Name = "Best band", NumberOfMembers = 5, NumberOfRecords = 15 });
bands.Add(new Band() { YearFormed = 1979, Name = "Great band", NumberOfMembers = 4, NumberOfRecords = 10 });
bands.Add(new Band() { YearFormed = 1979, Name = "Famous band", NumberOfMembers = 4, NumberOfRecords = 10 });

foreach (Band band in bands)
{
	Debug.WriteLine(band.Name);
}

This will print…

Best band
Famous band
Great band
Well known band

…so not only were the items sorted by their names but the non-unique values were rejected as well. The IComparer argument also provided a way to declare equality.

View all various C# language feature related posts here.

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