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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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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Keeping the key-values sorted by using a SortedDictionary with C# .NET

You can use the generic SortedDictionary of Key and Value to automatically keep the key value items sorted by their keys. Any time you add a new key value pair the dictionary will reorder the items. The SortedDictionary was optimised for frequent changes to its list of items. Keep in mind that the items will be sorted by their key and not their value.

Consider the following simple custom object:

public class Student
{
	public string Name { get; set; }
	public string SchoolName { get; set; }
}

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Return a default value from a Map in Java 8

Consider the following Employee class:

public class Employee
{
    private UUID id;
    private String name;
    private int age;

    public Employee(UUID id, String name, int age)
    {
        this.id = id;
        this.name = name;
        this.age = age;
    }
        
    public UUID getId()
    {
        return id;
    }

    public void setId(UUID id)
    {
        this.id = id;
    }

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }    
    
    public int getAge()
    {
        return age;
    }

    public void setAge(int age)
    {
        this.age = age;
    }
}

Let’s put some Employee objects into a hash map:

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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.

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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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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