C# Deconstruct and Discards

 Deconstruct and Discards

In the last post I mentioned the Deconstruct method and how it should not be mixed up with a Destructor. In this post I will show some examples and how you can use Deconstruct and especially in combination with something called Discards.

The Deconstruct method can be defined within a class to provide a way to retrieve a Tuple of the data that makes up the type and Discards can be used by the caller to effectively ignore the members of the Tuple that aren't of interest.

Sounds complicated? Hopefully the examples below will clear things out.

Implementing the Deconstruct method

Let's start with a simple container class holding attributes of a file in the filesystem.

public class FileInfo
{
  public string Name { get; }
  public int SizeInBytes { get; }
  public bool ReadOnly { get; }

  public FileInfo(string name, int sizeInBytes, bool readOnly)
  {
    Name = name;
    SizeInBytes = sizeInBytes;
    ReadOnly = readOnly;
  }

  public void Deconstruct(out string name, out int sizeInBytes, out bool readOnly)
  {
    name = Name;
    sizeInBytes = SizeInBytes;
    readOnly = ReadOnly;
  }
}

In this case the Deconstructor works as an alternative to using the public attributes. To use the Deconstruct method to extract the attributes declare a Tuple and assign on object instance of the FileInfo class to it.

var fileInfo = new FileInfo("MyFile.txt", 1024, true);
...
var (fileName, fileSize, readOnly) = fileInfo;
Console.WriteLine($"The file {fileName} is {fileSize} bytes and its read only attribute is set to {readOnly}");

Ok, this looks cool and all, but why should you use the Deconstructor instead of just the public attributes? That is actually a really good question. From what I can read between the lines in this blogpost from Microsoft the ability to add Deconstructors in this way was added as a way to create symmetry by adding Constructors and Deconstructors pairwise. For every Constructor you add to your class, you can now add a corresponding Deconstructor which returns the Constructor parameters. Whether you want to use Deconstructors or not is up to you and your development team.

Discard tuples members with Discards

It is possible to discard any members of the returned Tuple by using a single underscore, '_', instead of a parameter name. For example, if the only parameter of interest in the FileInfo object is the fileName the other two members of the Tuple can be discarded like this:

...
var (fileName, _, _) = fileInfo;
...

More resources

Deconstructing tuples and other types
What's new in C# 7
Discards