Destructuring assignment - object properties to variables in C#

Question

JavaScript has a nifty feature where you can assign several variables from properties in an object using one concise line. It's called destructuring assignment syntax which was added in ES6.

// New object
var o = {p1:'foo', p2:'bar', p3: 'baz'};
// Destructure
var {p1, p2} = o;
// Use the variables...
console.log(p1.toUpperCase()); // FOO
console.log(p2.toUpperCase()); // BAR

I want to do something similar with C#.

// New anonymous object
var o = new {p1="foo", p2="bar", p3="baz"};
// Destructure (wrong syntax as of C#6)
var {p1, p2} = o;
// Use the variables...
Console.WriteLine(p1.ToUpper()); // FOO
Console.WriteLine(p2.ToUpper()); // BAR

Is there a syntax to do this in C#?

@TechnicalSmile There is no requirement for this language feature (or any language feature for that matter). It's all syntactic sugar on top of zeros and ones :) In this case, I want to write less code and this is a feature that enables it. — styfle
– styfle, Commented Apr 20, 2021 at 16:21

Szer · Accepted Answer · 2016-03-02 17:31:15Z

36

Closest thing which could help you are Tuples.

C#7 maybe will have something like this:

public (int sum, int count) Tally(IEnumerable<int> values) 
{
    var res = (sum: 0, count: 0); // infer tuple type from names and values
    foreach (var value in values) { res.sum += value; res.count++; }
    return res;
}


(var sum, var count) = Tally(myValues); // deconstruct result
Console.WriteLine($"Sum: {sum}, count: {count}");

Link to discussion

Right now it is not possible.

answered Mar 2, 2016 at 17:31

Szer

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4 Comments

Lucas Trzesniewski Over a year ago

C# 7 Using Tuples. You can achieve something like this.

You can construct and destruct a Tuple. Snippet

var payLoad = (
    Username: "MHamzaRajput",
    Password: "password",
    Domain: "www.xyz.com",
    Age: "24" 
);

// Hint: You just need to follow the sequence. 

var (Username, Password, Domain, Age) = payLoad;
// or
var (username, password, _, _) = payLoad;

Console.WriteLine($"Username: {username} and Password: {password}");

Output

Username: MHamzaRajput and Password: password

edited Apr 8, 2020 at 18:25

answered Apr 8, 2020 at 9:10

M. Hamza Rajput

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2 Comments

styfle Over a year ago

I get error CS0128: A local variable named _ is already defined in this scope. Do you need two different names for the underscores?

M. Hamza Rajput Over a year ago

I have uploaded a snippet link you can test there.

KyleMit · Accepted Answer · 2023-01-04 17:00:56Z

9

The positional syntax for records comes with deconstruction by default (dotnet fiddle):

public record Person(string firstName, string lastName) {}

var person = new Person("Kyle", "Mit");
var (firstName, lastName) = person;
Console.WriteLine(firstName); // "Kyle"

Looking at the generated code via SharpLab, it just implements a regular Deconstruct method that you could add to your own type if you weren't using records:

[CompilerGenerated]
public void Deconstruct(out string firstName, out string lastName)
{
    firstName = this.firstName;
    lastName = this.lastName;
}

According to the docs on Deconstruction

C# doesn't offer built-in support for deconstructing non-tuple types other than the record and DictionaryEntry types. However, as the author of a class, a struct, or an interface, you can allow instances of the type to be deconstructed by implementing one or more Deconstruct methods.

answered Jan 4, 2023 at 17:00

KyleMit♦

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1 Comment

styfle Over a year ago

Thanks! I particularly like that you can add public void Deconstruct to any class so this best answers the question! (the dotnet fiddle was a nice too so I could confirm it works)

Will Harnett · Accepted Answer · 2022-08-01 16:58:45Z

I've been looking for something like this for a while now and I came across Deconstructor with Classes. This isn't as elegant of a solution as Javascript gives us, but it can definitely simplify a lot of things if done well.

Below is a small example (I have not run this code). I hope this helps others:

class Animal
{
    public string name;
    public string type;

    public Animal(
            string name, 
            string type
        )
    {
        this.name = name;
        this.type = type;
    }

    // Use out params with a 
    // method called Deconstruct
    public void Deconstruct(
            out string name, 
            out string type
        )
    {
        name = this.name;
        type = this.type;
    }
}

class Shelter
{
    public Animal[] animals;
    public int animalCapacity;
    public int numberOfAnimalsInShelter = 0;

    public Shelter(
            int animalCapacity
        )
    {
        this.animalCapacity = animalCapacity;
        animals = new Animal[animalCapacity];
    }

    public void AddAnimalToShelter(
            Animal animal
        )
    {
        animals[numberOfAnimalsInShelter] = animal;
        numberOfAnimalsInShelter++;
    }

    public void AnimalsInShelter()
    {
        for (int i = 0; i < animals.Length; i++)
            {
            // Here is how to use the Deconstructor 
            // method from the Animal class
                var (name, type) = animals[i];
                Console.WriteLine(name);
                Console.WriteLine(type);
            }
    }
}

Output should be the name and type for every animal that gets added to the shelter.

Records does have seamless Deconstruct learn.microsoft.com/en-us/dotnet/csharp/language-reference/…

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