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C# 9 Non-Destructive Mutation

There are some phrases in programming that feel like they belong in a superhero movie. Non-destructive mutation is just such as phrase. It refers to a concept in functional programming that means when you want to change the state of an object, you create a copy with the change – rather than changing the original.

Firstly, we need to use the new C# 9 record type, which is just a class preceded by the data keyword. Let’s continue the book example from the C# 9 Initializers and Immutability article.

public data class Book {
    public string Author { get; init; }
    public string Title { get; init; }
}

Book book = new Book {
    Author = "Mathew Lewis",
    Title = "The Monk"
};

Book anotherBook = book with { Title = "The Bravo of Venice" };

By creating the second book with the with keyword, we can choose to change one or more properties. The original book isn’t changed, we still have that. We also have our new book. That’s non-destructive mutation in C# 9 in a nutshell.

Now, the with keyword will create a shallow copy, going field by field and assigning the properties. That means reference type properties will end up being shared state by default. This matches what you would expect if you have used structs in C#. However, you can implement your own copy constructor (it just needs to be a constructor that takes an argument of the same type, a Book in our example). So, you can deep-copy if that’s what you need to do.