C# 11 Auto Default Structs in depth

I always had the feeling that Structs were the ignored son of C#. I don't want to cause problems in the Family. But, hey, it was not me picking favourites. 🤪

Classes were always the first to evolve and improve. Don't get me wrong. It's completely understandable. It's the most used, so the investment is understandable and unquestionable.

Now, I can tell you that it's changing. On the past few C# versions, we have seen Structs catching up. Struct Records and Parameterless constructors are steps on that direction. It's clear the effort to provide a similar experience between Classes and Structs. And that is good for the language. It brings consistency.

The next on the list is Auto Default Structs.

🙋‍♂️ What are we trying to solve?

Before C# 11, if you declare a structure and you have a Constructor, you would need to assign each field.

struct PlayerPosition
{
    public PlayerPosition(int x, int y)
    {
        X = x;
        Y = y;
    }

    public int X;
    public int Y;
}

If you don't initialize one of the fields, you will have the following compilation error.

  [CS0171] Field 'PlayerPosition.Y' must be fully assigned before control is returned to the caller.

That causes maintainability problems. Imagine that you add an extra field to the Struct. Now you need to change all the places where the Constructor is invoked. Besides the fact that this is a behaviour that doesn't mimic what we are used to in Classes.

🤔 But, why do we need to do that?

In the way that Structs work, if do not set the value, you could get the following result:

var player = new PlayerPosition();
player.X = 10;
player = new PlayerPosition();
player.X.Should().Be(0); // Assertion will fail. It will be 10 if X isn't initialized in constructor.

struct PlayerPosition
{
    public PlayerPosition(){}

    public int X;
    public int Y;
}

There's a brilliant thread about it here.

🌟 How Auto Default Structs can help?

Auto Default Structs are the way C# will fix that. Let's start by ignoring the feature name. Is super strange. 🙄

With C# 11, the compiler will ensure that all fields are initialized to their default value.

In other words, fields not explicitly initialized on the Constructor will be initialized with the default value. The initialization is inserted at the beginning of the Constructor.

Imagine the following code.

struct PlayerPosition
{
    public PlayerPosition(int x, int y)
    {
        X = x;
        Y = y;
    }

    public int X;
    public int Y;
    public int Z;
}

This code is now valid with C# 11. Why? Because the compiler will be doing something like:

struct PlayerPosition
{
    public PlayerPosition(int x, int y)
    {
        Z = default; // 👈 Here is the magic

        X = x;
        Y = y;
    }

    public int X;
    public int Y;
    public int Z;
}

👏 Takeaway

I like the fact that this is a step on uniformization between Classes and Structs. Structs, will for sure become a bit easier to maintain.

Will I be heavily using this? No. But when I use Structs, this will be useful for sure.

I hope that you liked it. Let me know if you have to give it a try.

In the meanwhile, follow me on Twitter (@gsferreira) and let's keep in touch. 😉