49. Init-only properties

We often find ourselves needing to create properties that can be set during the initialization of an object but remain immutable afterwards. For instance, consider a Point in a two-dimensional space. Once you’ve placed a point at some coordinate, you might not want it to move around.

Traditionally in C#, to make properties immutable after an object is created, you would have to make them read-only and then set their values in a constructor. However, with the advent of the init accessor in C#, we can now create properties that can only be set at the time of object initialization and stay read-only thereafter.

public class Point
{
    public int X { get; init; }
    public int Y { get; init; }
}

Notice how we’ve replaced the set accessor with the init accessor in the code example above. With this declaration, you can now set the X and Y properties when you create a new Point, but you can’t change them afterwards.

Point point = new Point { X = 5, Y = 10 };

point.X = 20;

(3,1): error CS8852: Init-only property or indexer 'Point.X' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

You might be wondering how init differs from readonly. While both keywords restrict you from modifying the value after initialization, the key difference is that readonly restricts the modification to the constructor only. In contrast, init allows the property to be set not only in the constructor but also in object initializers.

Remember the required keyword from the previous chapter? It works hand-in-hand with init. When a property is marked as required, it ensures that the property must be given a value when the object is initialized.

public class Point
{
    public required int X { get; init; }
    public required int Y { get; init; }
}

With this declaration, the compiler ensures that X and Y are given a value when a Point is created. If we fail to initialize all required attributes, the code does not compile:

Point point = new Point { X = 5 };

(1,19): error CS9035: Required member 'Point.Y' must be set in the object initializer or attribute constructor. (CS9035)

But if we do set all required attributes then it does compile.

Point point = new Point { X = 5, Y = 10 };

The init keyword is a powerful addition to C#’s toolbox, allowing us to create immutable objects with minimal and simple syntax. By ensuring the integrity of our objects, it greatly enhances code safety while also contributing to the readability and maintainability of our codebase.