Init

Suppose you have a property on a class that must be always set. For example, a Bird must always have a color. And you want to disallow setting it later or changing it.

With the init keyword, we can require that an initializer sets a property, so it is always set. We cannot use the set keyword alongside init, as this is redundant.

Example

Consider a Bird class that has a Color, and our requirement is that each Bird must always have a Color set. We specify init on the Color string property.

using System;

class Bird
{
    public string Color { get; init; }
}

class Program
{
    static void Main()
    {
        // Use the init property.
        var bird = new Bird() { Color = "Blue" };
        Console.WriteLine(bird.Color);
    }
}
Blue

Init error

The init keyword is unlike the set keyword in that it prohibits assigning the property later, after initialization. The C# compiler will issue an error in this case.

using System;

class Bird
{
    public string Color { get; init; }
}

class Program
{
    static void Main()
    {
        // With an init property, we must use an initializer.
        var bird = new Bird();
        bird.Color = "orange";
    }
}
... Program.cs(14,9): error CS8852:
Init-only property or indexer 'Bird.Color' can only be assigned in an object initializer,
or on 'this' or 'base ' in an instance constructor or an 'init' accessor.
...

Summary

We init, we have a keyword that acts like "set" but adds a constraint that the property must be set at initialization-time only. This can reduce incorrect usage of a property.