private init in C# props?

Question

What does private access modifier for init only setter in C# mean? I usually make setter as private for most of the properties in my domain classes. With init, does it matter?

The following seem to work without any problem, the compiler does not complain.

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

So how is the above different from the below. There is not private below.

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

Answer 1

It affects where the setter can be called from, just like normal setters.

Consider this code:

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

class Program
{
    static void Main()
    {
        var point = new Point { X = 10, Y = 20 };
    }
}

That fails with two compile-time errors:

The property or indexer 'Point.X' cannot be used in this context because the set accessor is inaccessible

(and the equivalent for Y).

The same initializer would be valid in the Point class, because the set accessors are accessible there. For example, you might want to use a public factory method, which is implemented with an object initializer:

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

    public static Point FromXY(int x, int y) =>
        new Point { X = x, Y = y };
}

(There are various reasons why that might not be ideal, or you might prefer to add parameters to a private constructor, but it's an example of where the private init-only setters are accessible.)

Likewise the code above would be valid without the private part of the set accessors, because then they can be called from anywhere (as part of an object initializer).