C# 12 features: ref readonly

Part 3 in a series on new language features in C# 12.

ref readonly modifier

A bit more of a niche feature - if you have code that is passing structs by ref, but you don’t need to (or want to) allow modification of the parameter, you can now indicate it is readonly.

    public static void ByRefReadonly(ref readonly Point p)
    {
        //p = new Point(2, 3);
        Console.WriteLine(p);

    }

In the code sample above, Point is a struct or record type. If you uncommented the line then the compiler will emit an error.

Interesting, but I’ve not come across a need to use this yet.

Further reading

https://learn.microsoft.com/dotnet/csharp/language-reference/keywords/method-parameters?WT.mc_id=DOP-MVP-5001655#ref-readonly-modifier

Example source

https://github.com/flcdrg/csharp-12/blob/main/02-collection-expressions/CollectionExpressions.cs