Why is this field declared as private and also readonly?

Question

In the following code:

public class MovieRepository : IMovieRepository
{
    private readonly IHtmlDownloader _downloader;

    public MovieRepository(IHtmlDownloader downloader)
    {
        _downloader = downloader;
    }

    public Movie FindMovieById(string id)
    {
        var idUri = ...build URI...;

        var html = _downloader.DownloadHtml(idUri);

        return ...parse ID HTML...;
    }

    public Movie FindMovieByTitle(string title)
    {
        var titleUri = ...build URI...;

        var html = _downloader.DownloadHtml(titleUri);

        return ...parse title HTML...;
    }
}

I asked for something to review my code, and someone suggested this approach. My question is why is the IHtmlDownloader variable readonly?

Answer 1

This ensures that the value of _downloader will not be changed after the constructor was executed. Fields marked as readonly can only be assigned a value from within the constructor(s) of a class.

Answer 2

If it's private and readonly, the benefit is that you can't inadvertently change it from another part of that class after it is initialized. The readonly modifier ensures the field can only be given a value during its initialization or in its class constructor.

If something functionally should not change after initialization, it's always good practice to use available language constructs to enforce that.

On a related note, C# 9 introduces the init accessor method for properties, which indicates the property value can only be set during object construction, e.g.:

class InitExample
{
     private double _seconds;

     public double Seconds
     {
         get { return _seconds; }
         init { _seconds = value; }
     }
}

Answer 3

A readonly field is useful for modelling data that should not change after it has been initialized. You can assign a value to a readonly field by using a initializer when you declare it or in a constructor, but thereafter you cannot change it.