Why is this variable 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

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 variable can only be given a value during class initialization and in a constructor.

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

Answer 2

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.