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

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.