Question

In my app, I have several comparison methods. I would like the user to be able to select the sorting method to use. Ideally, I would like to set a delegate and it is updated based on the user’s selection. This way, I can keep the code generic using List.Sort(delegate).

This is my first time attempting to use C# delegates and I’m running into syntax errors. Here is what I have so far:

The delegate:

private delegate int SortVideos(VideoData x, VideoData y);
private SortVideos sortVideos;

In the class constructor:

sortVideos = Sorting.VideoPerformanceDescending;

The comparison method in the public static Sorting class (which works when I call it directly):

public static int VideoPerformanceDescending(VideoData x, VideoData y)
{
    *code statements*
    *return -1, 0, or 1*
}

Failed syntax that complains of “some invalid arguments”:

videos.Sort(sortVideos);

Ultimately, I would like to change “sortVideos” to point to the method of choice. “videos” is a list of type VideoData. What am I doing wrong?

Answer 1

The List<T> accepts a delegate of type Comparison<T> so you cannot define your own delegate, you just need to reuse the delegate Comparison<T>.

private static Comparison<VideoData> sortVideos;

static void Main(string[] args)
{
    sortVideos = VideoPerformanceDescending;

    var videos = new List<VideoData>();

    videos.Sort(sortVideos);
}

Expanding the answer to also contemplate the user selection part, you could store the available options in a dictionary and then in the UI allow the user to select the sort algorithm by selecting the key to the dictionary.

private static Dictionary<string, Comparison<VideoData>> sortAlgorithms;

static void Main(string[] args)
{
    var videos = new List<VideoData>();

    var sortAlgorithms = new Dictionary<string, Comparison<VideoData>>();

    sortAlgorithms.Add("PerformanceAscending", VideoPerformanceAscending);
    sortAlgorithms.Add("PerformanceDescending", VideoPerformanceDescending);

    var userSort = sortAlgorithms[GetUserSortAlgorithmKey()];

    videos.Sort(userSort);
}

private static string GetUserSortAlgorithmKey()
{
    throw new NotImplementedException();
}

private static int VideoPerformanceDescending(VideoData x, VideoData y)
{
    throw new NotImplementedException();
}

private static int VideoPerformanceAscending(VideoData x, VideoData y)
{
    throw new NotImplementedException();
}