Use Delegate in Callback

You want to be notified when the download is complete so that you can perform some action, such as displaying a message to the user. You can use a callback delegate to achieve this:

   
  
using System;
using System.Net;
using System.Threading.Tasks;

class FileDownloader
{
    // Define a delegate for the callback
    public delegate void DownloadCompletedCallback(string message);

    // Method to download a file asynchronously
    public async Task DownloadFileAsync(string url, DownloadCompletedCallback callback)
    {
        using (var client = new WebClient())
        {
            try
            {
                // Download the file
                await client.DownloadFileTaskAsync(url, "downloadedFile.txt");

                // Invoke the callback to notify that the download is complete
                callback("Download completed successfully");
            }
            catch (Exception ex)
            {
                // Invoke the callback with the error message if an exception occurs
                callback("Download failed: " + ex.Message);
            }
        }
    }
      

🔸Callback Functions: When working with asynchronous operations, you might use callback functions to handle the result of the operation.

🔸Using generic delegates allows you to define callback functions that can handle different types of results.

🔸generic delegates provide a way to write more flexible and reusable code, especially in scenarios where you need to work with different types of data or operations.

   
  
class Program
{
    static void Main()
    {
        var fileDownloader = new FileDownloader();

        // Start the download and pass a callback function
        fileDownloader.DownloadFileAsync("https://www.example.com/file.txt", DownloadCompletedCallback);
        
        // Keep the program running to see the callback message
        Console.ReadLine();
    }

    // Callback function that will be invoked when the download is complete
    static void DownloadCompletedCallback(string message)
    {
        Console.WriteLine(message);
    }
}
  

In this example, the FileDownloader class defines a method DownloadFileAsync that downloads a file asynchronously. It takes a URL and a callback delegate as parameters. The method downloads the file and then invokes the callback, passing a message indicating whether the download was successful or not.

In the Main method, we create an instance of FileDownloader and start the download by calling DownloadFileAsync, passing the URL and the DownloadCompletedCallback method as the callback delegate. When the download is complete, the DownloadCompletedCallback method is invoked, and a message is displayed to the user.