[c#] How to download image from url

It is not necessary to use System.Drawing to find the image format in a URI. System.Drawing is not available for .NET Core unless you download the System.Drawing.Common NuGet package and therefore I don't see any good cross-platform answers to this question.

Also, my example does not use System.Net.WebClient since Microsoft explicitly discourage the use of System.Net.WebClient.

We don't recommend that you use the WebClient class for new development. Instead, use the System.Net.Http.HttpClient class.

Download an image from a URL and write it to a file (cross platform)*

*Without old System.Net.WebClient and System.Drawing.

This method will asynchronously download an image (or any file as long as the URI has a file extension) using the System.Net.Http.HttpClient and then write it to a file, using the same file extension as the image had in the URI.

Getting the file extension

First part of getting the file extension is removing all the unnecessary parts from the URI.
We use Uri.GetLeftPart() with UriPartial.Path to get everything from the Scheme up to the Path.
In other words, https://www.example.com/image.png?query&with.dots becomes https://www.example.com/image.png.

After that, we use Path.GetExtension() to get only the extension (in my previous example, .png).

var uriWithoutQuery = uri.GetLeftPart(UriPartial.Path);
var fileExtension = Path.GetExtension(uriWithoutQuery);

Downloading the image

From here it should be straight forward. Download the image with HttpClient.GetByteArrayAsync, create the path, ensure the directory exists and then write the bytes to the path with File.WriteAllBytesAsync() (or File.WriteAllBytes if you are on .NET Framework)

private async Task DownloadImageAsync(string directoryPath, string fileName, Uri uri)
{
    using var httpClient = new HttpClient();

    // Get the file extension
    var uriWithoutQuery = uri.GetLeftPart(UriPartial.Path);
    var fileExtension = Path.GetExtension(uriWithoutQuery);

    // Create file path and ensure directory exists
    var path = Path.Combine(directoryPath, $"{fileName}{fileExtension}");
    Directory.CreateDirectory(directoryPath);

    // Download the image and write to the file
    var imageBytes = await _httpClient.GetByteArrayAsync(uri);
    await File.WriteAllBytesAsync(path, imageBytes);
}

Note that you need the following using directives.

using System;
using System.IO;
using System.Threading.Tasks;
using System.Net.Http;

Example usage

var folder = "images";
var fileName = "test";
var url = "https://cdn.discordapp.com/attachments/458291463663386646/592779619212460054/Screenshot_20190624-201411.jpg?query&with.dots";

await DownloadImageAsync(folder, fileName, new Uri(url));

Notes

• It's bad practice to create a new HttpClient for every method call. It is supposed to be reused throughout the application. I wrote a short example of an ImageDownloader(50 lines) with more documentation that correctly reuses the HttpClient and properly disposes of it that you can find here.