How to use restsharp to download file

It seems SaveAs was discontinued. You can try this

var client = new RestClient("http://example.com")    
byte[] response = client.DownloadData(request);
File.WriteAllBytes(SAVE_PATH, response);

With RestSharp, it's right there in the readme:

var client = new RestClient("http://example.com");
client.DownloadData(request).SaveAs(path);

With HttpClient, it's a bit more involved. Have a look at this blog post.

Another option is Flurl.Http (disclaimer: I'm the author). It uses HttpClient under the hood and provides a fluent interface and lots of convenient helper methods, including:

await "http://example.com".DownloadFileAsync(folderPath, "foo.xml");

Get it on NuGet.