Download file using HttpClient wrapper asynchronously.

public static class HttpHelper{   private static readonly HttpClient _httpClient = new HttpClient();   public static async void DownloadFileAsync(string uri        , string outputPath)   {      Uri uriResult;      if (!Uri.TryCreate(uri, UriKind.Absolute, out uriResult))         throw new InvalidOperationException("URI is invalid.");      if (!File.Exists(outputPath))         throw new FileNotFoundException("File not found."            , nameof(outputPath));      byte[] fileBytes = await _httpClient.GetByteArrayAsync(uri);      File.WriteAllBytes(outputPath, fileBytes);   }}