download file from google drive using asp.net

Question

I am trying download file from google drive using the code below:

     public static Boolean downloadFile(string downloadurl, string _saveTo)
        {

            if (!String.IsNullOrEmpty(downloadurl))
            {
                try
                {
                  var x = service.HttpClient.GetByteArrayAsync(downloadurl);
                    byte[] arrBytes = x.Result;
                    System.IO.File.WriteAllBytes(_saveTo, arrBytes);
                    return true;
                }
                catch (Exception e)
                {
                    Console.WriteLine("An error occurred: " + e.Message);
                    return false;
                }
            }
            else
            {
                // The file doesn't have any content stored on Drive.
                return false;
            }
        }

On debugging the above code throwing exception as below:

?service.HttpClient.GetByteArrayAsync(downloadurl)
Id = 10, Status = WaitingForActivation, Method = "{null}", Result = "{Not yet computed}"
    AsyncState: null
    CancellationPending: false
    CreationOptions: None
    Exception: null
    Id: 10
    Result: null
    Status: WaitingForActivation

I am trying to do it from my Service account created using Google API Console.

and the exception detail is as follows:

System.NullReferenceException was caught
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=System.Net.Http
  StackTrace:
       at System.Net.Http.Headers.HttpRequestHeaders.AddHeaders(HttpHeaders sourceHeaders)
       at System.Net.Http.HttpClient.PrepareRequestMessage(HttpRequestMessage request)
       at System.Net.Http.HttpClient.SendAsync(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationToken cancellationToken)
       at System.Net.Http.HttpClient.GetAsync(Uri requestUri, HttpCompletionOption completionOption, CancellationToken cancellationToken)
       at System.Net.Http.HttpClient.GetContentAsync[T](Uri requestUri, HttpCompletionOption completionOption, T defaultValue, Func`2 readAs)
       at System.Net.Http.HttpClient.GetByteArrayAsync(Uri requestUri)
       at System.Net.Http.HttpClient.GetByteArrayAsync(String requestUri)

Answer 1

You can try this.
link

using Google.Apis.Authentication;
    using Google.Apis.Drive.v2;
    using Google.Apis.Drive.v2.Data;

    using System.Net;

    public class MyClass {

      public static System.IO.Stream DownloadFile(
          IAuthenticator authenticator, File file) {
        if (!String.IsNullOrEmpty(file.DownloadUrl)) {
          try {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(
                new Uri(file.DownloadUrl));
            authenticator.ApplyAuthenticationToRequest(request);
            HttpWebResponse response = (HttpWebResponse) request.GetResponse();
            if (response.StatusCode == HttpStatusCode.OK) {
              return response.GetResponseStream();
            } else {
              Console.WriteLine(
                  "An error occurred: " + response.StatusDescription);
              return null;
            }
          } catch (Exception e) {
            Console.WriteLine("An error occurred: " + e.Message);
            return null;
          }
        } else {
          // The file doesn't have any content stored on Drive.
          return null;
        }
      }
    }

Answer 2

Code using the Google .net client library

Service account:

string[] scopes = new string[] {DriveService.Scope.Drive}; // Full access

var keyFilePath = @"c:\file.p12" ;    // Downloaded from https://console.developers.google.com
var serviceAccountEmail = "xx@developer.gserviceaccount.com";  // found https://console.developers.google.com

//loading the Key file
var certificate = new X509Certificate2(keyFilePath, "notasecret", X509KeyStorageFlags.Exportable);
var credential = new ServiceAccountCredential( new ServiceAccountCredential.Initializer(serviceAccountEmail) {
                                                   Scopes = scopes}.FromCertificate(certificate));

create drive service

var service = new DriveService(new BaseClientService.Initializer() {HttpClientInitializer = credential,
                                                                            ApplicationName = "Drive API Sample",});

You can use files.list to list all of the files on drive.

FilesResource.ListRequest request = service.Files.List();
request.Q = "trashed=false";
title = 'hello'
FileList files = request.Execute();

loop though the items returned find the file you want it is a file resource you can pass it to the following method to down load your file

/// <summary>
        /// Download a file
        /// Documentation: https://developers.google.com/drive/v2/reference/files/get
        /// </summary>
        /// <param name="_service">a Valid authenticated DriveService</param>
        /// <param name="_fileResource">File resource of the file to download</param>
        /// <param name="_saveTo">location of where to save the file including the file name to save it as.</param>
        /// <returns></returns>
        public static Boolean downloadFile(DriveService _service, File _fileResource, string _saveTo)
        {

            if (!String.IsNullOrEmpty(_fileResource.DownloadUrl))
            {
                try
                {
                    var x = _service.HttpClient.GetByteArrayAsync(_fileResource.DownloadUrl );
                    byte[] arrBytes = x.Result;
                    System.IO.File.WriteAllBytes(_saveTo, arrBytes);
                    return true;                  
                }
                catch (Exception e)
                {
                    Console.WriteLine("An error occurred: " + e.Message);
                    return false;
                }
            }
            else
            {
                // The file doesn't have any content stored on Drive.
                return false;
            }
        }

code ripped from Google drive authentication C#