To access Google Drive Content we requires two things

 1.Refresh Token

2.Access Token(Valid for only one hour)
1.First get the Refresh Token Id to access Google Drive APIOffline.

How To Get Refresh Token ID?

 -First get Client id and Client Secret by registering you app for api access in

Google APIs Console.

-Login URL : https://accounts.google.com/o/oauth2/auth?response_type=code&client_id= @CLIENTID&client_secret= @CLIENTSECRET&redirect_uri=@REDIRECTURI &scope=@SCOPE&access_type=@ACCESSTYPE&approval_prompt= @APPROVALPROMPT&state=@STATE @CLIENTID = "111111.apps.googleusercontent.com"( will get from Google APIs Console) @CLIENTSECRET = "xXcsad3rg3aqwqg"(will get from Google APIs Console) @REDIRECTURI = "http://localhost:59364/Login.aspx"(will get from Google APIs Console) @SCOPE = "https://www.googleapis.com/auth/drive.file"(Scope) @ACCESSTYPE = "offline" @APPROVALPROMPT= "force" @STATE = "profile" - Above login URL will prompt the user for permission to access their Google Content via app and a code(Authentication_Code) request variable will be retuned in that URL like this: http://YOUR-SITE.com/YOUR-PATH/?state=profile&code=1/fFBGRNJru1FQd44AzqT3Zg

 

-Use that code to get Refresh Token and Access token(Access token will be valid for 1 hour).

2.Store that Refresh Token in database or in web.config file for future use.

3.Once we get the Refresh token no need to relogin or re-authenticate. Use that Refresh Token to get Access token using below code.

 

WebRequest request = WebRequest.Create("https://accounts.google.com/o/oauth2/token");     request.Method = "POST";     string postData = "client_id=" + @CLIENTID + "&client_secret=" + @CLIENTSECRET + "&refresh_token=" + @REFRESHTOKEN + "&grant_type=refresh_token"; 
    byte[] byteArray = Encoding.UTF8.GetBytes(postData);  
   request.ContentType = "application/x-www-form-urlencoded";
     request.ContentLength = byteArray.Length; 
    Stream dataStream = request.GetRequestStream();  
   dataStream.Write(byteArray, 0, byteArray.Length);
     dataStream.Close(); 
    WebResponse response = request.GetResponse();
     dataStream = response.GetResponseStream();  
   StreamReader reader = new StreamReader(dataStream); 
    string responseFromServer = reader.ReadToEnd();  
   reader.Close();
     dataStream.Close();  
   response.Close();

 

4.Get Files From Google Drive:

 

public DataTable RetrieveAllFiles(DriveService service)  
   {        
List<Google.Apis.Drive.v2.Data.File> result = new List<Google.Apis.Drive.v2.Data.File>();         FilesResource.ListRequest request = service.Files.List(); 
        string fileId = string.Empty;
         do    
     {       
      try    
         {     //Fetch all the files from the google drive.   
              FileList files = request.Fetch();     
            result.AddRange(files.Items);         
        request.PageToken = files.NextPageToken; 
                fileId = files.Items[0].Id;      
       }     
        catch (Exception e)  
           {   
              objError = new MapsLocator.BusinessLayer.ErrorLog();
                 objError.LogError(e);  
               request.PageToken = null;   
          }         } while (!String.IsNullOrEmpty(request.PageToken));   
            //Store all the files in a datatable.  
       DataTable dtResult = ConvertToDataTable(result); 
        return dtResult;
     }