AssertionFlowClient depreceated, trying to use ServiceAccountCredential, but it won't work - google-drive-api

Question

I've tried creating a DriveService using service account acting on behalf of another user.
I've copied this code from google documentation found here https://developers.google.com/drive/delegation
static DriveService BuildService() {
X509Certificate2 certificate = new X509Certificate2(SERVICE_ACCOUNT_PKCS12_FILE_PATH, "notasecret", X509KeyStorageFlags.Exportable);
var provider = new AssertionFlowClient(GoogleAuthenticationServer.Description, certificate)
{
ServiceAccountId = SERVICE_ACCOUNT_EMAIL,
Scope = DriveService.Scopes.Drive.GetStringValue(),
};
var auth = new OAuth2Authenticator<AssertionFlowClient>(provider, AssertionFlowClient.GetState);
return new DriveService(auth);
}
but I get this warning when trying to build the project:
Warning 4 'Google.Apis.Authentication.OAuth2.DotNetOpenAuth.AssertionFlowClient' is obsolete: 'AssertionFlowClient is not supported any more and it's going to be removed in 1.7.0-beta. Consider using ServiceAccountCredential from the new Google.Apis.Auth NuGet package.'
and I also get this error:
Error 11 Argument 1: cannot convert from 'Google.Apis.Authentication.OAuth2.OAuth2Authenticator' to 'Google.Apis.Services.BaseClientService.Initializer'
Then I googled ServiceAccountCredential and ended up with this code (derived from this page: https://code.google.com/p/google-api-dotnet-client/wiki/OAuth2#Service_Accounts)
static DriveService BuildService() {
X509Certificate2 certificate = new X509Certificate2(SERVICE_ACCOUNT_PKCS12_FILE_PATH, "notasecret", X509KeyStorageFlags.Exportable);
ServiceAccountCredential credential = new ServiceAccountCredential(
new ServiceAccountCredential.Initializer(SERVICE_ACCOUNT_EMAIL)
{
User = "someone#mydomain.mygbiz.com",
Scopes = new[] { DriveService.Scope.DriveFile }
}.FromCertificate(certificate));
var service = new DriveService(new BaseClientService.Initializer()
{
HttpClientInitializer = credential,
ApplicationName = "Drive API Sample",
});
return service;
}
When I try to build this code it seems all fine, but when I run it I get the following error.
A first chance exception of type 'System.Security.Cryptography.CryptographicException' occurred in mscorlib.dll
Additional information: Det går inte att hitta det begärda objektet.
(Translated : The requested object cannot be found)
If there is a handler for this exception, the program may be safely continued.
The error occurs on this line:
X509Certificate2 certificate = new X509Certificate2(SERVICE_ACCOUNT_PKCS12_FILE_PATH, "notasecret", X509KeyStorageFlags.Exportable);
Anyone have any ideas?
Update 2013 Oct 31
I have tried this code:
{
Console.WriteLine("Drive API - Service Account");
Console.WriteLine("==========================");
String serviceAccountEmail = "<some email>#developer.gserviceaccount.com";
var certificate = new X509Certificate2(#"key.p12", "notasecret", X509KeyStorageFlags.Exportable);
ServiceAccountCredential credential = new ServiceAccountCredential(
new ServiceAccountCredential.Initializer(serviceAccountEmail)
{
User = "<someuser>#<mydomain>.mygbiz.com",
Scopes = new[] { DriveService.Scope.Drive }
}.FromCertificate(certificate));
// Create the service.
var service = new DriveService(new BaseClientService.Initializer()
{
HttpClientInitializer = credential,
ApplicationName = "DrvMgr",
});
Console.WriteLine("Executing listing");
FileList UserFiles = service.Files.List().Execute();
I get this error message:
An unhandled exception of type 'Google.Apis.Auth.OAuth2.Responses.TokenResponseException' occurred in Google.Apis.dll
Additional information: Error:"access_denied", Description:"", Uri:""

Answer 1

It looks like the path to your p12 file is incorrect. See the Plus.ServiceAccount sample for a working solution.
I think that in this sample, the key.p12 was added to the project as a content file which is always copied to the output directory. Then mentioning "key.p12" file path from the code will result in grabbing this file from the output folder.