(Mono C#) Get XOAUTH2 Access Token from Google OAuth 2.0 Authorization Server

Obtains an OAUTH2 access token from the Google OAuth 2.0 Authorization Server. This is for Server to server applications using Google API's that need an access token. See https://developers.google.com/accounts/docs/OAuth2ServiceAccount

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// This example assumes the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

Chilkat.Http http = new Chilkat.Http();

// When a service account (Client ID) is created at https://code.google.com/apis/console/
// Google will generate a P12 key.  This is a PKCS12 (PFX) file that you will download
// and save.  The password to access the contents of this file is "notasecret".
// NOTE: The Chilkat Pfx API provides the ability to load a PFX/P12 and re-save
// with a different password.

// Begin by loading the downloaded .p12 into a Chilkat certificate object:
Chilkat.Cert cert = new Chilkat.Cert();
bool success = cert.LoadPfxFile("/myDir/API Project-1c43a291e2a1-notasecret.p12","notasecret");
if (success != true) {
    Debug.WriteLine(cert.LastErrorText);
    return;
}

// The next (and final) step is to request the access token.  Chilkat internally
// does all the work of forming the JWT header and JWT claim set, encoding and
// signing the JWT, and sending the access token request.
// The application need only provide the inputs: The iss, scope(s), sub, and the
// desired duration with a max of 3600 seconds (1 hour).
// 
// Each of these inputs is defined as follows 
// (see https://developers.google.com/accounts/docs/OAuth2ServiceAccount
// 
// iss: The email address of the service account.
// 
// scope: A space-delimited list of the permissions that the application requests.
// 
// sub: required if there is an email address, such as for a
//   Google Apps domain—if you use Google Apps for Work, where the administrator of the Google Apps domain 
//   can authorize an application to access user data on behalf of users in the Google Apps domain.
// 
// numSec: The number of seconds for which the access token will be valid (max 3600).

string iss = "761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com";
string scope = "https://mail.google.com/";
// Leave "sub" empty if there is no Google Apps email.
string s_sub = "";
int numSec = 3600;

string accessToken = http.G_SvcOauthAccessToken(iss,scope,s_sub,numSec,cert);
if (http.LastMethodSuccess != true) {
    Debug.WriteLine(http.LastErrorText);
}
else {
    Debug.WriteLine("access token: " + accessToken);
}