(VB.NET) Get Google API Access Token using JSON Private Key

Demonstrates how to get a Google API access token using a JSON service account private key.

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

' --------------------------------------------------------------------------------
' For a step-by-step guide for setting up your Google Workspace service account,
' see Setup Google Workspace Account for Sending SMTP GMail from a Service Account
' --------------------------------------------------------------------------------

' First load the JSON key into a string.
Dim fac As New Chilkat.FileAccess
Dim jsonKey As String = fac.ReadEntireTextFile("qa_data/googleApi/chilkat25-b4214220e565.json","utf-8")
If (fac.LastMethodSuccess <> True) Then
    Debug.WriteLine(fac.LastErrorText)
    Exit Sub
End If


' A Google service account JSON private key looks like this:

' {
'   "type": "service_account",
'   "project_id": "chilkat25",
'   "private_key_id": "b4214220f565881e19eeb97c2699bf5a0d1e3e0b",
'   "private_key": "-----BEGIN PRIVATE KEY-----\nMIIEvQ...NXcM=\n-----END PRIVATE KEY-----\n",
'   "client_email": "chilkatsvc@chilkat25.iam.gserviceaccount.com",
'   "client_id": "109122032928932715958",
'   "auth_uri": "https://accounts.google.com/o/oauth2/auth",
'   "token_uri": "https://oauth2.googleapis.com/token",
'   "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
'   "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/chilkatsvc%40chilkat25.iam.gserviceaccount.com",
'   "universe_domain": "googleapis.com"
' }


Dim gAuth As New Chilkat.AuthGoogle
gAuth.JsonKey = jsonKey

' Specify a scope.
gAuth.Scope = "https://mail.google.com/"

' Request an access token that is valid for this many seconds.
gAuth.ExpireNumSeconds = 3600

' When using a Google Workspace account with Gmail APIs, a service account can impersonate a user 
' via a process called domain-wide delegation — and the "sub" claim in the JWT is what enables this.
' Domain-wide delegation allows a Google Workspace administrator to authorize a service account to 
' act on behalf of any user in the domain, without user interaction.

' This is required for server-to-server access to user data — such as reading/sending Gmail from a background service.
' This is your company email address.
gAuth.SubEmailAddress = "info@chilkat.xyz"

' Connect to www.googleapis.com using TLS
Dim tlsSock As New Chilkat.Socket
Dim success As Boolean = tlsSock.Connect("www.googleapis.com",443,True,5000)
If (success <> True) Then
    Debug.WriteLine(tlsSock.LastErrorText)
    Exit Sub
End If


' Send the request to obtain the access token.
success = gAuth.ObtainAccessToken(tlsSock)
If (success <> True) Then
    Debug.WriteLine(gAuth.LastErrorText)
    Exit Sub
End If


' Examine the access token:
Dim accessToken As String = gAuth.AccessToken
Debug.WriteLine("Access Token: " & accessToken)

' Sample output:
' ya29.a0AW4XtxjGTD67Z8 .... IRw0218

' The access token allows us to send unlimited emails while it's valid. Once it expires, we must obtain and use a new one.


' -----------------------------------------------------------------------
Dim mailman As New Chilkat.MailMan

' Set the properties for the GMail SMTP server:
mailman.SmtpHost = "smtp.gmail.com"
mailman.SmtpPort = 587
mailman.StartTLS = True

mailman.SmtpUsername = "info@chilkat.xyz"
mailman.OAuth2AccessToken = accessToken

' Create a new email object
Dim email As New Chilkat.Email

email.Subject = "This is a test"
email.Body = "This is a test"
email.From = "Chilkat Test <info@chilkat.xyz>"
success = email.AddTo("Chilkat Software","info@chilkatsoft.com")
' To add more recipients, call AddTo, AddCC, or AddBcc once per recipient.

success = mailman.SendEmail(email)
If (success <> True) Then
    Debug.WriteLine(mailman.LastErrorText)
    Exit Sub
End If


success = mailman.CloseSmtpConnection()
If (success <> True) Then
    Debug.WriteLine("Connection to SMTP server not closed cleanly.")
End If


Debug.WriteLine("Successfully sent email using Gmail with a service account key.")