(VB.NET) Refresh GMail SMTP OAuth2 Access Token

Demonstrates how to refresh an access token for sending email via GMail (SMTP).

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

' See Get GMail SMTP Access Token 
' for sample code showing how to obtain a GMail SMTP access token for desktop apps.

' It is assumed we previously obtained an OAuth2 access token for GMail (one with a scope for sending email).

Dim jsonToken As New Chilkat.JsonObject
Dim success As Boolean = jsonToken.LoadFile("qa_data/tokens/_gmailSmtp.json")
If (success <> True) Then
    Debug.WriteLine("Failed to load _gmailSmtp.json")
    Exit Sub
End If


' The access token JSON looks something like this:

' 	{ 
' 	 "access_token": "abac.Ci-ZabcdF37-NUrmSzxbHxTsMbtlN-uPfabcdAQN_2ekzj0s5ofCzRabcdTDMGaa2Q",
' 	 "token_type": "Bearer",
' 	 "expires_in": 3600,
' 	 "refresh_token": "1/lEe5abcdNJ77eOSDBabcdkQGVmDalkwabcdR2Hatbo"
' 	}
' 

Dim oauth2 As New Chilkat.OAuth2

oauth2.TokenEndpoint = "https://www.googleapis.com/oauth2/v4/token"

' Replace these with actual values.
oauth2.ClientId = "GOOGLE-CLIENT-ID"
oauth2.ClientSecret = "GOOGLE-CLIENT-SECRET"

' Get the "refresh_token"
oauth2.RefreshToken = jsonToken.StringOf("refresh_token")

' Send the HTTP POST to refresh the access token..
success = oauth2.RefreshAccessToken()
If (success <> True) Then
    Debug.WriteLine(oauth2.LastErrorText)
    Exit Sub
End If



' The response contains a new access token, but we must keep
' our existing refresh token for when we need to refresh again in the future.
jsonToken.UpdateString("access_token",oauth2.AccessToken)

' Save the new JSON access token response to a file.
Dim sbJson As New Chilkat.StringBuilder
jsonToken.EmitCompact = False
jsonToken.EmitSb(sbJson)
sbJson.WriteFile("qa_data/tokens/_gmailSmtp.json","utf-8",False)

Debug.WriteLine("OAuth2 authorization granted!")
Debug.WriteLine("New Access Token = " & oauth2.AccessToken)