I want to get full message body. So I try:
Message gmailMessage = service.users().messages().get(\"me\", messageId).setFormat(\"full\").execute();
here is the solution in c# code gmail API v1 to read the email body content:
var request = _gmailService.Users.Messages.Get("me", mail.Id);
request.Format = UsersResource.MessagesResource.GetRequest.FormatEnum.Full;
and to solve the data error
var res = message.Payload.Body.Data.Replace("-", "+").Replace("_", "/");
byte[] bodyBytes = Convert.FromBase64String(res);
string val = Encoding.UTF8.GetString(bodyBytes);