Error BC30451 credential is not declared and folderId is not declared and Function in vb.net

Dear sir,

I have an error ` Error BC30451 'credential' is not declared and 'folderId' is not declared in vb.net`

this is method update file on Google Drive using Google Drive API v3 and 

Previously, the code was in C#, then I converted it to VB.NET, but in that code, there are missing variable declarations and function methods.

Please guide me

Thanks

Code in VB.NET

Public Function UploadFileToDrive(ByVal fileName As String, ByVal filePath As String, ByVal contentType As String) As String
        Dim fileMetadata = New Google.Apis.Drive.v3.Data.File()
        fileMetadata.Name = fileName
`Error folderid below line code
        fileMetadata.Parents = New List(Of String) From {folderId}
`Error Public function Getdriveservice as driveSERVICE and folderid below line code
        Dim service As DriveService = GetDriveService(credential)

        Dim query As String = "mimeType!='application/vnd.google-apps.folder' and trashed = false and name = '" & fileName & "'"
        Dim req As FilesResource.ListRequest
        req = service.Files.List()
        req.Q = query
        req.Fields = "files(id, name)"
        Dim result = req.Execute()
        If result.Files.Count = 1 Then
            Dim updatedFileMetadata As New Google.Apis.Drive.v3.Data.File()
            updatedFileMetadata.Name = fileName

            Dim updateRequest As FilesResource.UpdateMediaUpload
            Dim fileId As String = result.Files(0).Id
            Using stream = New FileStream(filePath, FileMode.OpenOrCreate)
                updateRequest = service.Files.Update(updatedFileMetadata, fileId, stream, contentType)
                updateRequest.Upload()
                Dim file = updateRequest.ResponseBody
                Return file.Id
            End Using
        Else
            Dim request As FilesResource.CreateMediaUpload
            Using stream = New FileStream(filePath, FileMode.Open)
                request = service.Files.Create(fileMetadata, stream, contentType)
                request.Fields = "id"
                request.Upload()
                Dim file = request.ResponseBody
                Return file.Id
            End Using
        End If
    End Function