Issue when writing multiple selected datagridview row to CSV

Im having a problem where, if I select multiple DataGridView rows, all the information is placed into one line of the .csv file, this is giving me issues when trying to load the file back up.

Instead of having to select and saving each row individually (They are selected so they write to another file), how can I change the code below, so that for each selected row, each row is written onto another line.

What I have tried:

Private Sub btnToSession_Click(sender As Object, e As EventArgs) Handles btnToSession.Click
    'PURPOSE: Write only selected DataGridView row to Session.csv
    Dim StrExport As String = ""
    For Each C As DataGridViewColumn In dataGVGeneral.Columns
        StrExport &= "" & C.HeaderText & ","
    Next
    StrExport = StrExport.Substring(0, StrExport.Length - 1)
    StrExport &= Environment.NewLine
    'Only the row which is selected will be writen
    For Each R As DataGridViewRow In dataGVGeneral.Rows
        If R.Selected = True Then
            For Each C As DataGridViewCell In R.Cells
                If Not C.Value Is Nothing Then
                    StrExport &= "" & C.Value.ToString & ","
                Else
                    StrExport &= "" & "" & ","
                End If
            Next
        Else
            StrExport = StrExport.Substring(0, StrExport.Length - 1)
            StrExport &= Environment.NewLine
        End If
    Next
    'ROLE: Given ({filePath}, False)- allows for the file to be completely overwritten with new data.
    Dim tw As System.IO.TextWriter = New System.IO.StreamWriter("E:\SAT\Work.io\Work.io\bin\Debug\Session.csv", False)
    tw.Write(StrExport)
    tw.Close()
End Sub

Tags: Visual Basic, DataGridView, CSV