[aljzazy]

كود :

Public Class Form1
   
   Private Sub Button1_Click_1(sender As System.Object, e As System.EventArgs) Handles Button1.Click
       Dim fileName As String = IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "MyFile.txt")

       Dim data As New List(Of String)

       If IO.File.Exists(fileName) Then
           data = IO.File.ReadAllLines(fileName).ToList
           data.AddRange(TextBox1.Lines)
       Else
           data = TextBox1.Lines.ToList
       End If
       IO.File.WriteAllLines(fileName, data.ToArray)
   End Sub
End Class