How do I search a string in all folder and sub-folder

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VB

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
    Dim path As String = FolderBrowserDialog1.SelectedPath
    Dim di As New DirectoryInfo(path)
    Dim yourSearchString As String
    yourSearchString = "my string"
    For Each Dir As String In Directory.GetDirectories(FolderBrowserDialog1.SelectedPath)
        ListBox1.Items.Clear()
        ListBox2.Items.Add(Dir)
    Next
    For Each fi As FileInfo In di.GetFiles()
        If File.ReadAllText(fi.FullName).Contains(yourSearchString) Then
            Console.WriteLine(fi.Name)
            ListBox1.Items.Add(fi.Name)
        End If
    Next
End Sub

What I have tried:

I want search a string in more file, and my code can search in one folder good, but i want this search in sub-folder in folder root.!
Who can help me, Please!!