Remove duplicate characters from strings and find the shortest - vb.net

Question

I can not figure out how to select from the result or the shortest line itself or its number
(Yes, the solution is needed in such ancient operators)
Imports System.IO
Public Class Form1
Sub readputh(ByRef s As String)
s = ""
OpenFileDialog1.Filter = "Textfiles (*.txt)|*.txt"
OpenFileDialog1.ShowDialog()
Do While s = ""
s = OpenFileDialog1.FileName
Loop
End Sub
Sub writeputh(ByRef s As String)
s = ""
SaveFileDialog1.Filter = "Textfiles (*.txt)|*.txt"
SaveFileDialog1.ShowDialog()
Do While s = ""
s = SaveFileDialog1.FileName
Loop
End Sub
Sub ch(ByVal Str As String, ByRef Res As String)
Dim a As Char
Res = Mid(Str, 1, 1)
For i = 2 To Len(Str)
a = CChar(Mid(Str, i, 1))
If InStr(Res, a) = 0 Then
Res = Res + a
End If
Next
End Sub
Sub resh(ByVal filename1 As String, ByVal filename2 As String, ByRef lb1 As ListBox, ByRef lb2 As ListBox)
Dim rf As StreamReader
Dim wf As StreamWriter
Dim s1, s2, s3 As String
s2 = ""
s3 = ""
Try
rf = New StreamReader(filename1)
wf = New StreamWriter(filename2, True)
Do While Not rf.EndOfStream()
s1 = rf.ReadLine()
lb1.Items.Add(s1)
ch(s1, s2)
wf.WriteLine(s2)
lb2.Items.Add(s2)
Loop
wf.Close()
rf.Close()
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim filename1, filename2 As String
readputh(filename1)
writeputh(filename2)
resh(filename1, filename2, ListBox1, ListBox2)
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
End
End Sub
End Class
Input file:
youtubeyoutubeyotube
dogdogdogdog
geeksforgeeks
Output file:
youtbe
dog
geksfor
But I expect Output file of only "dog"

Answer 1

I just couldn't handle the old code. One based functions? No! You can translate it back it you want but output is now dog.
Private Function GetOpenPath() As String
OpenFileDialog1.Filter = "Textfiles (*.txt)|*.txt"
If OpenFileDialog1.ShowDialog() = Windows.Forms.DialogResult.OK Then
Return OpenFileDialog1.FileName
Else
Return Nothing
End If
End Function
Private Function GetSavePath() As String
SaveFileDialog1.Filter = "Textfiles (*.txt)|*.txt"
If SaveFileDialog1.ShowDialog() = Windows.Forms.DialogResult.OK Then
Return SaveFileDialog1.FileName
Else
Return Nothing
End If
End Function
Private Function ch(ByVal Str As String) As String
Dim a As Char
Dim Res = Str.Substring(0, 1)
For i = 1 To Str.Length - 1
a = CChar(Str.Substring(i, 1))
If Res.IndexOf(a) = -1 Then
Res &= a
End If
Next
Return Res
End Function
Sub resh(ByVal filename1 As String, ByVal filename2 As String)
Dim lines = File.ReadAllLines(filename1)
Dim NewLines As New List(Of String)
For Each line In lines
Try
ListBox1.Items.Add(line)
Dim s2 = ch(line)
NewLines.Add(s2)
ListBox2.Items.Add(s2)
Catch ex As Exception
MsgBox(ex.Message)
End Try
Next
Dim Shortest = NewLines.OrderByDescending(Function(s) s.Length).Last()
File.WriteAllText(filename2, Shortest)
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim OpenPath = GetOpenPath()
Dim SavePath = GetSavePath()
If String.IsNullOrEmpty(OpenPath) OrElse String.IsNullOrEmpty(SavePath) Then
MessageBox.Show("You must provide a file. Try again.")
Return
End If
resh(OpenPath, SavePath)
End Sub