How not to overwrite files in VB.net - vb.net

Question

I want to check if the current file exists at the same time save video to different file but it only overwrites to an existing file.
Here is my code:
Private Sub ButtonVIDEO_Click(ByVal sender As System.Object, e As System.EventArgs) Handles ButtonVIDEO.Click
f = New Filters
cap = New Capture(f.VideoInputDevices(0), f.AudioInputDevices(0))
cap.PreviewWindow = PictureBox1
Dim Filename As String = "c:\\folder\MyFile"
Dim i As Integer = 0
Dim extension As String = ".mp4"
If ButtonVIDEO.BackColor = Color.Black Then
cap.Filename = Filename + extension
If File.Exists(Filename) Then
Do
i = i + 1
Loop While File.Exists(Filename + i.ToString() + extension)
Filename = Filename + i.ToString()
End If
cap.Cue()
cap.Start()
ButtonVIDEO.BackColor = Color.Red
ButtonVIDEO.Text = "PLAYING"
ButtonPHOTO.Hide()
End If
End Sub

Answer 1

You are assigning the file name to cap before searching for an unsused file name.
Also, there is an error in your logic, as you are creating the full file name with Filename + i.ToString() + extension but you also append i to Filename itself with Filename = Filename + i.ToString().
So, in then end, you are appending the number twice.
Private Sub ButtonVIDEO_Click(ByVal sender As System.Object, e As System.EventArgs) _
Handles ButtonVIDEO.Click
Dim Filename As String = "c:\folder\MyFile"
Dim i As Integer = 0
Dim extension As String = ".mp4"
Dim path As String = Filename & extension
f = New Filters
cap = New Capture(f.VideoInputDevices(0), f.AudioInputDevices(0))
cap.PreviewWindow = PictureBox1
If ButtonVIDEO.BackColor = Color.Black Then
If File.Exists(path) Then
Do
i = i + 1
path = Filename & i & extension
Loop While File.Exists(path)
End If
cap.Filename = path
cap.Cue()
cap.Start()
ButtonVIDEO.BackColor = Color.Red
ButtonVIDEO.Text = "PLAYING"
ButtonPHOTO.Hide()
End If
End Sub
The path should have only one backslash in c:\folder. In VB strings are concatenated with &, not +.
See answer to The difference between + and & for joining strings in VB.NET