Why does my file get locked?!

[MrPolite]

I am simply reading from a textfile, but it gets locked (cant write on it), until I close my program:

Dim sr As StreamReader = File.OpenText(strFullPath)
            Dim strLine As String
 
            Do
                strLine = sr.ReadLine()
                If strLine = Nothing Then
                    Exit Do
                End If
 
                ' Some other code here ... :P
            Loop
            sr.Close()

I'm closing the streamReader, but still the file is locked. I tried to do grabage collecting too, but that doesn't work.
Someone help plz