Visual basic "Unhandled exception" - vb.net

Question

Hello I'm getting this error, while trying to save serial number to XML File.
If the file doesn't exist, it saves the file fine, but if i change Registered tag to False in Xml file, and try again, it says "The Process Cannot acces the file ... because it is being used by another process".
In my main form i read the information from XML, and in my regform (which i open if registered tag in xml is false) i write to the file. is it because of that?! I don't think so.
Here is my Registration class:
Imports System.IO
Imports System.Xml
Public Class RegistrationClass
Public Property SerialNumber As String
Public Property Registered As Boolean = False
Public Sub Write_Reg(ByVal FileString As String, ByVal RegisterName As String, ByVal RegisterCompany As String, ByVal RegisterSerialNumber As String)
Dim Registered As Boolean = False
Dim Comment As String = "StroySoft 2012 Register Database"
Dim SerialNumber As String = "dev-xxx-123"
Dim ClientOS As String = Trim(My.Computer.Info.OSFullName)
If RegisterSerialNumber = SerialNumber Then
Dim settings As New XmlWriterSettings()
settings.Indent = True
' Initialize the XmlWriter.
Dim XmlWrt As XmlWriter = XmlWriter.Create(FileString, settings)
With XmlWrt
' Write the Xml declaration.
.WriteStartDocument()
' Write a comment.
.WriteComment(Comment)
' Write the root element.
.WriteStartElement("Data")
' Start our first person.
.WriteStartElement("Register")
' The person nodes.
.WriteStartElement("Name")
.WriteString(RegisterName.ToString())
.WriteEndElement()
.WriteStartElement("Company")
.WriteString(RegisterCompany.ToString())
.WriteEndElement()
.WriteStartElement("SerialNumber")
.WriteString(RegisterSerialNumber.ToString())
.WriteEndElement()
Registered = True
.WriteStartElement("Registered")
.WriteString(Registered)
.WriteEndElement()
.WriteStartElement("ClientOS")
.WriteString(ClientOS)
.WriteEndElement()
' The end of this person.
.WriteEndElement()
' Close the XmlTextWriter.
.WriteEndDocument()
.Close()
End With
MsgBox("Успешна регистрация! Благодарим Ви!")
MainForm.РегистрацияToolStripMenuItem.Visible = False
Else
MsgBox("Невалиден сериен номер!")
End If
End Sub
Public Sub Check_Reg(ByVal FileString As String)
If (System.IO.File.Exists(FileString)) Then
Dim document As XmlReader = New XmlTextReader(RegForm.RegFile)
While (document.Read())
Dim type = document.NodeType
If (type = XmlNodeType.Element) Then
If (document.Name = "Registered") Then
If document.ReadInnerXml.ToString() = "True" Then
Registered = True
Else
Registered = False
End If
End If
If (document.Name = "SerialNumber") Then
SerialNumber = document.ReadInnerXml.ToString()
End If
End If
End While
Else
MessageBox.Show("The filename you selected was not found.")
End If
End Sub
End Class

Answer 1

is it because of that?! I don't think so.
It's exactly because of that.
You should always make sure to properly dispose IDisposable resources such as Streams and Writers/Readers by wrapping them in a Using block. In your case I don't see you closing your reader. But if you wrap it in a Using block you shouldn't worry about it. Even if an exception is thrown the resource will be properly released.
Example:
Using XmlWrt As XmlWriter = XmlWriter.Create(FileString, settings)
...
End Using
You should do the same with your XmlReader:
Using document As XmlReader = XmlReader.Create(RegForm.RegFile)
...
End Using