How to write value to config file?

Hi,

I got problem Writing values to the config file (example: exeName.exe.config), currently i use this code:

'Set Config Key Value
    Public Function SetConfigValue(ByVal key As String) As Boolean
        Try
            If isFileExist(getConfigPath) Then
                Dim docConfig As New System.Xml.XmlDocument()
                Dim myStream As New IO.FileStream(getConfigPath, IO.FileMode.Open)
                docConfig.Load(myStream)
                Dim myNode As System.Xml.XmlNode = docConfig.SelectSingleNode("//appSettings/add[@key='" & key & "']")
                myNode.Attributes("value").Value = myNode.Attributes("value").Value & ";P"
                docConfig.Save(getConfigPath)
                'docConfig.Save("c:\test.txt")
                myStream.Close()
                Return True
            End If
        Catch e As Exception
            Console.WriteLine(e.ToString)
            Return False
        End Try
    End Function

'Get File Name
    Public Function getFileName(ByVal FileFullPath As String, Optional ByVal separator As String = "\") As String
        'EXAMPLE: input ="C:\winnt\system32\kernel.dll,
        'output = kernel.dll

        Dim intPos As Integer
        intPos = FileFullPath.LastIndexOfAny(separator)
        intPos += 1
        Return FileFullPath.Substring(intPos, (Len(FileFullPath) - intPos))
    End Function

'Get Current Exe Config File
    Public Function getConfigPath() As String
        Return getAppPath() & getEXEName() & ".config"
    End Function

    'Get Application Path
    Public Function getAppPath(Optional ByVal removeSeparator As Boolean = False)
        Dim tmp As String = System.Reflection.Assembly.GetExecutingAssembly.Location.ToString
        tmp = Replace(tmp, Dir(tmp), "", , , CompareMethod.Text)
        If removeSeparator And Right(tmp, 1) = "\" Then tmp = Left(tmp, Len(tmp) - 1)
        Return tmp
    End Function

'Get Current Exe Name
    Public Function getEXEName() As String
        Return getFileName(System.Reflection.Assembly.GetExecutingAssembly.CodeBase.ToString(), "/")
    End Function

the config file will be like this:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
   <appSettings>
         <add key="constring" value="data source=SERVER;initial catalog=db;user id=xx;password=xxx;persist security info=True;workstation id=SERVER;packet size=4096"/>         
   </appSettings>
</configuration>

so i try like:
MsgBox(SetConfigValue("constring"))

and i got this error:

System.IO.IOException: The process cannot access the file "H:\DotNet Projects\core\bin\core.exe.config" because it is being used by another process.
   at System.IO.__Error.WinIOError(Int32 errorCode, String str)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean useAsync, String msgPath, Boolean bFromProxy)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
   at System.Xml.XmlTextWriter..ctor(String filename, Encoding encoding)
   at System.Xml.XmlDOMTextWriter..ctor(String filename, Encoding encoding)
   at System.Xml.XmlDocument.Save(String filename)
   at core.modScripts.SetConfigValue(String key) in H:\DotNet Projects\core\modScripts.vb:line 64
'core.exe': Loaded 'c:\winnt\assembly\gac\system.data\1.0.3300.0__b77a5c561934e089\system.data.dll', No symbols loaded.
The program '[1984] core.exe' has exited with code 0 (0x0).


>>because it is being used by another process.

How can i overcome this? thks