如何修复'文件正在被另一个进程使用'错误

我创建了一个程序，可以修改。exe文件的内容。通过datagridview配置文件(app Config)，并在datagridview上显示键值对。问题是，我有一个保存设置，它用用户键入的新值替换旧值。它保存了，下次我打开它时，文件将被覆盖，但是当我试图加载配置文件以更新用户设置时，我得到一个"文件被另一个进程使用"的错误。

这是代码:

    private XmlDocument m_XmlDoc;
    private FileStream fIn;
    private StreamReader sr;
    private StreamWriter sw;
    private OrderedDictionary m_Settings;
    private void ProgramConfig_Load(object sender, EventArgs e)
    {
        try
        {
            loadconfigfile(GatewayConfiguration.Properties.Settings.Default.Config);
            BindingList<KeyValueType> list = new BindingList<KeyValueType>();
            for (index = 0; index < m_Settings.Count; index++)
            {
                list.Add(new KeyValueType(keys[index], values[index].ToString()));
            }
            var source = new BindingSource();
            source.DataSource = list;
            dataGridView1.DataSource = source;
        }
        catch (Exception ex)
        {
            textBox1.Text = ex.Message;
        }
    }
    public void loadconfigfile(string configfile)
    {
        if (File.Exists(configfile))
        {
            m_XmlDoc = new XmlDocument();
            GatewayConfiguration.Properties.Settings.Default.Config = configfile;
            GatewayConfiguration.Properties.Settings.Default.Save();
            // Error Occurs here at the fIn, telling me that the file is currently in use and cannot be accessed.
            fIn = new FileStream(configfile, FileMode.Open, FileAccess.ReadWrite);
            sr = new StreamReader(fIn);
            sw = new StreamWriter(fIn);
            try
            {
                m_XmlDoc.LoadXml(sr.ReadToEnd());
                loadAppSettings();
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        else
        {
            throw new FileNotFoundException(configfile + " does not exist.");
        }
    }
    private void loadAppSettings()
    {
        m_Settings = new OrderedDictionary();
        XmlNodeList nl = m_XmlDoc.GetElementsByTagName("setting");
        foreach (XmlNode node in nl)
        {
            try
            {
                m_Settings.Add(node.Attributes["name"].Value, node.ChildNodes[0].InnerText);
            }
            catch (Exception)
            {
            }
        }
    }
    private void SaveAppSettings_Click(object sender, EventArgs e)
    {
        // saves
        MessageBoxButtons buttons = MessageBoxButtons.YesNo;
        DialogResult result = MessageBox.Show("Overwrite the old values with the new values?", "Save Settings?", buttons);
        if (result == DialogResult.No)
        {
            return;
        }
        int index = 0;
        string[] keys = new string[m_Settings.Keys.Count];
        m_Settings.Keys.CopyTo(keys, 0);
        for (index = 0; index < dataGridView1.Rows.Count; index++)
        {
            if ((string)dataGridView1[2, index].Value != string.Empty)
            {
                setAppSetting(keys[index], (string)dataGridView1[2, index].Value);
            }
        }
        // Updates datagrid by loading configfile again
        loadconfigfile(GatewayConfiguration.Properties.Settings.Default.Config);
        textBox1.Text = "Settings Saved. You may now exit.";
        m_savecounter++;
        dataGridView1.Update(); 
        dataGridView1.Refresh();
    }

在SaveAppSettings下的loadconfigfile函数中出现错误。它告诉我它无法访问该文件，因为该文件正在被另一个进程使用。在我再次打开文件并将其显示给用户之前，我需要做些什么吗?

许多谢谢,

Tf.rz

using(fIn = new FileStream(configfile, FileMode.Open, FileAccess.ReadWrite))
{
   // working with file stream
}

fIn.Close();
sr.Close();
sw.Close();