private static readonly object _xmlLock = new object();
private static XDocument _document = XDocument.Load("data.xml");

public static void UpdateXml(string elementName, string value)
{
    lock (_xmlLock)
    {
        var element = _document.Root?.Element(elementName);
        if (element != null)
            element.Value = value;
        else
            _document.Root?.Add(new XElement(elementName, value));
        
        _document.S*e("data.xml");
    }
}

public static string ReadXml(string elementName)
{
    lock (_xmlLock)
    {
        return _document.Root?.Element(elementName)?.Value;
    }
}

private static volatile XDocument _currentDoc = XDocument.Load("data.xml");

public static void UpdateXmlSafe(string elementName, string value)
{
    XDocument oldDoc, newDoc;
    do
    {
        oldDoc = _currentDoc;
        newDoc = new XDocument(oldDoc); // 克隆当前文档

        var element = newDoc.Root?.Element(elementName);
        if (element != null)
            element.Value = value;
        else
            newDoc.Root?.Add(new XElement(elementName, value));

    } while (Interlocked.CompareExchange(ref _currentDoc, newDoc, oldDoc) != oldDoc);

    // 可选：异步保存到文件
    Task.Run(() => newDoc.S*e("data.xml"));
}

public static string ReadXmlSafe(string elementName)
{
    var doc = _currentDoc; // 读取volatile引用
    return doc.Root?.Element(elementName)?.Value;
}

private static ReaderWriterLockSlim _rwLock = new ReaderWriterLockSlim();
private static XDocument _sharedDoc = XDocument.Load("data.xml");

public static string ReadWithRwLock(string elementName)
{
    _rwLock.EnterReadLock();
    try
    {
        return _sharedDoc.Root?.Element(elementName)?.Value;
    }
    finally
    {
        _rwLock.ExitReadLock();
    }
}

public static void WriteWithRwLock(string elementName, string value)
{
    _rwLock.EnterWriteLock();
    try
    {
        var element = _sharedDoc.Root?.Element(elementName);
        if (element != null)
            element.Value = value;
        else
            _sharedDoc.Root?.Add(new XElement(elementName, value));
        
        _sharedDoc.S*e("data.xml");
    }
    finally
    {
        _rwLock.ExitWriteLock();
    }
}