.NET (Core and Framework)

private void XMLBugSave() 
{ 
    // Everything works fine if you use a file other than ..\..\app.config... 

    // const string strFile = @"d:\temp\testbug.xml"; 
    const string strFile = @"..\..\app.config"; 

    XmlTextReader xr = new XmlTextReader(strFile); 
    xdoc = new XmlDocument(); 
    xdoc.Load(xr); 
    xr.Close(); 

    Random rnd = new Random(); 
    string strNodeName = "NODE_" + rnd.Next().ToString(); 
    Console.WriteLine("Attempting to add Element " + strNodeName); 

    xdoc.DocumentElement.AppendChild(xdoc.CreateNode(XmlNodeType.Element, strNodeName, null)); 

    OpenFileDialog ofd = new OpenFileDialog(); 
    // If you click "Cancel" in the following Dialog, then everything with the XML file works as 
    // expected. If you click "Open" then everything will seem to work at runtime - even 
    // reopening the file and looking at the result "verifies" that the save worked but in 
    // reality, nothing is written to the file. Very odd. 
    ofd.ShowDialog(); 

    StreamWriter sw = new StreamWriter(strFile); 
    sw.AutoFlush = true; 
    xdoc.Save(sw); 
    sw.Flush(); 
    sw.Close(); 

    // If you go back to the IDE after pressing open with app.config already opened in 
    // an IDE window, then nothing appears to have changed. You can change app.config in the 
    // IDE and save it out and your saves will persist. 
    MessageBox.Show("Verify file..."); 

    xr = new XmlTextReader(strFile); 
    xdoc = new XmlDocument(); 
    xdoc.Load(xr); 
    xr.Close(); 

    XmlNode xn = xdoc.SelectSingleNode(@"/configuration/" + strNodeName); 

    // This always seems to verify whether or not the file was written. 
    Console.WriteLine(xn == null ? "unverified" : "verified"); 
}