问题:

        // Remove element with ID of 1
        var userIds = from user in document.Descendants("Id")
                       where user.Value == "1"
                       select user;

        userIds.Remove();

        SaveAndDisplay(document);

        // Add element back
        var newElement = new XElement("Id", "0", 
            new XElement("Balance", "3000"));
        document.Add(newElement);

        SaveAndDisplay(document);

The add element back block is the problem. As when it gets to the add it states:

This operation would create an incorrectly structured document.

What stupid mistake am I making?

Edit:

Yes, I was reading as an XDocument, not XElement. Any advice on when to favour one or the other?

回答1:

It looks like you are trying to add a new element as a child of your document's root. If so, then you need to change your Add statement to the following.

var newElement = new XElement("Id", "0", new XElement("Balanace", "3000"));
document.Root.Add(newElement);

Adding directly to the document adds another root element, which violates the XML structure.

回答2:

You're effectively trying to add a new root element, which these objects don't like. I assume document is an XDocument? Place it further inside the root node, by adding it to the root node. Use:

document.Root.Add(newElement) or document.FirstNode.Add(newElement)