How to delete specific nodes from an XElement?

You can try this approach:

var nodes = xRelation.Elements().Where(x => x.Element("Conditions") != null).ToList();

foreach(var node in nodes)
    node.Remove();

Basic idea: you can't delete elements of collection you're currently iterating.
So first you have to create list of nodes to delete and then delete these nodes.

研究发现，只有Elements()这种筛选的 IEnumerable<XElement>才有问题，没法进行Remove。

但是如果是Elements("add")或Elements("remove")，可以不用ToList()就正常删除的

另外还发现，如果有2个相同的节点，通过Elements("add")操作where过滤出来之后，只能删除一个

只有用了ToList()，然后再遍历，才可以正常删除