Add sibling after a specific XML node C#

Question

My xml document looks like this:

<root>
<parent1>
    <element1>
    </element1>
</parent1>
<parent1>
    <element1>
    </element1>
</parent1>
<parent1>
    <element1>
    </element1>
</parent1>
<parent2>
    <element1>
    </element1>
</parent2>
<parent2>
    <element1>
    </element1>
</parent2>
<parent3>
    <element2>
    </element2>
</parent3>
</root>

I would like to add a new parent1 node after the last parent1 node before the first parent2 node.

Like this:

<root>
    <parent1>
        <element1>
        </element1>
    </parent1>
    <parent1>
        <element1>
        </element1>
    </parent1>
    <parent1>
        <element1>
        </element1>
    </parent1>
    <parent1>
        <element1>
        </element1>
    </parent1>
    <parent2>
        <element1>
        </element1>
    </parent2>
    <parent2>
        <element1>
        </element1>
    </parent2>
    <parent3>
        <element2>
        </element2>
    </parent3>
</root>

I tried with the XmlDocument SelectSingleNode and AppendChild methods but it only adds the new child after the last parent3 node.

EDIT:

I tried with this code:

string newNode = "<parent1/><element1>1</element1></parent1/>";

XmlDocumentFragment xmlDocFrag = xmlDocument.CreateDocumentFragment();
xmlDocFrag.InnerXml = newNode;


XmlElement element = (XmlElement)xmlDocument.SelectSingleNode(@"//root/parent1[last()]");


element.InsertAfter(xmlDocFrag, element); //--> this throws an error
element.AppendChild(xmlDocFrag); //--> this works but it inserts the new node inside the last parent1 node

Answer 1

you were pretty close, but the method you needed is InsertAfter which has to be used on parent. Take a look at this example (comments are in code)

string xml = @"
    <root>
    <parent1>
        <element1>
        </element1>
    </parent1>
    <parent1>
        <element1>
        </element1>
    </parent1>
    <parent1>
        <element1>
        </element1>
    </parent1>
    <parent2>
        <element1>
        </element1>
    </parent2>
    <parent2>
        <element1>
        </element1>
    </parent2>
    <parent3>
        <element2>
        </element2>
    </parent3>
    </root>
    ";

XmlDocument xmlDocument = new XmlDocument();
xmlDocument.LoadXml(xml);

//you had invalid xml here. <parent1/> instead of <parent1> and </parent1/> instead of </parent1>
string newNode = "<parent1><element1>1</element1></parent1>";

XmlDocumentFragment xmlDocFrag = xmlDocument.CreateDocumentFragment();
xmlDocFrag.InnerXml = newNode;

XmlElement element = (XmlElement)xmlDocument.SelectSingleNode(@"//root/parent1[last()]");

//once you found last parent1 element, find it's parent
XmlNode parent = element.ParentNode;
//now, use that parent element and it's InsertAfter method to add new node as sibling to your found element
parent.InsertAfter(xmlDocFrag, element);