I want to find the element of multiple Nodes in a XML-File by XPath. The Path to the Nodes is:

(/chapter/block/procedure/step/action/table/tgroup/tbody/row/entry/p/formfield)

I want to get the Element field\_id from the Childnode formfield. There are multiple formfields in the XML-File. All have the same path but are located in different rows (/row). 

I've tried:

```
XmlDocument doc = new XmlDocument();
doc.Load("xmlfile.xml");
XmlNode node = doc.DocumentElement.SelectSingleNode("/chapter/block/procedure/step/action/table/tgroup/tbody/row/entry/p/formfield");
string attr = node.Attributes["field_id"]?.InnerText;
Console.WriteLine(attr);

```

This gives me only the field\_id from the first formfield. I've tried multiple other ways to get every ID but I'm always getting a System.NullReferenceException. How can I get every field\_id?