I can retrieve the contents of a relatedlinks field by creating an XmlDocument from the returned value as below:

XmlDocument links = new XmlDocument();
links.LoadXml(part.GetProperty("products").Value);

and then traversing the XmlDocument. However, I cannot use C# and the API to create the field in the first place.I am using code that looks like:

// Create XmlDocument to hold products
doc = new XmlDocument();

// Insert declaration as first node
XmlDeclaration declaration = doc.CreateXmlDeclaration("1.0", null, "yes");
doc.AppendChild(declaration);

// Create the root node
rootElement = doc.CreateElement("links");
doc.AppendChild(rootElement);

// Look up the Product Id
int productId = productsXref[thisProduct];
                    
// Add Product link to current Part
linkElement = doc.CreateElement("link");
rootElement.AppendChild(linkElement);

// Link Type is internal
linkAttribute = doc.CreateAttribute("type");
linkAttribute.Value = "internal";
linkElement.SetAttributeNode(linkAttribute);

// Link Title
linkAttribute = doc.CreateAttribute("title");
linkAttribute.Value = thisProduct;
linkElement.SetAttributeNode(linkAttribute);

// Link Link
linkAttribute = doc.CreateAttribute("link");
linkAttribute.Value = productId.ToString();
linkElement.SetAttributeNode(linkAttribute);

// Link NewWindow
linkAttribute = doc.CreateAttribute("newwindow");
linkAttribute.Value = "1";
linkElement.SetAttributeNode(linkAttribute);

DocumentBeingWorkedOn.getProperty("relatedLink").Value = doc;

The last statement above consistently fails with nul object exception.

Can anyone help? I'm new to Umbraco.