below is the code that works best for my use-case.

public static string[] hcoConnectors = new string[]
{
"Evertz 7800R2x2-ACS-3G",
"Snell Wilcox HCO51 RollCall",
"Snell Wilcox IQHCO3193-1A"
};

private void RunSafe(IEngine engine)
{
IDms thisDms = engine.GetDms();

//This gets all the services in the DMS
ICollection<IDmsService> allServices = thisDms.GetServices();

List<IDmsElement> allStandaloneElementsForHCOs = new List<IDmsElement>();

// Collect all HCO elements
foreach (string connector in hcoConnectors)
{
Element[] elementsForConnector = engine.FindElementsByProtocol(connector);
foreach (Element element in elementsForConnector)
{
IDmsElement myElement = thisDms.GetElement(element.ElementName);
if (myElement != null)
{
allStandaloneElementsForHCOs.Add(myElement);
}
}
}

// Collect all element IDs associated with services
HashSet<(int DataMinerId, int ElementId)> serviceElementIds = new HashSet<(int, int)>();
foreach (IDmsService service in allServices)
{
ServiceParamSettings[] parameters = service.ParameterSettings.IncludedParameters;
foreach (ServiceParamSettings parameter in parameters)
{
int dataMinerId = parameter.DataMinerID;
int elementId = parameter.ElementID;
serviceElementIds.Add((dataMinerId, elementId));
}
}

// Build email content for unused HCO elements
StringBuilder emailBody = new StringBuilder();
emailBody.AppendLine("Overview of Unused HCO Elements:");
emailBody.AppendLine("——————————–");
bool hasUnusedElements = false;

foreach (IDmsElement hcoElement in allStandaloneElementsForHCOs)
{
if (!serviceElementIds.Contains((hcoElement.AgentId, hcoElement.Id)))
{
hasUnusedElements = true;
emailBody.AppendLine($"Element: {hcoElement.Name} (DataMinerId: {hcoElement.AgentId}, ElementId: {hcoElement.Id})");
}
}

// Send email if there are unused elements
if (hasUnusedElements)
{
string subject = "Unused HCO Elements Report – " + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss");
string body = emailBody.ToString();
string to = "jeroen.geldhof@skyline.be";

engine.SendEmail(body, subject, to);
engine.GenerateInformation("Email sent with overview of unused HCO elements.");
}
else
{
engine.GenerateInformation("No unused HCO elements found.");
}
}