Call an OPC UA method in C#

The code

var objectId = new NodeId("Methods", (ushort)nsIndex);
var methodId = new NodeId("Methods/sqrt(x)", (ushort)nsIndex);

try
{
    IList<object> outputs = session.Call(objectId, methodId, 16.0);

    Console.WriteLine($"outputs: {outputs.Count}");
    foreach (var o in outputs) Console.WriteLine($"  {o}");
}
catch (ServiceResultException ex)
{
    Console.WriteLine($"call refused: {ex.StatusCode} - {ex.Message}");
}

What it prints

outputs: 1
  4

Notes

• session.Call is the convenience wrapper; the underlying CallRequest lets you batch several method calls in one round trip if you need to.
• A refused call raises ServiceResultException. Read ex.StatusCode rather than the message: the code is what tells you whether it was arguments, permissions or state.
• Argument types must match the method's declared InputArguments. Passing 16 rather than 16.0 sends an Int32 where a Double is expected.
• Check the Executable attribute if a method exists but will not run: some methods are only callable in certain machine states.

What breaks first

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.