Decode custom structures in C#

The code

using Opc.Ua.Client.ComplexTypes;

// build CLR types from the server's DataTypeDefinitions at runtime
var typeSystem = new ComplexTypeSystem(session);
await typeSystem.Load();

var nodeId = new NodeId(7001, (ushort)nsIndex);
DataValue dv = session.ReadValue(nodeId);

if (dv.Value is ExtensionObject eo && eo.Body is BaseComplexType complex)
{
    Console.WriteLine($"decoded as {complex.TypeId}");
    foreach (var prop in complex.GetPropertyEnumerator())
        Console.WriteLine($"  {prop.Name} = {complex[prop.Name]}");
}
else
{
    Console.WriteLine("still an opaque ExtensionObject - type system not loaded?");
}

What it prints

decoded as ns=4;i=3001
  Identifier = JOB-2026-0912
  State = 2
  PartsPlanned = 500
  PartsProduced = 431

Notes

• ComplexTypeSystem.Load() reads the server's DataTypeDefinition attributes and emits CLR types dynamically. Call it once after the session is up.
• If types still will not decode, the server may only publish a legacy binary type dictionary. That is a genuine gap between the two mechanisms, not a client defect.
• Unions and optional fields break first. Test those specifically.
• The reference stack can also generate types at build time from a NodeSet2 file, which is preferable when the model is stable and known.

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.