Write a value to an OPC UA server in C#

The code

// umati is read-only. Run the reference server locally instead:
//   dotnet run --project Applications/ConsoleReferenceServer

var nodeId = new NodeId("MyVariable", (ushort)nsIndex);

// read the declared DataType first - do not guess
var dtValue = session.ReadValue(nodeId, Attributes.DataType);
Console.WriteLine($"declared DataType: {dtValue.Value}");

var toWrite = new WriteValueCollection
{
    new WriteValue
    {
        NodeId = nodeId,
        AttributeId = Attributes.Value,
        Value = new DataValue(new Variant((double)42.5)),
    },
};

session.Write(null, toWrite, out StatusCodeCollection results, out _);

foreach (var sc in results)
    Console.WriteLine(StatusCode.IsGood(sc)
        ? "write accepted"
        : $"write rejected: {sc}");

// always read back in a test - some servers accept and discard
Console.WriteLine($"read back: {session.ReadValue(nodeId).Value}");

What it prints

declared DataType: i=11
write accepted
read back: 42.5

Notes

• Write returns a StatusCodeCollection with one entry per operation. Check every entry, a batch can partially succeed.
• The Variant type must match the node's DataType. Casting to double explicitly, as above, avoids C# inferring int.
• Read AccessLevel before writing rather than diagnosing the refusal afterwards.
• Read back in tests. A Good write status does not prove the server kept the value.

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.