Passing structs as server method arguments

[MxBurger]

In this discussion someone asked how to pass structs as parameter on opcua-methods.
#882

Now a few years later as the library-api has evolved, I want to ask again what's the correct way to construct struct-types and pass them as method-parameters. I am also communicating with a S7-1500 plc, if that's somehow relevant.

[kevinherron]

Depends how much you know about the type ahead of time and whether you've code generated or hand coded types or not.

If it's totally dynamic then it's a bit of work:

    UaMethod method = ... // get UaMethod instance from UaObjectNode etc...

    NodeId dataTypeId = ... // get from method input arguments or browse
    DataType dataType = client.getDataTypeTree().getDataType(dataTypeId);
    
    DynamicStructType struct = DynamicStructType.newInstance(dataType);
    // have to add values for all members here
    struct.getMembers().put("Foo", 42);
    struct.getMembers().put("Bar", "Hello, World!");

    Variant[] inputs = new Variant[]{Variant.ofStruct(struct)};

    method.call(inputs);

Something along these lines...

[kevinherron]

if this blows up in the channels with an error about codecs or something, it might be necessary to eagerly encode the structure here instead:

    // note: DynamicEncodingContext because we're using DynamicStructType. 
    // Static is for code- or hand-generated structs.
    ExtensionObject xo = ExtensionObject.encode(client.getDynamicEncodingContext(), struct);
    Variant[] inputs = new Variant[]{Variant.ofExtensionObject(xo)};

[MxBurger]

Hello, it worked. Thank you.

[MxBurger]

Thank you for your fast response. It worked right ahead.

[jchirantan]

Hi @kevinherron, I just tried your solution, but I am getting following error during encoding:

org.eclipse.milo.opcua.stack.core.UaSerializationException: no codec registered for encodingId=ns=2;i=5054
org.eclipse.milo.opcua.stack.core.UaSerializationException: no codec registered for encodingId=ns=2;i=5054
at org.eclipse.milo.opcua.stack.core.encoding.binary.OpcUaDefaultBinaryEncoding.encode(OpcUaDefaultBinaryEncoding.java:63)
at org.eclipse.milo.opcua.stack.core.types.builtin.ExtensionObject.encodeBinary(ExtensionObject.java:243)
at org.eclipse.milo.opcua.stack.core.types.builtin.ExtensionObject.encode(ExtensionObject.java:211)

I tried without early encoding as well but still getting same error

I am using Eclipse Milo version 1.1.0

[kevinherron]

You'll have to share more information or a way to reproduce. no codec registered for encodingId=ns=2;i=5054 means the codec for that type isn't found for some reason, it doesn't really have anything to do with calling methods, it's more a general issue.

[jchirantan]

Hi @kevinherron,

I am using UACPP Server and in that, I am trying to call vectorAdd method (This method adds 2 vector arguments and outputs a vector).

This is the dictionary structure for vector

<opc:StructuredType BaseType="ua:ExtensionObject" Name="Vector">
<opc:Field TypeName="opc:Double" Name="X"/>
<opc:Field TypeName="opc:Double" Name="Y"/>
<opc:Field TypeName="opc:Double" Name="Z"/>
</opc:StructuredType>

Can you help with this?

[jchirantan]

Also, can we create a flow like UAExpert, where we will automatically read the datatype dictionary and we will get to know which all fields are required to be provided?
If not then why?

[kevinherron]

Create a new discussion for troubleshooting this issue.

Use UaExpert to connect to that server, then browse the DataTypes folder to locate the DataType Node in question and see if it has a non-null DataTypeDefinition attribute. Post screenshots in the new discussion of what you find.