docs: document three approaches for decoding ComplexTypes from OPC UA servers

[Copilot]

Users were unclear on how to decode ExtensionObject values into their own custom .NET types when reading complex types from an OPC UA server. The missing piece: custom IEncodeable implementations must be registered with the session's EncodeableFactory before reading — the ComplexTypeSystem alone is not sufficient.

Changes to Docs/ComplexTypes.md

• New section: "Approaches for Working with Custom Types" — documents all three supported strategies side-by-side:

  • Approach 1 — Hand-written IEncodeable + EncodeableFactory registration: complete example including the critical registration step, assembly-level bulk registration, multi-type chaining, and a callout that BinaryEncodingId must exactly match the server's Binary Encoding NodeId
  • Approach 2 — Source-generated IEncodeable (new in 1.6): brief example + cross-reference to SourceGeneratedDataTypes.md
  • Approach 3 — Runtime IStructure via ComplexTypeSystem: already documented; now surfaced as an explicit choice with links to the existing sections

• Updated troubleshooting ("Values Still Encoded as ExtensionObject"): covers the registration requirement for all three approaches, not just ComplexTypeSystem.LoadAsync()

• "See Also" link to SourceGeneratedDataTypes.md added

Key usage pattern (Approach 1)

// Register before any reads — BinaryEncodingId on the type must match the server
session.Factory.Builder
    .AddEncodeableType(typeof(UDT_SemVer))
    .Commit();

DataValue result = await session.ReadValueAsync("ns=3;s=...");
if (result.WrappedValue.TryGet(out ExtensionObject ext) &&
    ext.Body is UDT_SemVer semVer)
{
    Console.WriteLine($"Major={semVer.Major} Minor={semVer.Minor} Patch={semVer.Patch}");
}

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

No code changes — documentation only. The three-approach structure maps directly to the options described in the issue thread and makes the EncodeableFactory registration requirement explicit, which was the root cause of the confusion.

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ marcschier
❌ Copilot
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.24%. Comparing base (64ca85f) to head (2d94ab9).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3664      +/-   ##
==========================================
+ Coverage   73.13%   73.24%   +0.11%     
==========================================
  Files         516      516              
  Lines      110789   110789              
  Branches    18172    18172              
==========================================
+ Hits        81023    81149     +126     
+ Misses      24515    24392     -123     
+ Partials     5251     5248       -3     

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