Improve SimConnect struct writing and allow empty unit with SetAsync

[bstudtma]

Summary

This pull request introduces several minor improvements and fixes to the SimConnect.NET library, focusing on more robust handling of SimConnect data definitions and struct writing, as well as improved string encoding for SimConnect interoperability. The changes primarily address consistency, caching, and encoding issues, making struct operations safer and more efficient. This mainly cleans up a few issues detected by GitHub Copilot during the previous commit.

Changes Made

SimConnect struct writing and definition handling:

• Added a cache (defToWriter) in SimVarManager to store delegates and layout sizes for struct writers keyed by definition ID, ensuring efficient and consistent struct writing.
• Refactored SetStructAsync to use the cached writer delegate and layout size from defToWriter, improving reliability and preventing redundant layout computation. Throws a clear error if the writer is missing.

String encoding improvements:

• Updated string field writing logic in SimVarFieldWriter to encode strings as fixed-size, null-terminated ANSI (Latin1) strings, reserving the last byte for the terminator and avoiding unnecessary allocations. This aligns with SimConnect's expectations for string fields.

Consistency and error handling:

• Standardized error messages to refer to [SimConnect] attributes instead of [SimVar] in factories, clarifying attribute usage in exception messages.
• Improved parameter validation in SetAsync by switching to ArgumentNullException for the unit parameter, ensuring correct exception semantics. For example, this allows setting SimVar "Initial Position" which doesn't require a unit type. SimConnect uses a default unit if one isn't provided.

SimConnect data type inference:

• Added support for inferring SimConnectDataType.InitPosition from the SimConnectDataInitPosition type, expanding the range of supported struct types.

Author Information

Discord Username: bstudtma

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Checklist:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

[Copilot]

Pull Request Overview

This PR improves SimConnect struct writing reliability and flexibility by introducing caching for struct writers, fixing string encoding to match SimConnect's ANSI expectations, and allowing empty units in SetAsync. The changes enhance performance through cached delegates, improve correctness of string marshaling, and fix consistency issues in error messages.

Key Changes:

• Introduced defToWriter cache in SimVarManager to store struct writer delegates and layout sizes, eliminating redundant layout computations in SetStructAsync
• Fixed string field encoding to use Latin1 with explicit null-termination and avoid unnecessary allocations
• Relaxed unit parameter validation in SetAsync to allow null values for SimVars that don't require units

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/SimConnect.NET/SimVar/SimVarManager.cs	Added writer caching mechanism, changed unit validation to allow null, added InitPosition type inference
src/SimConnect.NET/SimVar/Internal/SimVarFieldWriterFactory.cs	Updated error message to reference correct attribute name
src/SimConnect.NET/SimVar/Internal/SimVarFieldWriter.cs	Rewrote string encoding to use Latin1 with explicit null-termination and eliminate array allocation
src/SimConnect.NET/SimVar/Internal/SimVarFieldReaderFactory.cs	Updated error messages to reference correct attribute name