feat: add support for struct simvar marshalling

[bstudtma]

Summary

Added support to GetAsync to retrieve an entire structure at once.

Example:

var snapshot = await client.SimVars.GetAsync<MyApp.AircraftSnapshot>();
Console.WriteLine($"Altitude: {snapshot.AltitudeFeet:F0} ft");
Console.WriteLine($"Airspeed: {snapshot.IndicatedAirspeedKnots:F0} kts");
Console.WriteLine($"Title: {snapshot.Title}");

This will populate a struct like:

[StructLayout(LayoutKind.Sequential)]
public struct AircraftSnapshot
{
    /// <summary>
    /// Gets or sets the pressure altitude of the aircraft in feet.
    /// </summary>
    [SimConnect("PLANE ALTITUDE", "feet", SimConnectDataType.Integer32)]
    public int AltitudeFeet;

    /// <summary>
    /// Gets or sets the indicated airspeed in knots.
    /// </summary>
    [SimConnect("AIRSPEED INDICATED", "knots", SimConnectDataType.FloatDouble)]
    public double IndicatedAirspeedKnots;

    /// <summary>
    /// Gets or sets the aircraft title (name).
    /// </summary>
    [SimConnect("TITLE", null, SimConnectDataType.String260, order: 0)]
    public string Title;
}

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Changes Made

• Added support to GetAsync to return a struct.
• Added a corresponding test case.

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Additional Information

• May need some polish but seems to be working.

• order attribute is not required. If set, attributes with an order are sorted first; otherwise, ordering falls back to struct field order.

• Tried to make attributes optional where possible.

  • Best practice is to set name, unit, and SimConnectDataType explicitly.
  • If any are null, the code attempts to look up the info from the registry.
  • Name is required.

• Overloads the existing GetAsync method.

• [StructLayout(LayoutKind.Sequential)] may not be necessary based on late testing.

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Author Information

Discord Username: bstudtma

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Checklist

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

[Copilot]

Pull Request Overview

This PR adds support for struct-based SimVar marshalling, allowing developers to retrieve multiple SimVars in a single request by defining structs annotated with SimConnect attributes.

• Added struct marshalling capability through a new GetAsync<T>() overload that accepts struct types
• Introduced SimConnectAttribute for annotating struct fields with SimVar metadata
• Created infrastructure classes for struct binding, field ordering, and data type sizing

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
SimVarManager.cs	Added new GetAsync<T>() overload for struct marshalling with reflection-based field binding and data parsing
SimConnectAttribute.cs	New attribute class for annotating struct fields with SimVar name, unit, and data type information
SimVarStructBinder.cs	Internal utility for validating struct layouts and building SimConnect data definitions from annotated fields
SimVarDataTypeSizing.cs	Utility class providing size calculations and offset computations for SimConnect data types
SimVarTests.cs	Added test case demonstrating struct marshalling with a Position struct containing latitude, longitude, and altitude