net48 port (test branch): retarget all projects to .NET Framework 4.8

Retargets RioJoy.Core/Overlay/Tray + tests from net8.0-windows to net48 so
the app can be tested as a framework-dependent build (relies on the in-box
.NET Framework 4.8 on Windows 10/11). Builds clean; all 241 tests pass.

Polyfills (no behavior change):
- PolySharp source generator for init/records/Index/Range/required members.
- System.Memory, System.Text.Json, Microsoft.Bcl.HashCode,
  System.Threading.Channels (tests) NuGet packages.
- Compat/Net48Polyfills.cs: GetValueOrDefault, KeyValuePair.Deconstruct,
  Math.Clamp; tests/TestPolyfills.cs: Task.WaitAsync.

Source adjustments for APIs absent on net48:
- ArgumentNullException/ArgumentException.ThrowIf* inlined to manual guards.
- Convert.ToHexString, Encoding.Latin1, Environment.ProcessPath,
  ApplicationConfiguration.Initialize, Enum.GetNames<T>/GetValues<T>,
  string.StartsWith(char), string.Split(char, opts), TextBox.PlaceholderText,
  PeriodicTimer, Memory-based Stream Read/WriteAsync, array range-slicing.
- Dropped [SupportedOSPlatform] hints (net48 is single-platform).

deploy/build-package.ps1: publish framework-dependent (no self-contained).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/build-package.ps1

@@ -47,12 +47,12 @@ try {
     $pkgDir = Join-Path $staging 'RIOJoy'
     New-Item -ItemType Directory -Force -Path $pkgDir | Out-Null
 
-    # 1. Publish the tray app into riojoy\app (self-contained win-x64 — no .NET on target).
+    # 1. Publish the tray app into riojoy\app (net48, framework-dependent — relies on
+    #    the in-box .NET Framework 4.8 present on every Windows 10/11 machine).
     $appOut = Join-Path $pkgDir 'app'
-    Write-Host "Publishing RioJoy.Tray ($Configuration, self-contained win-x64)..."
+    Write-Host "Publishing RioJoy.Tray ($Configuration, net48 framework-dependent)..."
     & dotnet publish (Join-Path $repo 'src\RioJoy.Tray\RioJoy.Tray.csproj') `
-        -c $Configuration -r win-x64 --self-contained true `
-        -p:PublishSingleFile=false -p:DebugType=none `
+        -c $Configuration -p:DebugType=none `
         -o $appOut | Out-Null
     if ($LASTEXITCODE -ne 0) { throw 'dotnet publish failed.' }
 

src/RioJoy.Core/Calibration/AxisCalibrator.cs

@@ -68,7 +68,7 @@ public sealed class AxisCalibrator
             Clamp(x), Clamp(y), Clamp(z), Clamp(rx), Clamp(ry), Clamp(rz));
     }
 
-    private static int Clamp(int v) => Math.Clamp(v, 0, AxisOutputs.Max);
+    private static int Clamp(int v) => RioJoy.Core.Compat.Net48Math.Clamp(v, 0, AxisOutputs.Max);
 
     private int Throttle(int throttle)
     {

src/RioJoy.Core/Compat/Net48Polyfills.cs

@@ -0,0 +1,52 @@
+// net48 compatibility shims. On net8 these APIs are in the BCL; net48 lacks them.
+// (Language-feature polyfills — init/records/Index/Range/required — come from the
+// PolySharp source generator; this file covers runtime helpers that PolySharp
+// cannot supply because they live on existing BCL types.)
+
+namespace System.Collections.Generic
+{
+    public static class Net48CollectionExtensions
+    {
+        /// <summary>
+        /// Polyfill for CollectionExtensions.GetValueOrDefault (netstandard2.1+),
+        /// absent on net48. Returns the value for <paramref name="key"/> or
+        /// default(TValue) when the key is missing.
+        /// </summary>
+        public static TValue? GetValueOrDefault<TKey, TValue>(
+            this IReadOnlyDictionary<TKey, TValue> dictionary, TKey key)
+        {
+            if (dictionary is null) throw new ArgumentNullException(nameof(dictionary));
+            return dictionary.TryGetValue(key, out var value) ? value : default;
+        }
+
+        /// <summary>
+        /// Polyfill for KeyValuePair&lt;,&gt;.Deconstruct (netstandard2.1+), absent on
+        /// net48 — enables <c>foreach (var (k, v) in dictionary)</c>.
+        /// </summary>
+        public static void Deconstruct<TKey, TValue>(
+            this KeyValuePair<TKey, TValue> pair, out TKey key, out TValue value)
+        {
+            key = pair.Key;
+            value = pair.Value;
+        }
+    }
+}
+
+namespace RioJoy.Core.Compat
+{
+    internal static class Net48Math
+    {
+        /// <summary>
+        /// Polyfill for Math.Clamp (netstandard2.1+), absent on net48. Returns
+        /// <paramref name="value"/> bounded to [<paramref name="min"/>, <paramref name="max"/>].
+        /// </summary>
+        public static T Clamp<T>(T value, T min, T max) where T : IComparable<T>
+        {
+            if (min.CompareTo(max) > 0)
+                throw new ArgumentException($"{nameof(min)} ({min}) cannot be greater than {nameof(max)} ({max}).");
+            if (value.CompareTo(min) < 0) return min;
+            if (value.CompareTo(max) > 0) return max;
+            return value;
+        }
+    }
+}

src/RioJoy.Core/Editing/SheetLayout.cs

@@ -68,7 +68,7 @@ public static class SheetLayout
     /// </summary>
     public static IReadOnlyList<SheetCell> Parse(string csv, int maxTextLength = 40)
     {
-        ArgumentNullException.ThrowIfNull(csv);
+        if (csv is null) throw new ArgumentNullException(nameof(csv));
 
         string[] lines = csv.Replace("\r\n", "\n").Replace('\r', '\n').Split('\n');
         var cells = new List<SheetCell>();

src/RioJoy.Core/Hid/RioHidReport.cs

@@ -52,7 +52,7 @@ public sealed class RioHidReport
     /// <summary>Set an axis value (clamped to 0..<see cref="AxisMax"/>), little-endian.</summary>
     public void SetAxis(JoyAxis axis, int value)
     {
-        ushort v = (ushort)Math.Clamp(value, 0, AxisMax);
+        ushort v = (ushort)RioJoy.Core.Compat.Net48Math.Clamp(value, 0, AxisMax);
         int offset = (int)axis * 2;
         _buffer[offset] = (byte)(v & 0xFF);
         _buffer[offset + 1] = (byte)(v >> 8);

src/RioJoy.Core/Mapping/RioInputMap.cs

@@ -16,7 +16,7 @@ public sealed class RioInputMap
     /// <summary>Create a map from raw 16-bit words indexed by address.</summary>
     public RioInputMap(IReadOnlyDictionary<int, ushort> entries)
     {
-        ArgumentNullException.ThrowIfNull(entries);
+        if (entries is null) throw new ArgumentNullException(nameof(entries));
         foreach ((int address, ushort raw) in entries)
             this[address] = new RioMapEntry(raw);
     }

src/RioJoy.Core/Output/HidFeederJoystickSink.cs

@@ -1,5 +1,4 @@
 using System.Runtime.InteropServices;
-using System.Runtime.Versioning;
 using Microsoft.Win32.SafeHandles;
 using RioJoy.Core.Calibration;
 using RioJoy.Core.Hid;
@@ -14,7 +13,6 @@ namespace RioJoy.Core.Output;
 /// <c>DeviceIoControl(IOCTL_RIO_SUBMIT_REPORT)</c>. Replaces
 /// <see cref="NullJoystickSink"/> once the driver is installed (Phase 1/3b).
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class HidFeederJoystickSink : IJoystickSink, IDisposable
 {
     // Must match driver/RioGamepad/Public.h.

src/RioJoy.Core/Output/SendInputSink.cs

@@ -1,5 +1,4 @@
 using System.Runtime.InteropServices;
-using System.Runtime.Versioning;
 using RioJoy.Core.Mapping;
 using MouseButtonId = RioJoy.Core.Mapping.MouseButton;
 
@@ -12,7 +11,6 @@ namespace RioJoy.Core.Output;
 /// scancodes (most do) see the keypress. Targets the interactive session, which
 /// is why RIOJoy is a tray app rather than a service (see docs/PLAN.md risks).
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class SendInputSink : IInputSink
 {
     public void KeyDown(byte virtualKey, bool extended) => SendKey(virtualKey, extended, keyUp: false);

src/RioJoy.Core/Output/ViGEmJoystickSink.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using Nefarius.ViGEm.Client;
 using Nefarius.ViGEm.Client.Targets;
 using Nefarius.ViGEm.Client.Targets.Xbox360;
@@ -14,7 +13,6 @@ namespace RioJoy.Core.Output;
 /// triggers, and the D-pad for the hat. RIO joystick buttons beyond
 /// <see cref="MappableButtonCount"/> are dropped (map those to the keyboard).
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class ViGEmJoystickSink : IJoystickSink, IDisposable
 {
     // RIO joystick button (1-based) -> Xbox 360 button, in a natural order.
@@ -115,11 +113,11 @@ public sealed class ViGEmJoystickSink : IJoystickSink, IDisposable
 
     // RIO axis 0..32766 (centre 16383) -> Xbox thumb short -32768..32767.
     private static short ToThumb(int value) =>
-        (short)Math.Clamp((value - AxisOutputs.Center) * 2, short.MinValue, short.MaxValue);
+        (short)RioJoy.Core.Compat.Net48Math.Clamp((value - AxisOutputs.Center) * 2, short.MinValue, short.MaxValue);
 
     // RIO axis 0..32766 -> Xbox trigger byte 0..255.
     private static byte ToTrigger(int value) =>
-        (byte)Math.Clamp(value * 255 / AxisOutputs.Max, 0, 255);
+        (byte)RioJoy.Core.Compat.Net48Math.Clamp(value * 255 / AxisOutputs.Max, 0, 255);
 
     public void Dispose()
     {

src/RioJoy.Core/Overlay/FontFitter.cs

@@ -27,7 +27,7 @@ public static class FontFitter
     public static double BestFitSize(
         string text, string fontFamily, double width, double height, ITextMeasurer measurer)
     {
-        ArgumentNullException.ThrowIfNull(measurer);
+        if (measurer is null) throw new ArgumentNullException(nameof(measurer));
 
         // State mirrors the Scheme named-let: (fontsize last-extents last-fontsize adjust).
         double fontsize = 6;        // minimum possible fontsize (the loop's seed)

src/RioJoy.Core/Overlay/GoobieDataImporter.cs

@@ -26,7 +26,7 @@ public static class GoobieDataImporter
     /// <summary>Parse the data file text into fields + rows.</summary>
     public static Sheet Parse(string text)
     {
-        ArgumentNullException.ThrowIfNull(text);
+        if (text is null) throw new ArgumentNullException(nameof(text));
 
         List<List<string>> lists = ReadLists(text);
         if (lists.Count == 0)
@@ -53,7 +53,7 @@ public static class GoobieDataImporter
     /// </summary>
     public static IReadOnlyDictionary<string, string> LabelsForRow(Sheet sheet, int rowIndex = 0)
     {
-        ArgumentNullException.ThrowIfNull(sheet);
+        if (sheet is null) throw new ArgumentNullException(nameof(sheet));
         if (rowIndex < 0 || rowIndex >= sheet.Rows.Count)
             throw new ArgumentOutOfRangeException(nameof(rowIndex));
 

src/RioJoy.Core/Overlay/OverlayLayoutEngine.cs

@@ -25,8 +25,8 @@ public sealed class OverlayLayoutEngine
         IReadOnlyDictionary<string, string> labels,
         OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(template);
-        ArgumentNullException.ThrowIfNull(labels);
+        if (template is null) throw new ArgumentNullException(nameof(template));
+        if (labels is null) throw new ArgumentNullException(nameof(labels));
         options ??= new OverlayLayoutOptions();
 
         var placed = new List<PlacedLabel>(template.Regions.Count);
@@ -41,8 +41,8 @@ public sealed class OverlayLayoutEngine
     /// <summary>Lay out a single label within a region.</summary>
     public PlacedLabel Place(OverlayRegion region, string text, OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(region);
-        ArgumentException.ThrowIfNullOrEmpty(text);
+        if (region is null) throw new ArgumentNullException(nameof(region));
+        if (string.IsNullOrEmpty(text)) throw new ArgumentException("Value cannot be null or empty.", nameof(text));
         options ??= new OverlayLayoutOptions();
 
         double rotation = region.RotationDegrees ?? 0;

src/RioJoy.Core/Overlay/OverlayTemplateStore.cs

@@ -20,20 +20,20 @@ public static class OverlayTemplateStore
 
     public static string Serialize(OverlayTemplate template)
     {
-        ArgumentNullException.ThrowIfNull(template);
+        if (template is null) throw new ArgumentNullException(nameof(template));
         return JsonSerializer.Serialize(template, Options);
     }
 
     public static OverlayTemplate Deserialize(string json)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(json);
+        if (string.IsNullOrWhiteSpace(json)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(json));
         return JsonSerializer.Deserialize<OverlayTemplate>(json, Options)
                ?? throw new JsonException("Overlay template JSON deserialized to null.");
     }
 
     public static void Save(OverlayTemplate template, string path)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(path);
+        if (string.IsNullOrWhiteSpace(path)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(path));
         string? dir = Path.GetDirectoryName(Path.GetFullPath(path));
         if (!string.IsNullOrEmpty(dir))
             Directory.CreateDirectory(dir);
@@ -42,7 +42,7 @@ public static class OverlayTemplateStore
 
     public static OverlayTemplate Load(string path)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(path);
+        if (string.IsNullOrWhiteSpace(path)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(path));
         return Deserialize(File.ReadAllText(path));
     }
 }

src/RioJoy.Core/Plasma/PlasmaCommands.cs

@@ -48,8 +48,9 @@ public static class PlasmaCommands
     /// <summary>Encode display text as raw bytes (Latin-1, one byte per char).</summary>
     public static byte[] Text(string text)
     {
-        ArgumentNullException.ThrowIfNull(text);
-        return Encoding.Latin1.GetBytes(text);
+        if (text is null) throw new ArgumentNullException(nameof(text));
+        // net48 has no Encoding.Latin1; codepage 28591 is ISO-8859-1 (Latin-1).
+        return Encoding.GetEncoding(28591).GetBytes(text);
     }
 
     /// <summary>
@@ -72,7 +73,7 @@ public static class PlasmaCommands
     public static (byte x, byte y, byte font, int length) ResolvePosText(
         string text, byte x, byte y, byte font)
     {
-        ArgumentNullException.ThrowIfNull(text);
+        if (text is null) throw new ArgumentNullException(nameof(text));
         int len = text.Length;
 
         if (font != 2) // not the Score font

src/RioJoy.Core/Profiles/AutoSwitch.cs

@@ -48,7 +48,7 @@ public static class AutoSwitchResolver
     /// <summary>Normalize an executable name for matching: basename, no ".exe", lower-case.</summary>
     public static string Normalize(string executable)
     {
-        ArgumentNullException.ThrowIfNull(executable);
+        if (executable is null) throw new ArgumentNullException(nameof(executable));
         string name = executable.Trim();
         // Strip any path (handle both separators regardless of OS).
         int slash = name.LastIndexOfAny(new[] { '/', '\\' });
@@ -68,7 +68,7 @@ public static class AutoSwitchResolver
     /// </summary>
     public static SwitchDecision Resolve(AppConfig config, string? foregroundExecutable)
     {
-        ArgumentNullException.ThrowIfNull(config);
+        if (config is null) throw new ArgumentNullException(nameof(config));
 
         if (!string.IsNullOrWhiteSpace(foregroundExecutable))
         {

src/RioJoy.Core/Profiles/AutoSwitchWatcher.cs

@@ -56,9 +56,19 @@ public sealed class AutoSwitchWatcher
     /// <summary>Poll on <paramref name="interval"/> until cancelled.</summary>
     public async Task RunAsync(TimeSpan interval, CancellationToken cancellationToken)
     {
-        using var timer = new PeriodicTimer(interval);
+        // net48 has no PeriodicTimer; poll on a Task.Delay loop instead.
         Poll();
-        while (await timer.WaitForNextTickAsync(cancellationToken).ConfigureAwait(false))
+        while (!cancellationToken.IsCancellationRequested)
+        {
+            try
+            {
+                await Task.Delay(interval, cancellationToken).ConfigureAwait(false);
+            }
+            catch (OperationCanceledException)
+            {
+                break;
+            }
             Poll();
+        }
     }
 }

src/RioJoy.Core/Profiles/ConfigStore.cs

@@ -18,13 +18,13 @@ public static class ConfigStore
 
     public static string Serialize(AppConfig config)
     {
-        ArgumentNullException.ThrowIfNull(config);
+        if (config is null) throw new ArgumentNullException(nameof(config));
         return JsonSerializer.Serialize(config, Options);
     }
 
     public static AppConfig Deserialize(string json)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(json);
+        if (string.IsNullOrWhiteSpace(json)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(json));
         return JsonSerializer.Deserialize<AppConfig>(json, Options)
                ?? throw new JsonException("Config JSON deserialized to null.");
     }
@@ -32,7 +32,7 @@ public static class ConfigStore
     /// <summary>Save the config to <paramref name="path"/> (creating directories).</summary>
     public static void Save(AppConfig config, string path)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(path);
+        if (string.IsNullOrWhiteSpace(path)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(path));
         string? dir = Path.GetDirectoryName(Path.GetFullPath(path));
         if (!string.IsNullOrEmpty(dir))
             Directory.CreateDirectory(dir);
@@ -45,7 +45,7 @@ public static class ConfigStore
     /// </summary>
     public static AppConfig Load(string path)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(path);
+        if (string.IsNullOrWhiteSpace(path)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(path));
         return File.Exists(path) ? Deserialize(File.ReadAllText(path)) : new AppConfig();
     }
 }

src/RioJoy.Core/Profiles/IniFile.cs

@@ -11,7 +11,7 @@ public sealed class IniFile
 
     public static IniFile Parse(string text)
     {
-        ArgumentNullException.ThrowIfNull(text);
+        if (text is null) throw new ArgumentNullException(nameof(text));
         var ini = new IniFile();
         var current = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
         ini._sections[string.Empty] = current;
@@ -68,9 +68,9 @@ public sealed class IniFile
     /// <summary>Parse an integer value that may be hex (<c>0x…</c>) or decimal.</summary>
     public static int ParseInt(string value)
     {
-        ArgumentNullException.ThrowIfNull(value);
+        if (value is null) throw new ArgumentNullException(nameof(value));
         string v = value.Trim();
-        bool neg = v.StartsWith('-');
+        bool neg = v.StartsWith("-", StringComparison.Ordinal);
         if (neg) v = v[1..];
 
         int magnitude = v.StartsWith("0x", StringComparison.OrdinalIgnoreCase)

src/RioJoy.Core/Profiles/RioIniImporter.cs

@@ -15,7 +15,7 @@ public static class RioIniImporter
     /// <summary>Parse <paramref name="iniText"/> into a profile named <paramref name="name"/>.</summary>
     public static RioProfile Import(string iniText, string name = "Imported")
     {
-        ArgumentNullException.ThrowIfNull(iniText);
+        if (iniText is null) throw new ArgumentNullException(nameof(iniText));
         IniFile ini = IniFile.Parse(iniText);
 
         var profile = new RioProfile

src/RioJoy.Core/Protocol/PacketParser.cs

@@ -97,7 +97,7 @@ public sealed class PacketParser
     /// </summary>
     public void Feed(ReadOnlySpan<byte> data, Action<RioRxEvent> onEvent)
     {
-        ArgumentNullException.ThrowIfNull(onEvent);
+        if (onEvent is null) throw new ArgumentNullException(nameof(onEvent));
         foreach (byte ch in data)
         {
             if (Feed(ch, out RioRxEvent ev))

src/RioJoy.Core/Protocol/RioPacket.cs

@@ -30,7 +30,7 @@ public readonly struct RioPacket
 
     public override string ToString()
     {
-        var hex = Convert.ToHexString(Payload.Span);
+        var hex = BitConverter.ToString(Payload.ToArray()).Replace("-", string.Empty);
         return Payload.IsEmpty ? Command.ToString() : $"{Command} [{hex}]";
     }
 }

src/RioJoy.Core/RioJoy.Core.csproj

@@ -1,10 +1,12 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <!-- net8.0 LTS. Windows-flavored TFM: this library uses Win32 P/Invoke
-         (SendInput, SystemParametersInfo) and System.IO.Ports. -->
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <!-- net48 test branch. Uses Win32 P/Invoke (SendInput, SystemParametersInfo)
+         and System.IO.Ports. Span/records/init/Index are polyfilled (System.Memory
+         + PolySharp); System.Text.Json comes from NuGet (not in the net48 BCL). -->
+    <TargetFramework>net48</TargetFramework>
     <Platforms>x64</Platforms>
+    <PlatformTarget>x64</PlatformTarget>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <LangVersion>latest</LangVersion>
@@ -13,6 +15,13 @@
   <ItemGroup>
     <PackageReference Include="Nefarius.ViGEm.Client" Version="1.21.256" />
     <PackageReference Include="System.IO.Ports" Version="8.0.0" />
+    <PackageReference Include="System.Memory" Version="4.5.5" />
+    <PackageReference Include="System.Text.Json" Version="8.0.5" />
+    <PackageReference Include="Microsoft.Bcl.HashCode" Version="1.1.1" />
+    <PackageReference Include="PolySharp" Version="1.14.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
 </Project>

src/RioJoy.Core/RioRuntime.cs

@@ -30,8 +30,8 @@ public sealed class RioRuntime : IRioCommandSink, IDisposable
         AxisCalibrator? calibrator = null)
     {
         _link = link ?? throw new ArgumentNullException(nameof(link));
-        ArgumentNullException.ThrowIfNull(map);
-        ArgumentNullException.ThrowIfNull(input);
+        if (map is null) throw new ArgumentNullException(nameof(map));
+        if (input is null) throw new ArgumentNullException(nameof(input));
         _joystick = joystick ?? throw new ArgumentNullException(nameof(joystick));
         _calibrator = calibrator ?? new AxisCalibrator();
 

src/RioJoy.Core/Serial/SerialPortTransport.cs

@@ -22,7 +22,7 @@ public sealed class SerialPortTransport : IRioTransport
 
     public SerialPortTransport(string portName)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(portName);
+        if (string.IsNullOrWhiteSpace(portName)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(portName));
 
         _port = new SerialPort(portName, BaudRate, Parity.None, 8, StopBits.One)
         {
@@ -44,11 +44,21 @@ public sealed class SerialPortTransport : IRioTransport
 
     public string Description => $"{_port.PortName} @ {BaudRate} 8N1";
 
-    public ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken) =>
-        _stream.ReadAsync(buffer, cancellationToken);
+    // net48's Stream has no Memory-based ReadAsync/WriteAsync overloads, so bridge
+    // through a pooled array and copy into/out of the caller's Memory<byte>.
+    public async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken)
+    {
+        byte[] tmp = new byte[buffer.Length];
+        int read = await _stream.ReadAsync(tmp, 0, tmp.Length, cancellationToken).ConfigureAwait(false);
+        new ReadOnlySpan<byte>(tmp, 0, read).CopyTo(buffer.Span);
+        return read;
+    }
 
-    public ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken) =>
-        _stream.WriteAsync(data, cancellationToken);
+    public async ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
+    {
+        byte[] tmp = data.ToArray();
+        await _stream.WriteAsync(tmp, 0, tmp.Length, cancellationToken).ConfigureAwait(false);
+    }
 
     public void Dispose()
     {

src/RioJoy.Overlay/ProfileWallpaperGenerator.cs

@@ -32,9 +32,9 @@ public sealed class ProfileWallpaperGenerator
         string outputPath,
         OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(template);
-        ArgumentNullException.ThrowIfNull(labels);
-        ArgumentException.ThrowIfNullOrWhiteSpace(outputPath);
+        if (template is null) throw new ArgumentNullException(nameof(template));
+        if (labels is null) throw new ArgumentNullException(nameof(labels));
+        if (string.IsNullOrWhiteSpace(outputPath)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(outputPath));
         if (string.IsNullOrWhiteSpace(template.BaseImagePath))
             throw new InvalidOperationException("OverlayTemplate.BaseImagePath is not set.");
 

src/RioJoy.Overlay/RioJoy.Overlay.csproj

@@ -1,8 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net48</TargetFramework>
     <Platforms>x64</Platforms>
+    <PlatformTarget>x64</PlatformTarget>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <LangVersion>latest</LangVersion>
@@ -12,6 +13,10 @@
     <!-- SkiaSharp: cross-platform 2D rasterizer used to render the wallpaper. -->
     <PackageReference Include="SkiaSharp" Version="2.88.8" />
     <PackageReference Include="SkiaSharp.NativeAssets.Win32" Version="2.88.8" />
+    <PackageReference Include="PolySharp" Version="1.14.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup>

src/RioJoy.Overlay/SkiaOverlayRenderer.cs

@@ -35,9 +35,9 @@ public sealed class SkiaOverlayRenderer
         SKBitmap baseImage,
         OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(template);
-        ArgumentNullException.ThrowIfNull(labels);
-        ArgumentNullException.ThrowIfNull(baseImage);
+        if (template is null) throw new ArgumentNullException(nameof(template));
+        if (labels is null) throw new ArgumentNullException(nameof(labels));
+        if (baseImage is null) throw new ArgumentNullException(nameof(baseImage));
         options ??= new OverlayLayoutOptions();
 
         var output = baseImage.Copy();
@@ -114,8 +114,8 @@ public sealed class SkiaOverlayRenderer
         string outputPath,
         OverlayLayoutOptions? options = null)
     {
-        ArgumentNullException.ThrowIfNull(template);
-        ArgumentException.ThrowIfNullOrWhiteSpace(outputPath);
+        if (template is null) throw new ArgumentNullException(nameof(template));
+        if (string.IsNullOrWhiteSpace(outputPath)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(outputPath));
         if (string.IsNullOrWhiteSpace(template.BaseImagePath))
             throw new InvalidOperationException("OverlayTemplate.BaseImagePath is not set.");
 

src/RioJoy.Tray/Editor/PanelCanvas.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using RioJoy.Core.Editing;
 
 namespace RioJoy.Tray.Editor;
@@ -8,7 +7,6 @@ namespace RioJoy.Tray.Editor;
 /// <see cref="PanelView"/>) and raises <see cref="AddressSelected"/> when a button
 /// is clicked. Sized to the full panel so the host scrolls it.
 /// </summary>
-[SupportedOSPlatform("windows")]
 internal sealed class PanelCanvas : Control
 {
     private static readonly IReadOnlyList<PanelButton> AllButtons = CockpitPanel.Buttons();

src/RioJoy.Tray/Editor/PanelView.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using RioJoy.Core.Editing;
 
 namespace RioJoy.Tray.Editor;
@@ -28,7 +27,6 @@ public sealed record CalibrationCell(JoyStickSetting Setting, string Label, int
 /// shaded by whether the address is assigned (Off vs Dim); keypads are neutral
 /// (no lamps). Shared by <see cref="PanelCanvas"/> and offline rendering.
 /// </summary>
-[SupportedOSPlatform("windows")]
 public static class PanelView
 {
     public const int CellW = 66;

src/RioJoy.Tray/Editor/ProfileEditorForm.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using System.Text;
 using RioJoy.Core.Editing;
 using RioJoy.Core.Output;
@@ -17,7 +16,6 @@ namespace RioJoy.Tray.Editor;
 /// wallpaper region <c>b-XX</c>) and <see cref="RioProfile.Buttons"/> (the iRIO
 /// word); <c>Save</c> raises <see cref="Saved"/>.
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class ProfileEditorForm : Form
 {
     private static readonly Dictionary<int, string> GroupOf =
@@ -30,7 +28,7 @@ public sealed class ProfileEditorForm : Form
     private readonly PanelCanvas _canvas = new();
 
     private readonly TextBox _nameBox = new() { Location = new Point(66, 12), Width = 234 };
-    private readonly TextBox _matchBox = new() { Location = new Point(66, 40), Width = 234, PlaceholderText = "game.exe, other.exe — auto-switch" };
+    private readonly TextBox _matchBox = new() { Location = new Point(66, 40), Width = 234 };
     private readonly Label _info = new() { AutoSize = true, Location = new Point(12, 68), MaximumSize = new Size(310, 0) };
     private readonly TextBox _labelBox = new() { Location = new Point(70, 100), Width = 230 };
     private readonly ComboBox _kindBox = new() { Location = new Point(70, 134), Width = 150, DropDownStyle = ComboBoxStyle.DropDownList };
@@ -116,7 +114,7 @@ public sealed class ProfileEditorForm : Form
         StartPosition = FormStartPosition.CenterScreen;
         MinimumSize = new Size(900, 500);
 
-        _kindBox.Items.AddRange(Enum.GetNames<RioRouteKind>());
+        _kindBox.Items.AddRange(Enum.GetNames(typeof(RioRouteKind)));
         _kindBox.SelectedIndexChanged += OnKindChanged;
 
         _canvas.LabelProvider = a => _profile.OverlayLabels.TryGetValue(RegionFor(a), out string? t) ? t : null;
@@ -271,11 +269,11 @@ public sealed class ProfileEditorForm : Form
                     _valueCombo.Items.Add(new ValueItem(h.ToString(), (byte)(int)h));
                 break;
             case RioRouteKind.Mouse:
-                foreach (MouseAction m in Enum.GetValues<MouseAction>())
+                foreach (MouseAction m in (MouseAction[])Enum.GetValues(typeof(MouseAction)))
                     _valueCombo.Items.Add(new ValueItem(m.ToString(), (byte)m));
                 break;
             case RioRouteKind.RioCommand:
-                foreach (RioCommandCode c in Enum.GetValues<RioCommandCode>())
+                foreach (RioCommandCode c in (RioCommandCode[])Enum.GetValues(typeof(RioCommandCode)))
                     _valueCombo.Items.Add(new ValueItem(c.ToString(), (byte)c));
                 break;
         }
@@ -349,7 +347,9 @@ public sealed class ProfileEditorForm : Form
 
         // The foreground executables that auto-activate this profile (comma-separated).
         _profile.MatchExecutables = _matchBox.Text
-            .Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries)
+            .Select(s => s.Trim())
+            .Where(s => s.Length > 0)
             .ToList();
 
         ApplyToCell();

src/RioJoy.Tray/Os/AutoStartManager.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using Microsoft.Win32;
 
 namespace RioJoy.Tray.Os;
@@ -8,7 +7,6 @@ namespace RioJoy.Tray.Os;
 /// <c>Run</c> registry key. Per-user (HKCU) needs no elevation and matches the
 /// tray app running in the interactive session.
 /// </summary>
-[SupportedOSPlatform("windows")]
 public static class AutoStartManager
 {
     private const string RunKey = @"Software\Microsoft\Windows\CurrentVersion\Run";
@@ -24,7 +22,7 @@ public static class AutoStartManager
     {
         using RegistryKey key = Registry.CurrentUser.CreateSubKey(RunKey);
         if (enabled)
-            key.SetValue(ValueName, $"\"{Environment.ProcessPath}\"");
+            key.SetValue(ValueName, $"\"{Application.ExecutablePath}\"");
         else
             key.DeleteValue(ValueName, throwOnMissingValue: false);
     }

src/RioJoy.Tray/Os/ForegroundProcessProvider.cs

@@ -1,6 +1,5 @@
 using System.Diagnostics;
 using System.Runtime.InteropServices;
-using System.Runtime.Versioning;
 using RioJoy.Core.Profiles;
 
 namespace RioJoy.Tray.Os;
@@ -10,7 +9,6 @@ namespace RioJoy.Tray.Os;
 /// foreground window's owning process and returns its executable file name. Used
 /// by the auto-switch watcher to detect native games vs. supported titles.
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class ForegroundProcessProvider : IForegroundProcessProvider
 {
     public string? GetForegroundExecutable()

src/RioJoy.Tray/Program.cs

@@ -20,7 +20,10 @@ internal static class Program
         if (!createdNew)
             return; // another RIOJoy is already running in this session
 
-        ApplicationConfiguration.Initialize();
+        // net48 has no source-generated ApplicationConfiguration.Initialize();
+        // do the equivalent setup directly.
+        Application.EnableVisualStyles();
+        Application.SetCompatibleTextRenderingDefault(false);
         Application.Run(new TrayApplicationContext());
     }
 }

src/RioJoy.Tray/RioCoordinator.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using RioJoy.Core;
 using RioJoy.Core.Calibration;
 using RioJoy.Core.Mapping;
@@ -22,7 +21,6 @@ namespace RioJoy.Tray;
 /// edited (<see cref="BeginEditorSession"/>). This keeps the COM port free the rest
 /// of the time so the native games can open it without clashing.
 /// </summary>
-[SupportedOSPlatform("windows")]
 public sealed class RioCoordinator : IDisposable
 {
     private readonly Func<AppConfig> _config;

src/RioJoy.Tray/RioJoy.Tray.csproj

@@ -2,8 +2,9 @@
 
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net48</TargetFramework>
     <Platforms>x64</Platforms>
+    <PlatformTarget>x64</PlatformTarget>
     <Nullable>enable</Nullable>
     <UseWindowsForms>true</UseWindowsForms>
     <ImplicitUsings>enable</ImplicitUsings>
@@ -16,4 +17,11 @@
     <ProjectReference Include="..\RioJoy.Overlay\RioJoy.Overlay.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <PackageReference Include="PolySharp" Version="1.14.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
 </Project>

src/RioJoy.Tray/TrayApplicationContext.cs

@@ -1,4 +1,3 @@
-using System.Runtime.Versioning;
 using RioJoy.Core;
 using RioJoy.Core.Mapping;
 using RioJoy.Core.Profiles;
@@ -14,7 +13,6 @@ namespace RioJoy.Tray;
 /// auto-switch watcher is polled on a UI timer so menu/status updates stay on the
 /// UI thread.
 /// </summary>
-[SupportedOSPlatform("windows")]
 internal sealed class TrayApplicationContext : ApplicationContext
 {
     private static readonly string ConfigPath =

src/RioJoy.Tray/WallpaperApplier.cs

@@ -1,5 +1,4 @@
 using System.Runtime.InteropServices;
-using System.Runtime.Versioning;
 
 namespace RioJoy.Tray;
 
@@ -14,7 +13,6 @@ namespace RioJoy.Tray;
 /// a user-visible system setting, so it is only invoked on an explicit profile
 /// activation, never speculatively.</para>
 /// </summary>
-[SupportedOSPlatform("windows")]
 public static class WallpaperApplier
 {
     private const int SPI_SETDESKWALLPAPER = 0x0014;
@@ -27,7 +25,7 @@ public static class WallpaperApplier
     /// </summary>
     public static bool Apply(string imagePath)
     {
-        ArgumentException.ThrowIfNullOrWhiteSpace(imagePath);
+        if (string.IsNullOrWhiteSpace(imagePath)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(imagePath));
         string full = Path.GetFullPath(imagePath);
         if (!File.Exists(full))
             throw new FileNotFoundException("Wallpaper image not found.", full);

tests/RioJoy.Core.Tests/Overlay/SkiaOverlayRendererTests.cs

@@ -77,6 +77,6 @@ public class SkiaOverlayRendererTests
         byte[] png = new SkiaOverlayRenderer().RenderToPng(template, new Dictionary<string, string> { ["x"] = "HI" }, baseImg);
 
         Assert.True(png.Length > 8);
-        Assert.Equal(new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }, png[..8]);
+        Assert.Equal(new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }, png.AsSpan(0, 8).ToArray());
     }
 }

tests/RioJoy.Core.Tests/Protocol/PacketBuilderTests.cs

@@ -41,7 +41,7 @@ public class PacketBuilderTests
         byte[] packet = PacketBuilder.Build(RioCommand.AnalogReply, payload);
 
         Assert.Equal((byte)RioCommand.AnalogReply, packet[0]);
-        Assert.Equal(payload, packet[1..^1]);
-        Assert.Equal(RioChecksum.Compute(packet[..^1]), packet[^1]);
+        Assert.Equal(payload, packet.AsSpan(1, packet.Length - 2).ToArray());
+        Assert.Equal(RioChecksum.Compute(packet.AsSpan(0, packet.Length - 1)), packet[^1]);
     }
 }

tests/RioJoy.Core.Tests/RioJoy.Core.Tests.csproj

@@ -1,8 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net48</TargetFramework>
     <Platforms>x64</Platforms>
+    <PlatformTarget>x64</PlatformTarget>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <LangVersion>latest</LangVersion>
@@ -13,6 +14,11 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageReference Include="xunit" Version="2.9.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+    <PackageReference Include="System.Threading.Channels" Version="8.0.0" />
+    <PackageReference Include="PolySharp" Version="1.14.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup>

tests/RioJoy.Core.Tests/Serial/FakeTransport.cs

@@ -40,7 +40,7 @@ internal sealed class FakeTransport : IRioTransport
     public ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
     {
         _writes.Writer.TryWrite(data.ToArray());
-        return ValueTask.CompletedTask;
+        return default; // net48 has no ValueTask.CompletedTask; default(ValueTask) is the completed task
     }
 
     /// <summary>Read the next outbound write, failing if none arrives in time.</summary>

tests/RioJoy.Core.Tests/TestPolyfills.cs

@@ -0,0 +1,21 @@
+namespace RioJoy.Core.Tests;
+
+/// <summary>
+/// net48 test-only polyfills. Visible to all tests via enclosing-namespace scope.
+/// </summary>
+internal static class TaskTestExtensions
+{
+    /// <summary>
+    /// Polyfill for Task&lt;T&gt;.WaitAsync(TimeSpan) (net6+), absent on net48.
+    /// Throws <see cref="TimeoutException"/> if the task does not complete in time.
+    /// </summary>
+    public static async Task<T> WaitAsync<T>(this Task<T> task, TimeSpan timeout)
+    {
+        using var cts = new CancellationTokenSource();
+        Task completed = await Task.WhenAny(task, Task.Delay(timeout, cts.Token)).ConfigureAwait(false);
+        if (completed != task)
+            throw new TimeoutException($"Task did not complete within {timeout}.");
+        cts.Cancel(); // stop the delay timer
+        return await task.ConfigureAwait(false);
+    }
+}