Replace netstandard2.1 target with netstandard2.0 (#37)

* Initial plan

* Replace netstandard2.1 with netstandard2.0, add polyfills for missing APIs

Co-authored-by: HarryCordewener <5649138+HarryCordewener@users.noreply.github.com>

* Fix .gitignore and add polyfill limitation documentation

Co-authored-by: HarryCordewener <5649138+HarryCordewener@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: HarryCordewener <5649138+HarryCordewener@users.noreply.github.com>

Author: Copilot

.gitignore

@@ -340,4 +340,5 @@ ASALocalRun/
 .localhistory/
 
 # BeatPulse healthcheck temp database
-healthchecksdb
+healthchecksdb
+TestResults/

CHANGELOG.md

@@ -11,7 +11,7 @@ All notable changes to this project will be documented in this file.
   - `UsePipe(IDuplexPipe)` — wires `OnNegotiation` to `pipe.Output`; leaves read loop to the caller
   - `UseStream(Stream)` — wires `OnNegotiation` via `PipeWriter.Create(stream)`; leaves read loop to the caller
   - `ReadFromPipeAsync(TelnetInterpreter, PipeReader, CancellationToken)` — static helper that drives the standard read loop
-- Added `System.IO.Pipelines` as an explicit package reference for `net8.0` and `netstandard2.1` targets
+- Added `System.IO.Pipelines` as an explicit package reference for `net8.0` and `netstandard2.0` targets
 
 ## [2.3.0] - 2026-02-13
 

TelnetNegotiationCore.Functional/TelnetNegotiationCore.Functional.fsproj

@@ -1,16 +1,16 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.1;net8.0;net10.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net8.0;net10.0</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
   </PropertyGroup>
 
   <ItemGroup>
     <Compile Include="MSDPLibrary.fs" />
   </ItemGroup>
 
-  <!-- System.Text.Json for netstandard2.1 -->
-  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.1'">
+  <!-- System.Text.Json for netstandard2.0 -->
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
     <PackageReference Include="System.Text.Json" Version="10.0.2" />
   </ItemGroup>
 

TelnetNegotiationCore/Handlers/MSDPServerHandler.cs

@@ -120,7 +120,7 @@ await ExecuteOnAsync(item, (var) =>
 
     private async ValueTask ExecuteOnAsync(string item, Func<string, ValueTask> function)
     {
-        var items = item.StartsWith('[')
+        var items = item.StartsWith("[", StringComparison.Ordinal)
             ? JsonSerializer.Deserialize<string[]>(item)
             : [item];
 

TelnetNegotiationCore/Helpers/NetStandard20Polyfills.cs

@@ -0,0 +1,58 @@
+#if NETSTANDARD2_0
+using System.Collections.Generic;
+using System.Linq;
+
+namespace TelnetNegotiationCore;
+
+/// <summary>
+/// Polyfills for APIs available in netstandard2.1+ but missing in netstandard2.0.
+/// </summary>
+internal static class NetStandard20Polyfills
+{
+	/// <summary>
+	/// Polyfill for <see cref="HashSet{T}.TryGetValue"/> (added in netstandard2.1).
+	/// Note: Returns the input value rather than the stored value. This is correct for
+	/// default equality comparers but may differ from the real API with custom comparers.
+	/// </summary>
+	public static bool TryGetValue<T>(this HashSet<T> set, T equalValue, out T actualValue)
+	{
+		if (set.Contains(equalValue))
+		{
+			actualValue = equalValue;
+			return true;
+		}
+		actualValue = default!;
+		return false;
+	}
+
+	/// <summary>
+	/// Polyfill for <see cref="Dictionary{TKey,TValue}.TryAdd"/> (added in netstandard2.1).
+	/// </summary>
+	public static bool TryAdd<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key, TValue value)
+	{
+		if (!dictionary.ContainsKey(key))
+		{
+			dictionary.Add(key, value);
+			return true;
+		}
+		return false;
+	}
+
+	/// <summary>
+	/// Polyfill for <see cref="Enumerable.ToHashSet{T}(IEnumerable{T})"/> (added in netstandard2.1).
+	/// </summary>
+	public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source)
+	{
+		return new HashSet<T>(source);
+	}
+
+	/// <summary>
+	/// Polyfill for KeyValuePair deconstruction (added in netstandard2.1).
+	/// </summary>
+	public static void Deconstruct<TKey, TValue>(this KeyValuePair<TKey, TValue> kvp, out TKey key, out TValue value)
+	{
+		key = kvp.Key;
+		value = kvp.Value;
+	}
+}
+#endif

TelnetNegotiationCore/Interpreters/TelnetMSSPInterpreter.cs

@@ -81,7 +81,11 @@ private void StoreClientMSSPDetails(string variable, IEnumerable<string> value)
 		{
 			dynamic valueToSet = value.Count() > 1 ? value : value.First();
 
-			if (!_msspConfig().Extended.TryAdd(variable, valueToSet))
+			if (!_msspConfig().Extended.ContainsKey(variable))
+			{
+				_msspConfig().Extended.Add(variable, valueToSet);
+			}
+			else
 			{
 				_msspConfig().Extended[variable] = valueToSet;
 			}

TelnetNegotiationCore/Interpreters/TelnetNAWSInterpreter.cs

@@ -104,8 +104,8 @@ private async ValueTask CompleteNAWSAsync(StateMachine<State, Trigger>.Transitio
 			Array.Reverse(height);
 		}
 
-		ClientWidth = BitConverter.ToInt16(width);
-		ClientHeight = BitConverter.ToInt16(height);
+		ClientWidth = BitConverter.ToInt16(width, 0);
+		ClientHeight = BitConverter.ToInt16(height, 0);
 
 		_logger.LogDebug("Negotiated for: {clientWidth} width and {clientHeight} height", ClientWidth, ClientHeight);
 

TelnetNegotiationCore/Interpreters/TelnetSafeInterpreter.cs

@@ -95,7 +95,7 @@ private static byte[] TelnetSafeBytesInternal(ReadOnlySpan<byte> input)
 
 		return result;
 #else
-		// Fallback for netstandard2.1 and .NET 8: ArrayPool worst-case buffer + IndexOf loop
+		// Fallback for netstandard2.0 and .NET 8: ArrayPool worst-case buffer + IndexOf loop
 		// with CopyTo block copies. MemoryExtensions.Split<T>(T value) returning
 		// SpanSplitEnumerator<T> was introduced in .NET 9 and has no available polyfill.
 		var pooled = ArrayPool<byte>.Shared.Rent(input.Length * 2);
@@ -123,7 +123,7 @@ private static byte[] TelnetSafeBytesInternal(ReadOnlySpan<byte> input)
 				remaining = remaining[(iacPos + 1)..];
 			}
 
-			return pooled[..writePos].ToArray();
+			return pooled.AsSpan(0, writePos).ToArray();
 		}
 		finally
 		{

TelnetNegotiationCore/Protocols/MCCPProtocol.cs

@@ -8,7 +8,7 @@
 using TelnetNegotiationCore.Attributes;
 using TelnetNegotiationCore.Models;
 using TelnetNegotiationCore.Plugins;
-#if NETSTANDARD2_1
+#if NETSTANDARD2_0
 using ICSharpCode.SharpZipLib.Zip.Compression.Streams;
 #endif
 
@@ -36,7 +36,7 @@ public class MCCPProtocol : TelnetProtocolPluginBase
 {
     private bool _mccp2Enabled = false;
     private bool _mccp3Enabled = false;
-#if NETSTANDARD2_1
+#if NETSTANDARD2_0
     private Stream? _compressionStream;
 #else
     private ZLibStream? _compressionStream;
@@ -242,8 +242,8 @@ public byte[] DecompressData(byte[] data)
 
         try
         {
-#if NETSTANDARD2_1
-            // Use SharpZipLib for .NET Standard 2.1
+#if NETSTANDARD2_0
+            // Use SharpZipLib for .NET Standard 2.0
             using var compressedStream = new MemoryStream(data);
             using var zlibStream = new InflaterInputStream(compressedStream);
             using var outputStream = new MemoryStream();
@@ -371,7 +371,7 @@ private async ValueTask OnWillMCCP3Async(IProtocolContext context)
         await context.SendNegotiationAsync(new byte[] { (byte)Trigger.IAC, (byte)Trigger.SB, (byte)Trigger.MCCP3, (byte)Trigger.IAC, (byte)Trigger.SE });
         // Start compression
         _compressionBuffer = new MemoryStream();
-#if NETSTANDARD2_1
+#if NETSTANDARD2_0
         _compressionStream = new DeflaterOutputStream(_compressionBuffer);
 #else
         _compressionStream = new ZLibStream(_compressionBuffer, CompressionMode.Compress);
@@ -396,7 +396,7 @@ private async ValueTask CompleteMCCP2NegotiationAsync(IProtocolContext context)
 
         // Initialize compression stream for server-to-client compression
         _compressionBuffer = new MemoryStream();
-#if NETSTANDARD2_1
+#if NETSTANDARD2_0
         _compressionStream = new DeflaterOutputStream(_compressionBuffer);
 #else
         _compressionStream = new ZLibStream(_compressionBuffer, CompressionMode.Compress);

TelnetNegotiationCore/Protocols/NAWSProtocol.cs

@@ -278,8 +278,8 @@ private async ValueTask CompleteNAWSAsync(StateMachine<State, Trigger>.Transitio
             Array.Reverse(height);
         }
 
-        ClientWidth = BitConverter.ToInt16(width);
-        ClientHeight = BitConverter.ToInt16(height);
+        ClientWidth = BitConverter.ToInt16(width, 0);
+        ClientHeight = BitConverter.ToInt16(height, 0);
 
         context.Logger.LogDebug("Negotiated for: {clientWidth} width and {clientHeight} height", ClientWidth, ClientHeight);