Fix target-gated `#if`s to cover every applicable TFM

src/BizHawk.Client.Common/Api/Classes/UserDataApi.cs

@@ -2,19 +2,26 @@
 using System.Collections.Generic;
 using System.Linq;
 
+#if NET5_0_OR_GREATER
+using KeyCollectionType = System.Collections.Generic.IReadOnlySet<string>;
+#else
+using KeyCollectionType = System.Collections.Generic.IReadOnlyCollection<string>;
+#endif
+
 namespace BizHawk.Client.Common
 {
 	public sealed class UserDataApi : IUserDataApi
 	{
 		private readonly IMovieSession _movieSession;
 
-#if NET6_0
-		public IReadOnlySet<string> Keys
-			=> throw new NotImplementedException();
-#else
-		public IReadOnlyCollection<string> Keys
-			=> _movieSession.UserBag.Keys.ToList();
-#endif
+		public KeyCollectionType Keys
+		{
+			get
+			{
+				ICollection<string> keys = _movieSession.UserBag.Keys;
+				return (keys as KeyCollectionType) ?? keys.ToList();
+			}
+		}
 
 		public UserDataApi(IMovieSession movieSession) => _movieSession = movieSession;
 

src/BizHawk.Client.Common/Api/Interfaces/IUserDataApi.cs

@@ -4,7 +4,7 @@ namespace BizHawk.Client.Common
 {
 	public interface IUserDataApi : IExternalApi
 	{
-#if NET6_0
+#if NET5_0_OR_GREATER
 		IReadOnlySet<string> Keys { get; }
 #else
 		IReadOnlyCollection<string> Keys { get; }

src/BizHawk.Client.Common/movie/bk2/Bk2Controller.cs

@@ -100,7 +100,7 @@ namespace BizHawk.Client.Common
 					else if (key.IsAxis)
 					{
 						var commaIndex = mnemonic.IndexOf(',', iterator);
-#if NET6_0_OR_GREATER
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
 						var val = int.Parse(mnemonic.AsSpan(start: iterator, length: commaIndex - iterator));
 #else
 						var axisValueString = mnemonic.Substring(startIndex: iterator, length: commaIndex - iterator);

src/BizHawk.Client.Common/tools/Watch/WatchList/WatchList.cs

@@ -149,7 +149,7 @@ namespace BizHawk.Client.Common
 		/// </param>
 		public void InsertRange(int index, IEnumerable<Watch> collection)
 		{
-#if NET6_0
+#if NET6_0_OR_GREATER
 			if (collection.TryGetNonEnumeratedCount(out var n) && n is 0) return;
 #else
 			if (collection is ICollection<Watch> hasCount && hasCount.Count is 0) return;

src/BizHawk.Common/Extensions/CollectionExtensions.cs

@@ -146,7 +146,7 @@ namespace BizHawk.Common.CollectionExtensions
 				? countable.Count == n
 				: collection.Take(n + 1).Count() == n;
 
-#if !NET6_0
+#if !(NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_0_OR_GREATER)
 		/// <summary>
 		/// Returns the value at <paramref name="key"/>.
 		/// If the key is not present, returns default(TValue).

src/BizHawk.Common/MemoryBlock/MemoryViewStream.cs

@@ -60,7 +60,7 @@ namespace BizHawk.Common
 		private byte* CurrentPointer()
 			=> (byte*)Z.SS(_ptr + _pos);
 
-#if NET6_0
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
 		public override int Read(Span<byte> buffer)
 #else
 		public int Read(Span<byte> buffer)
@@ -110,7 +110,7 @@ namespace BizHawk.Common
 		public override void SetLength(long value)
 			=> throw new IOException();
 
-#if NET6_0
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
 		public override void Write(ReadOnlySpan<byte> buffer)
 #else
 		public void Write(ReadOnlySpan<byte> buffer)

src/BizHawk.Common/Util.cs

@@ -5,7 +5,7 @@ using System.IO;
 using System.IO.Compression;
 using System.Linq;
 using System.Reflection;
-#if NET6_0
+#if NETCOREAPP3_0_OR_GREATER
 using System.Runtime.CompilerServices;
 #endif
 using System.Threading;
@@ -81,7 +81,7 @@ namespace BizHawk.Common
 			return a.All(kvp => b.TryGetValue(kvp.Key, out var bVal) && comparer.Equals(kvp.Value, bVal));
 		}
 
-#if NET6_0
+#if NETCOREAPP3_0_OR_GREATER
 		public static string DescribeIsNull<T>(T? obj, [CallerArgumentExpression("obj")] string? expr = default)
 #else
 		public static string DescribeIsNull<T>(T? obj, string expr)
@@ -89,7 +89,7 @@ namespace BizHawk.Common
 			where T : class
 			=> $"{expr} is {(obj is null ? "null" : "not null")}";
 
-#if NET6_0
+#if NETCOREAPP3_0_OR_GREATER
 		public static string DescribeIsNullValT<T>(T? boxed, [CallerArgumentExpression("boxed")] string? expr = default)
 #else
 		public static string DescribeIsNullValT<T>(T? boxed, string expr)

src/BizHawk.Common/checksums/MD5Checksum.cs

@@ -19,7 +19,7 @@ namespace BizHawk.Common
 
 		public /*static readonly*/const string EmptyFile = "D41D8CD98F00B204E9800998ECF8427E";
 
-#if NET6_0
+#if NET5_0_OR_GREATER
 		public static byte[] Compute(ReadOnlySpan<byte> data)
 			=> MD5.HashData(data);
 #else

src/BizHawk.Common/checksums/SHA1Checksum.cs

@@ -25,17 +25,9 @@ namespace BizHawk.Common
 
 		public /*static readonly*/const string Zero = "0000000000000000000000000000000000000000";
 
-#if NET6_0
+#if NET5_0_OR_GREATER
 		public static byte[] Compute(ReadOnlySpan<byte> data)
 			=> SHA1.HashData(data);
-
-		public static byte[] ComputeConcat(ReadOnlySpan<byte> dataA, ReadOnlySpan<byte> dataB)
-		{
-			using var impl = IncrementalHash.CreateHash(HashAlgorithmName.SHA1);
-			impl.AppendData(dataA);
-			impl.AppendData(dataB);
-			return impl.GetHashAndReset();
-		}
 #else
 		private static unsafe byte[] UnmanagedImpl(byte[] buffer)
 		{
@@ -88,7 +80,17 @@ namespace BizHawk.Common
 
 		public static byte[] Compute(ReadOnlySpan<byte> data)
 			=> Compute(data.ToArray());
+#endif
 
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
+		public static byte[] ComputeConcat(ReadOnlySpan<byte> dataA, ReadOnlySpan<byte> dataB)
+		{
+			using var impl = IncrementalHash.CreateHash(HashAlgorithmName.SHA1);
+			impl.AppendData(dataA);
+			impl.AppendData(dataB);
+			return impl.GetHashAndReset();
+		}
+#else
 		public static byte[] ComputeConcat(ReadOnlySpan<byte> dataA, ReadOnlySpan<byte> dataB)
 			=> ComputeConcat(dataA.ToArray(), dataB.ToArray());
 #endif

src/BizHawk.Common/checksums/SHA256Checksum.cs

@@ -19,7 +19,7 @@ namespace BizHawk.Common
 
 		public /*static readonly*/const string EmptyFile = "E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649B934CA495991B7852B855";
 
-#if NET6_0
+#if NET5_0_OR_GREATER
 		public static byte[] Compute(ReadOnlySpan<byte> data)
 			=> SHA256.HashData(data);
 #else

src/BizHawk.Tests/Client.Common/config/SerializationStabilityTests.cs

@@ -20,7 +20,7 @@ namespace BizHawk.Tests.Client.Common.config
 
 		private const string ZWINDER_SER = @"{""CurrentUseCompression"":false,""CurrentBufferSize"":256,""CurrentTargetFrameLength"":500,""CurrentStoreType"":0,""RecentUseCompression"":false,""RecentBufferSize"":128,""RecentTargetFrameLength"":2000,""RecentStoreType"":0,""GapsUseCompression"":false,""GapsBufferSize"":64,""GapsTargetFrameLength"":125,""GapsStoreType"":0,""AncientStateInterval"":5000,""AncientStoreType"":0}";
 
-#if NET6_0
+#if NET5_0_OR_GREATER
 		private static readonly IReadOnlySet<Type> KnownGoodFromStdlib = new HashSet<Type>
 #else
 		private static readonly ICollection<Type> KnownGoodFromStdlib = new HashSet<Type>