.github/workflows/dotnet.yml

@@ -23,6 +23,7 @@ jobs:
       uses: actions/setup-dotnet@v3
       with:
         dotnet-version: |
+          3.1.x
           6.0.x
           7.0.x
 
@@ -35,6 +36,9 @@ jobs:
     - name: Build
       run: dotnet build --no-restore --configuration Release
 
+    - name: Test .NET Core 3.1
+      run: dotnet test --no-build --verbosity normal --configuration Release --framework netcoreapp3.1
+
     - name: Test .NET 6
       run: dotnet test --no-build --verbosity normal --configuration Release --framework net6.0
 

CHANGELOG.md

@@ -46,8 +46,6 @@ TypeInitializationException.
 
 - X10D: Removed `IEnumerable<T>.ConcatOne` - this functionality already exists with `Append`.
 - X10D: Removed `Endianness` enum.
-- X10D: Removed .NET Standard 2.1 target.
-- X10D.Hosting: Removed .NET Standard 2.1 target.
 - X10D.DSharpPlus: Complete sunset of library. This library will not be updated to support DSharpPlus v5.0.0 (#83).
 - X10D.Unity: Complete sunset of library. This library will not be updated effective immediately (#86).
 

X10D.Hosting/X10D.Hosting.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net7.0;net6.0</TargetFrameworks>
+        <TargetFrameworks>net7.0;net6.0;netstandard2.1</TargetFrameworks>
     </PropertyGroup>
 
     <ItemGroup>

X10D.Tests/X10D.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net7.0;net6.0</TargetFrameworks>
+        <TargetFrameworks>net7.0;net6.0;netcoreapp3.1</TargetFrameworks>
         <IsPackable>false</IsPackable>
         <IsTestProject>true</IsTestProject>
         <CoverletOutputFormat>json,cobertura</CoverletOutputFormat>

X10D.Tests/src/Collections/ByteTests.cs

@@ -47,6 +47,7 @@ internal class ByteTests
         });
     }
 
+#if NET5_0_OR_GREATER
     [Test]
     public void UnpackInternal_Fallback_ShouldUnpackToSpanCorrectly()
     {
@@ -91,6 +92,7 @@ internal class ByteTests
             Assert.That(bits[7], Is.True);
         });
     }
+#endif
 
     [Test]
     public void Unpack_ShouldRepackEqually()

X10D.Tests/src/Collections/Int16Tests.cs

@@ -1,4 +1,6 @@
+#if NET5_0_OR_GREATER
 using System.Runtime.Intrinsics.X86;
+#endif
 using NUnit.Framework;
 using X10D.Collections;
 
@@ -82,6 +84,7 @@ internal class Int16Tests
         });
     }
 
+#if NET5_0_OR_GREATER
     [Test]
     public void UnpackInternal_Ssse3_ShouldUnpackToSpanCorrectly()
     {
@@ -112,6 +115,7 @@ internal class Int16Tests
             }
         });
     }
+#endif
 
     [Test]
     public void Unpack_ShouldRepackEqually()

X10D.Tests/src/Collections/Int32Tests.cs

@@ -1,4 +1,6 @@
+#if NET5_0_OR_GREATER
 using System.Runtime.Intrinsics.X86;
+#endif
 using NUnit.Framework;
 using X10D.Collections;
 
@@ -83,6 +85,7 @@ internal class Int32Tests
         });
     }
 
+#if NET5_0_OR_GREATER
     [Test]
     public void UnpackInternal_Ssse3_ShouldUnpackToSpanCorrectly()
     {
@@ -142,6 +145,7 @@ internal class Int32Tests
             }
         });
     }
+#endif
 
     [Test]
     public void Unpack_ShouldRepackEqually()

X10D.Tests/src/Core/IntrinsicTests.cs

@@ -1,3 +1,4 @@
+#if NET6_0_OR_GREATER
 using System.Runtime.Intrinsics;
 using System.Runtime.Intrinsics.X86;
 using NUnit.Framework;
@@ -222,3 +223,4 @@ internal class IntrinsicTests
         Assert.That(result, Is.EqualTo(expectedResult));
     }
 }
+#endif

X10D.Tests/src/Core/SpanTest.cs

@@ -1,5 +1,7 @@
+#if NET5_0_OR_GREATER
 using System.Runtime.Intrinsics.Arm;
 using System.Runtime.Intrinsics.X86;
+#endif
 using NUnit.Framework;
 using X10D.Core;
 
@@ -207,6 +209,7 @@ internal class SpanTest
         Assert.That(actual, Is.EqualTo(expected));
     }
 
+#if NET5_0_OR_GREATER
     [Test]
     public void PackByteInternal_Sse2_ShouldReturnCorrectByte_GivenReadOnlySpan_Using()
     {
@@ -238,6 +241,7 @@ internal class SpanTest
 
         Assert.That(actual, Is.EqualTo(expected));
     }
+#endif
 
     [Test]
     public void PackInt16_ShouldReturnSameAsPackByte_WhenSpanHasLength8()
@@ -264,6 +268,7 @@ internal class SpanTest
         Assert.That(actual, Is.EqualTo(expected));
     }
 
+#if NET5_0_OR_GREATER
     [Test]
     public void PackInt16Internal_Sse2_ShouldReturnCorrectInt16_GivenReadOnlySpan_Using()
     {
@@ -282,6 +287,7 @@ internal class SpanTest
 
         Assert.That(actual, Is.EqualTo(expected));
     }
+#endif
 
     [Test]
     public void PackInt32Internal_Fallback_ShouldReturnCorrectInt32_GivenReadOnlySpan()
@@ -298,6 +304,7 @@ internal class SpanTest
         Assert.That(actual, Is.EqualTo(expected));
     }
 
+#if NET5_0_OR_GREATER
     [Test]
     public void PackInt32Internal_Sse2_ShouldReturnCorrectInt32_GivenReadOnlySpan()
     {
@@ -357,6 +364,7 @@ internal class SpanTest
 
         Assert.That(actual, Is.EqualTo(expected));
     }
+#endif
 
     [Test]
     public void PackInt32_ShouldReturnSameAsPackByte_WhenSpanHasLength8_UsingReadOnlySpan()

X10D.Tests/src/Drawing/ColorTests.cs

@@ -207,7 +207,9 @@ internal class ColorTests
             Assert.That(Color.Plum.GetClosestConsoleColor(), Is.EqualTo(ConsoleColor.DarkGray));
             Assert.That(Color.PowderBlue.GetClosestConsoleColor(), Is.EqualTo(ConsoleColor.DarkGray));
             Assert.That(Color.Purple.GetClosestConsoleColor(), Is.EqualTo(ConsoleColor.DarkMagenta));
+#if NET6_0_OR_GREATER
             Assert.That(Color.RebeccaPurple.GetClosestConsoleColor(), Is.EqualTo(ConsoleColor.DarkMagenta));
+#endif
             Assert.That(Color.Red.GetClosestConsoleColor(), Is.EqualTo(ConsoleColor.Red));
             Assert.That(Color.RosyBrown.GetClosestConsoleColor(), Is.EqualTo(ConsoleColor.DarkGray));
             Assert.That(Color.RoyalBlue.GetClosestConsoleColor(), Is.EqualTo(ConsoleColor.DarkCyan));

X10D.Tests/src/Text/CoreTests.cs

@@ -6,6 +6,7 @@ namespace X10D.Tests.Text;
 [TestFixture]
 internal class CoreTests
 {
+#if NET5_0_OR_GREATER
     [Test]
     public void ToJsonShouldNotBeEmpty()
     {
@@ -23,4 +24,5 @@ internal class CoreTests
         CollectionAssert.AreEqual(source, target);
         CollectionAssert.AreEquivalent(source, target);
     }
+#endif
 }

X10D.Tests/src/Text/RuneTests.cs

@@ -1,3 +1,4 @@
+#if NET5_0_OR_GREATER
 using System.Text;
 using NUnit.Framework;
 using X10D.Text;
@@ -89,3 +90,4 @@ internal class RuneTests
         Assert.That(repeated, Is.EqualTo("𐀀𐀀𐀀𐀀𐀀𐀀"));
     }
 }
+#endif

X10D.Tests/src/Text/StringTests.cs

@@ -1,5 +1,7 @@
 using System.Text;
+#if NET5_0_OR_GREATER
 using System.Text.Json.Serialization;
+#endif
 using NUnit.Framework;
 using X10D.Text;
 
@@ -416,6 +418,7 @@ internal class StringTests
         Assert.Throws<ArgumentException>(() => _ = " ".EnumParse<DayOfWeek>());
     }
 
+#if NET5_0_OR_GREATER
     [Test]
     public void FromJson_ShouldDeserializeCorrectly_GivenJsonString()
     {
@@ -431,6 +434,7 @@ internal class StringTests
             Assert.That(target.Values[2], Is.EqualTo(3));
         });
     }
+#endif
 
     [Test]
     public void GetBytes_ShouldReturnUtf8Bytes_GivenHelloWorld()
@@ -1002,9 +1006,11 @@ internal class StringTests
         });
     }
 
+#if NET5_0_OR_GREATER
     private struct SampleStructure
     {
         [JsonPropertyName("values")]
         public int[] Values { get; set; }
     }
+#endif
 }

X10D.Tests/src/Time/DateOnlyTests.cs

@@ -1,3 +1,4 @@
+#if NET6_0_OR_GREATER
 using NUnit.Framework;
 using X10D.Time;
 
@@ -229,3 +230,4 @@ internal class DateOnlyTests
         Assert.That(date.ToUnixTimeSeconds(time), Is.EqualTo(946684800));
     }
 }
+#endif

X10D.Tests/src/Time/DoubleTests.cs

@@ -1,3 +1,4 @@
+#if NET5_0_OR_GREATER
 using NUnit.Framework;
 using X10D.Time;
 
@@ -51,3 +52,4 @@ internal class DoubleTests
         Assert.That((_negativeOne).Weeks() < TimeSpan.Zero);
     }
 }
+#endif

X10D/X10D.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net7.0;net6.0</TargetFrameworks>
+        <TargetFrameworks>net7.0;net6.0;netstandard2.1</TargetFrameworks>
     </PropertyGroup>
 
     <ItemGroup>

X10D/src/Collections/ByteExtensions.cs

@@ -1,9 +1,12 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
+using X10D.CompilerServices;
+
+#if NETCOREAPP3_0_OR_GREATER
 using System.Runtime.Intrinsics;
 using System.Runtime.Intrinsics.X86;
-using X10D.CompilerServices;
+#endif
 
 namespace X10D.Collections;
 
@@ -43,11 +46,13 @@ public static class ByteExtensions
             throw new ArgumentException(ExceptionMessages.DestinationSpanLengthTooShort, nameof(destination));
         }
 
+#if NETCOREAPP3_0_OR_GREATER
         if (Sse3.IsSupported)
         {
             UnpackInternal_Ssse3(value, destination);
             return;
         }
+#endif
 
         UnpackInternal_Fallback(value, destination);
     }
@@ -61,6 +66,7 @@ public static class ByteExtensions
         }
     }
 
+#if NETCOREAPP3_0_OR_GREATER
     [MethodImpl(CompilerResources.MethodImplOptions)]
     internal unsafe static void UnpackInternal_Ssse3(this byte value, Span<bool> destination)
     {
@@ -81,4 +87,5 @@ public static class ByteExtensions
             Sse2.StoreScalar((long*)pDestination, correctness.AsInt64());
         }
     }
+#endif
 }

X10D/src/Collections/DictionaryExtensions.cs

@@ -1,5 +1,7 @@
 using System.Diagnostics.Contracts;
+#if NET6_0_OR_GREATER
 using System.Runtime.InteropServices;
+#endif
 using System.Web;
 
 namespace X10D.Collections;
@@ -45,10 +47,23 @@ public static class DictionaryExtensions
             throw new ArgumentNullException(nameof(updateValueFactory));
         }
 
+#if NET6_0_OR_GREATER
         ref var value = ref CollectionsMarshal.GetValueRefOrAddDefault(dictionary, key, out bool exists);
         // DO NOT CHANGE. reassigning value is necessary to mutate the dictionary, due to ref return above.
         // mutation of the dictionary is INTENDED BEHAVIOUR. this is not a mistake.
         return value = exists ? updateValueFactory(key, value!) : addValue;
+#else
+        if (dictionary.TryGetValue(key, out TValue? old))
+        {
+            TValue updated = updateValueFactory(key, old);
+            dictionary[key] = updated;
+
+            return updated;
+        }
+
+        dictionary.Add(key, addValue);
+        return addValue;
+#endif
     }
 
     /// <summary>
@@ -142,10 +157,25 @@ public static class DictionaryExtensions
             throw new ArgumentNullException(nameof(updateValueFactory));
         }
 
+#if NET6_0_OR_GREATER
         ref TValue? value = ref CollectionsMarshal.GetValueRefOrAddDefault(dictionary, key, out bool exists);
         // DO NOT CHANGE. reassigning value is necessary to mutate the dictionary, due to ref return above.
         // mutation of the dictionary is INTENDED BEHAVIOUR. this is not a mistake.
         return value = exists ? updateValueFactory(key, value!) : addValueFactory(key);
+#else
+        if (dictionary.TryGetValue(key, out TValue? old))
+        {
+            TValue updated = updateValueFactory(key, old);
+            dictionary[key] = updated;
+
+            return updated;
+        }
+
+        TValue add = addValueFactory(key);
+        dictionary.Add(key, add);
+
+        return add;
+#endif
     }
 
     /// <summary>
@@ -254,10 +284,25 @@ public static class DictionaryExtensions
             throw new ArgumentNullException(nameof(updateValueFactory));
         }
 
+#if NET6_0_OR_GREATER
         ref TValue? value = ref CollectionsMarshal.GetValueRefOrAddDefault(dictionary, key, out bool exists);
         // DO NOT CHANGE. reassigning value is necessary to mutate the dictionary, due to ref return above.
         // mutation of the dictionary is INTENDED BEHAVIOUR. this is not a mistake.
         return value = exists ? updateValueFactory(key, value!, factoryArgument) : addValueFactory(key, factoryArgument);
+#else
+        if (dictionary.TryGetValue(key, out TValue? old))
+        {
+            TValue updated = updateValueFactory(key, old, factoryArgument);
+            dictionary[key] = updated;
+
+            return updated;
+        }
+
+        TValue add = addValueFactory(key, factoryArgument);
+        dictionary.Add(key, add);
+
+        return add;
+#endif
     }
 
     /// <summary>

X10D/src/Collections/Int16Extensions.cs

@@ -1,9 +1,12 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
+using X10D.CompilerServices;
+
+#if NETCOREAPP3_0_OR_GREATER
 using System.Runtime.Intrinsics;
 using System.Runtime.Intrinsics.X86;
-using X10D.CompilerServices;
+#endif
 
 namespace X10D.Collections;
 
@@ -43,11 +46,13 @@ public static class Int16Extensions
             throw new ArgumentException(ExceptionMessages.DestinationSpanLengthTooShort, nameof(destination));
         }
 
+#if NETCOREAPP3_0_OR_GREATER
         if (Sse3.IsSupported)
         {
             UnpackInternal_Ssse3(value, destination);
             return;
         }
+#endif
 
         UnpackInternal_Fallback(value, destination);
     }
@@ -61,6 +66,7 @@ public static class Int16Extensions
         }
     }
 
+#if NETCOREAPP3_0_OR_GREATER
     [MethodImpl(CompilerResources.MethodImplOptions)]
     internal unsafe static void UnpackInternal_Ssse3(this short value, Span<bool> destination)
     {
@@ -83,4 +89,5 @@ public static class Int16Extensions
             Sse2.Store((byte*)pDestination, correctness);
         }
     }
+#endif
 }

X10D/src/Collections/Int32Extensions.cs

@@ -1,9 +1,12 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
+using X10D.CompilerServices;
+
+#if NETCOREAPP3_0_OR_GREATER
 using System.Runtime.Intrinsics;
 using System.Runtime.Intrinsics.X86;
-using X10D.CompilerServices;
+#endif
 
 namespace X10D.Collections;
 
@@ -43,6 +46,7 @@ public static class Int32Extensions
             throw new ArgumentException(ExceptionMessages.DestinationSpanLengthTooShort, nameof(destination));
         }
 
+#if NETCOREAPP3_0_OR_GREATER
         if (Avx2.IsSupported)
         {
             UnpackInternal_Avx2(value, destination);
@@ -54,6 +58,7 @@ public static class Int32Extensions
             UnpackInternal_Ssse3(value, destination);
             return;
         }
+#endif
 
         UnpackInternal_Fallback(value, destination);
     }
@@ -67,6 +72,7 @@ public static class Int32Extensions
         }
     }
 
+#if NETCOREAPP3_0_OR_GREATER
     [MethodImpl(CompilerResources.MethodImplOptions)]
     internal static unsafe void UnpackInternal_Ssse3(this int value, Span<bool> destination)
     {
@@ -124,4 +130,5 @@ public static class Int32Extensions
             Avx.Store((byte*)pDestination, correctness);
         }
     }
+#endif
 }

X10D/src/CompilerServices/CompilerResources.cs

@@ -4,6 +4,10 @@ namespace X10D.CompilerServices;
 
 internal static class CompilerResources
 {
+#if NETCOREAPP3_0_OR_GREATER
     public const MethodImplOptions MethodImplOptions = System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining |
                                                        System.Runtime.CompilerServices.MethodImplOptions.AggressiveOptimization;
+#else
+    public const MethodImplOptions MethodImplOptions = System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining;
+#endif
 }

X10D/src/Core/EnumExtensions.cs

@@ -20,7 +20,11 @@ public static class EnumExtensions
     public static T Next<T>(this T value)
         where T : struct, Enum
     {
+#if NET5_0_OR_GREATER
         T[] values = Enum.GetValues<T>();
+#else
+        T[] values = Enum.GetValues(typeof(T)).Cast<T>().ToArray();
+#endif
         int index = Array.IndexOf(values, value) + 1;
         index %= values.Length;
         return values[index];
@@ -40,7 +44,11 @@ public static class EnumExtensions
     public static T NextUnchecked<T>(this T value)
         where T : struct, Enum
     {
+#if NET5_0_OR_GREATER
         T[] values = Enum.GetValues<T>();
+#else
+        T[] values = Enum.GetValues(typeof(T)).Cast<T>().ToArray();
+#endif
         int index = Array.IndexOf(values, value) + 1;
         return values[index];
     }
@@ -58,7 +66,11 @@ public static class EnumExtensions
     public static T Previous<T>(this T value)
         where T : struct, Enum
     {
+#if NET5_0_OR_GREATER
         T[] values = Enum.GetValues<T>();
+#else
+        T[] values = Enum.GetValues(typeof(T)).Cast<T>().ToArray();
+#endif
         int index = Array.IndexOf(values, value) - 1;
         int length = values.Length;
 
@@ -82,7 +94,11 @@ public static class EnumExtensions
     public static T PreviousUnchecked<T>(this T value)
         where T : struct, Enum
     {
+#if NET5_0_OR_GREATER
         T[] values = Enum.GetValues<T>();
+#else
+        T[] values = Enum.GetValues(typeof(T)).Cast<T>().ToArray();
+#endif
         int index = Array.IndexOf(values, value) - 1;
         return values[index];
     }

X10D/src/Core/IntrinsicExtensions.cs

@@ -1,3 +1,4 @@
+#if NETCOREAPP3_0_OR_GREATER
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
@@ -187,3 +188,4 @@ public static class IntrinsicExtensions
         return Sse2.Shuffle(vector.AsDouble(), vector.AsDouble(), 0b01).AsUInt64();
     }
 }
+#endif

X10D/src/Core/IntrinsicUtility.cs

@@ -1,3 +1,5 @@
+#if NETCOREAPP3_0_OR_GREATER
+
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
@@ -182,22 +184,34 @@ public static class IntrinsicUtility
     [MethodImpl(CompilerResources.MethodImplOptions)]
     internal static Vector64<T> GetUninitializedVector64<T>() where T : struct
     {
+#if NET6_0_OR_GREATER
         Unsafe.SkipInit(out Vector64<T> output);
         return output;
+#else
+        return default;
+#endif
     }
 
     [MethodImpl(CompilerResources.MethodImplOptions)]
     internal static Vector128<T> GetUninitializedVector128<T>() where T : struct
     {
+#if NET6_0_OR_GREATER
         Unsafe.SkipInit(out Vector128<T> output);
         return output;
+#else
+        return default;
+#endif
     }
 
     [MethodImpl(CompilerResources.MethodImplOptions)]
     internal static Vector256<T> GetUninitializedVector256<T>() where T : struct
     {
+#if NET6_0_OR_GREATER
         Unsafe.SkipInit(out Vector256<T> output);
         return output;
+#else
+        return default;
+#endif
     }
 
     [Pure]
@@ -301,3 +315,5 @@ public static class IntrinsicUtility
         return Avx2.Add(high, ac);
     }
 }
+
+#endif

X10D/src/Core/RandomExtensions.cs

@@ -11,6 +11,10 @@ namespace X10D.Core;
 /// </summary>
 public static class RandomExtensions
 {
+#if !NET6_0_OR_GREATER
+    private static readonly Random Shared = new();
+#endif
+
     /// <summary>
     ///     Returns a random value that defined in a specified enum.
     /// </summary>
@@ -501,6 +505,10 @@ public static class RandomExtensions
 
     internal static Random GetShared()
     {
+#if NET6_0_OR_GREATER
         return Random.Shared;
+#else
+        return Shared;
+#endif
     }
 }

X10D/src/Core/SpanExtensions.cs

@@ -2,10 +2,13 @@ using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using X10D.CompilerServices;
+
+#if NETCOREAPP3_0_OR_GREATER
 using System.Runtime.Intrinsics;
 using System.Runtime.Intrinsics.X86;
 using System.Runtime.Intrinsics.Arm;
-using X10D.CompilerServices;
+#endif
 
 #if NET7_0_OR_GREATER
 using System.Diagnostics;
@@ -18,6 +21,7 @@ namespace X10D.Core;
 /// </summary>
 public static class SpanExtensions
 {
+#if NETCOREAPP3_0_OR_GREATER
     private const ulong IntegerPackingMagic = 0x0102040810204080;
 
     [ExcludeFromCodeCoverage]
@@ -31,6 +35,7 @@ public static class SpanExtensions
     {
         get => Vector256.Create(IntegerPackingMagic);
     }
+#endif
 
     /// <summary>
     ///     Returns a value indicating whether a specific enumeration value is contained with the current span of elements.
@@ -66,6 +71,7 @@ public static class SpanExtensions
     [MethodImpl(CompilerResources.MethodImplOptions)]
     public static bool Contains<T>(this ReadOnlySpan<T> span, T value) where T : struct, Enum
     {
+#if NET6_0_OR_GREATER
         unsafe
         {
 #pragma warning disable CS8500
@@ -108,6 +114,17 @@ public static class SpanExtensions
                 // dotcover enable
             }
         }
+#else
+        foreach (var it in span)
+        {
+            if (EqualityComparer<T>.Default.Equals(it, value))
+            {
+                return true;
+            }
+        }
+
+        return false;
+#endif
     }
 
     /// <summary>
@@ -144,6 +161,7 @@ public static class SpanExtensions
             return PackByteInternal_Fallback(source);
         }
 
+#if NETCOREAPP3_0_OR_GREATER
         if (!BitConverter.IsLittleEndian)
         {
             return PackByteInternal_Fallback(source);
@@ -158,6 +176,7 @@ public static class SpanExtensions
         {
             return PackByteInternal_AdvSimd(source);
         }
+#endif
 
         return PackByteInternal_Fallback(source);
     }
@@ -198,10 +217,12 @@ public static class SpanExtensions
                     goto default;
                 }
 
+#if NETCOREAPP3_0_OR_GREATER
                 if (Sse2.IsSupported)
                 {
                     return PackInt16Internal_Sse2(source);
                 }
+#endif
 
                 goto default;
             case < 16:
@@ -248,6 +269,7 @@ public static class SpanExtensions
                 return PackInt16(source);
 
             case 32:
+#if NETCOREAPP3_0_OR_GREATER
                 if (!BitConverter.IsLittleEndian)
                 {
                     goto default;
@@ -267,6 +289,7 @@ public static class SpanExtensions
                 {
                     return PackInt32Internal_AdvSimd(source);
                 }
+#endif
                 goto default;
 
             default:
@@ -360,6 +383,7 @@ public static class SpanExtensions
         return result;
     }
 
+#if NETCOREAPP3_0_OR_GREATER
     [Pure]
     [MethodImpl(CompilerResources.MethodImplOptions)]
     internal static byte PackByteInternal_Sse2(this ReadOnlySpan<bool> source)
@@ -475,6 +499,7 @@ public static class SpanExtensions
         }
     }
 
+#if NET5_0_OR_GREATER
     // dotcover disable
     //NOSONAR
     [Pure]
@@ -492,4 +517,6 @@ public static class SpanExtensions
     }
     //NOSONAR
     // dotcover enable
+#endif
+#endif
 }

X10D/src/Drawing/ColorExtensions.cs

@@ -72,9 +72,17 @@ public static class ColorExtensions
         double blue = color.B;
         var delta = double.MaxValue;
 
+#if NET5_0_OR_GREATER
         foreach (ConsoleColor consoleColor in Enum.GetValues<ConsoleColor>())
+#else
+        foreach (ConsoleColor consoleColor in Enum.GetValues(typeof(ConsoleColor)))
+#endif
         {
+#if NET5_0_OR_GREATER
             string name = Enum.GetName(consoleColor)!;
+#else
+            string name = Enum.GetName(typeof(ConsoleColor), consoleColor)!;
+#endif
             Color currentColor = Color.FromName(name == "DarkYellow" ? "Orange" : name); // bug fix
             double r = currentColor.R - red;
             double g = currentColor.G - green;

X10D/src/IO/DecimalExtensions.cs

@@ -93,7 +93,13 @@ public static class DecimalExtensions
 
     private static void GetBits(decimal value, Span<int> destination)
     {
+#if NET5_0_OR_GREATER
         _ = decimal.GetBits(value, destination);
+#else
+        Span<byte> buffer = stackalloc byte[16];
+        MemoryMarshal.Write(buffer, ref value);
+        WriteBits(destination, buffer);
+#endif
     }
 
 #if !NET5_0_OR_GREATER

X10D/src/IO/DoubleExtensions.cs

@@ -43,7 +43,17 @@ public static class DoubleExtensions
     /// <returns><see langword="true" /> if the conversion was successful; otherwise, <see langword="false" />.</returns>
     public static bool TryWriteBigEndianBytes(this double value, Span<byte> destination)
     {
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.TryWriteDoubleBigEndian(destination, value);
+#else
+        if (BitConverter.IsLittleEndian)
+        {
+            long tmp = BinaryPrimitives.ReverseEndianness(BitConverter.DoubleToInt64Bits(value));
+            return MemoryMarshal.TryWrite(destination, ref tmp);
+        }
+
+        return MemoryMarshal.TryWrite(destination, ref value);
+#endif
     }
 
     /// <summary>
@@ -54,6 +64,16 @@ public static class DoubleExtensions
     /// <returns><see langword="true" /> if the conversion was successful; otherwise, <see langword="false" />.</returns>
     public static bool TryWriteLittleEndianBytes(this double value, Span<byte> destination)
     {
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.TryWriteDoubleLittleEndian(destination, value);
+#else
+        if (BitConverter.IsLittleEndian)
+        {
+            return MemoryMarshal.TryWrite(destination, ref value);
+        }
+
+        long tmp = BinaryPrimitives.ReverseEndianness(BitConverter.DoubleToInt64Bits(value));
+        return MemoryMarshal.TryWrite(destination, ref tmp);
+#endif
     }
 }

X10D/src/IO/SingleExtensions.cs

@@ -45,7 +45,17 @@ public static class SingleExtensions
     /// <returns><see langword="true" /> if the conversion was successful; otherwise, <see langword="false" />.</returns>
     public static bool TryWriteBigEndianBytes(this float value, Span<byte> destination)
     {
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.TryWriteSingleBigEndian(destination, value);
+#else
+        if (BitConverter.IsLittleEndian)
+        {
+            int tmp = BinaryPrimitives.ReverseEndianness(BitConverter.SingleToInt32Bits(value));
+            return MemoryMarshal.TryWrite(destination, ref tmp);
+        }
+
+        return MemoryMarshal.TryWrite(destination, ref value);
+#endif
     }
 
     /// <summary>
@@ -56,6 +66,16 @@ public static class SingleExtensions
     /// <returns><see langword="true" /> if the conversion was successful; otherwise, <see langword="false" />.</returns>
     public static bool TryWriteLittleEndianBytes(this float value, Span<byte> destination)
     {
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.TryWriteSingleLittleEndian(destination, value);
+#else
+        if (BitConverter.IsLittleEndian)
+        {
+            return MemoryMarshal.TryWrite(destination, ref value);
+        }
+
+        int tmp = BinaryPrimitives.ReverseEndianness(BitConverter.SingleToInt32Bits(value));
+        return MemoryMarshal.TryWrite(destination, ref tmp);
+#endif
     }
 }

X10D/src/IO/StreamExtensions.Reading.cs

@@ -1,4 +1,5 @@
 using System.Buffers.Binary;
+using System.Runtime.InteropServices;
 
 namespace X10D.IO;
 
@@ -39,7 +40,11 @@ public static partial class StreamExtensions
             buffer.Reverse();
         }
 
+#if NET5_0_OR_GREATER
         return new decimal(buffer);
+#else
+        return new decimal(buffer.ToArray());
+#endif
     }
 
     /// <summary>
@@ -77,7 +82,11 @@ public static partial class StreamExtensions
             buffer.Reverse();
         }
 
+#if NET5_0_OR_GREATER
         return new decimal(buffer);
+#else
+        return new decimal(buffer.ToArray());
+#endif
     }
 
     /// <summary>
@@ -101,7 +110,16 @@ public static partial class StreamExtensions
 
         Span<byte> buffer = stackalloc byte[8];
         _ = stream.Read(buffer);
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.ReadDoubleBigEndian(buffer);
+#else
+        if (BitConverter.IsLittleEndian)
+        {
+            buffer.Reverse();
+        }
+
+        return MemoryMarshal.Read<double>(buffer);
+#endif
     }
 
     /// <summary>
@@ -126,7 +144,16 @@ public static partial class StreamExtensions
 
         Span<byte> buffer = stackalloc byte[8];
         _ = stream.Read(buffer);
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.ReadDoubleLittleEndian(buffer);
+#else
+        if (!BitConverter.IsLittleEndian)
+        {
+            buffer.Reverse();
+        }
+
+        return MemoryMarshal.Read<double>(buffer);
+#endif
     }
 
     /// <summary>
@@ -296,7 +323,16 @@ public static partial class StreamExtensions
 
         Span<byte> buffer = stackalloc byte[4];
         _ = stream.Read(buffer);
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.ReadSingleBigEndian(buffer);
+#else
+        if (BitConverter.IsLittleEndian)
+        {
+            buffer.Reverse();
+        }
+
+        return MemoryMarshal.Read<float>(buffer);
+#endif
     }
 
     /// <summary>
@@ -322,7 +358,16 @@ public static partial class StreamExtensions
 
         Span<byte> buffer = stackalloc byte[4];
         _ = stream.Read(buffer);
+#if NET5_0_OR_GREATER
         return BinaryPrimitives.ReadSingleLittleEndian(buffer);
+#else
+        if (!BitConverter.IsLittleEndian)
+        {
+            buffer.Reverse();
+        }
+
+        return MemoryMarshal.Read<float>(buffer);
+#endif
     }
 
     /// <summary>

X10D/src/Linq/EnumerableExtensions.cs

@@ -1,4 +1,6 @@
+#if NET5_0_OR_GREATER
 using System.Runtime.InteropServices;
+#endif
 
 namespace X10D.Linq;
 
@@ -387,9 +389,11 @@ public static class EnumerableExtensions
                 span = array;
                 break;
 
+#if NET5_0_OR_GREATER
             case List<TSource> list:
                 span = CollectionsMarshal.AsSpan(list);
                 break;
+#endif
 
             default:
                 span = default;

X10D/src/Math/MathUtility.cs

@@ -253,7 +253,15 @@ public static class MathUtility
     public static float Pulse(float value, float lowerBound, float upperBound)
     {
         bool result = lowerBound <= value && value <= upperBound;
+#if NET6_0_OR_GREATER
         return Unsafe.As<bool, int>(ref result);
+#else
+        unsafe
+        {
+            var pResult = (int*)&result;
+            return *pResult;
+        }
+#endif
     }
 
     /// <summary>
@@ -269,7 +277,15 @@ public static class MathUtility
     public static double Pulse(double value, double lowerBound, double upperBound)
     {
         bool result = lowerBound <= value && value <= upperBound;
+#if NET6_0_OR_GREATER
         return Unsafe.As<bool, int>(ref result);
+#else
+        unsafe
+        {
+            var pResult = (int*)&result;
+            return *pResult;
+        }
+#endif
     }
 
     /// <summary>

X10D/src/Numerics/UInt32Extensions.cs

@@ -24,7 +24,20 @@ public static class UInt32Extensions
     [MethodImpl(CompilerResources.MethodImplOptions)]
     public static int PopCount(this uint value)
     {
+#if NETCOREAPP3_1_OR_GREATER
         return BitOperations.PopCount(value);
+#else
+        const uint c1 = 0x_55555555u;
+        const uint c2 = 0x_33333333u;
+        const uint c3 = 0x_0F0F0F0Fu;
+        const uint c4 = 0x_01010101u;
+
+        value -= (value >> 1) & c1;
+        value = (value & c2) + ((value >> 2) & c2);
+        value = (((value + (value >> 4)) & c3) * c4) >> 24;
+
+        return (int)value;
+#endif
     }
 
     /// <summary>
@@ -69,6 +82,17 @@ public static class UInt32Extensions
     [MethodImpl(CompilerResources.MethodImplOptions)]
     public static uint RoundUpToPowerOf2(this uint value)
     {
+#if NET6_0_OR_GREATER
         return BitOperations.RoundUpToPowerOf2(value);
+#else
+        // Based on https://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
+        --value;
+        value |= value >> 1;
+        value |= value >> 2;
+        value |= value >> 4;
+        value |= value >> 8;
+        value |= value >> 16;
+        return value + 1;
+#endif
     }
 }

X10D/src/Numerics/UInt64Extensions.cs

@@ -24,7 +24,20 @@ public static class UInt64Extensions
     [MethodImpl(CompilerResources.MethodImplOptions)]
     public static int PopCount(this ulong value)
     {
+#if NETCOREAPP3_1_OR_GREATER
         return BitOperations.PopCount(value);
+#else
+        const ulong c1 = 0x_55555555_55555555ul;
+        const ulong c2 = 0x_33333333_33333333ul;
+        const ulong c3 = 0x_0F0F0F0F_0F0F0F0Ful;
+        const ulong c4 = 0x_01010101_01010101ul;
+
+        value -= (value >> 1) & c1;
+        value = (value & c2) + ((value >> 2) & c2);
+        value = (((value + (value >> 4)) & c3) * c4) >> 56;
+
+        return (int)value;
+#endif
     }
 
     /// <summary>
@@ -69,6 +82,18 @@ public static class UInt64Extensions
     [MethodImpl(CompilerResources.MethodImplOptions)]
     public static ulong RoundUpToPowerOf2(this ulong value)
     {
+#if NET6_0_OR_GREATER
         return BitOperations.RoundUpToPowerOf2(value);
+#else
+        // Based on https://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
+        --value;
+        value |= value >> 1;
+        value |= value >> 2;
+        value |= value >> 4;
+        value |= value >> 8;
+        value |= value >> 16;
+        value |= value >> 32;
+        return value + 1;
+#endif
     }
 }

X10D/src/Text/EnumerableExtensions.cs

@@ -55,10 +55,12 @@ public static class EnumerableExtensions
             throw new ArgumentNullException(nameof(pattern));
         }
 
+#if NET6_0_OR_GREATER
         if (source.TryGetNonEnumeratedCount(out int count) && count == 0)
         {
             yield break;
         }
+#endif
 
         var options = RegexOptions.Compiled | (ignoreCase ? RegexOptions.IgnoreCase : RegexOptions.None);
         var regex = new Regex(pattern, options, Regex.InfiniteMatchTimeout);

X10D/src/Text/Extensions.cs

@@ -1,3 +1,4 @@
+#if NET5_0_OR_GREATER
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 using System.Text.Json;
@@ -24,3 +25,4 @@ public static class Extensions
         return JsonSerializer.Serialize(value, options);
     }
 }
+#endif

X10D/src/Text/RuneExtensions.cs

@@ -1,3 +1,4 @@
+#if NETCOREAPP3_0_OR_GREATER
 using System.Diagnostics;
 using System.Diagnostics.Contracts;
 using System.Globalization;
@@ -105,10 +106,11 @@ public static class RuneExtensions
 #if NET7_0_OR_GREATER
                 throw new UnreachableException(message);
 #else
-                throw new InvalidOperationException(message);
+            throw new InvalidOperationException(message);
 #endif
             //NOSONAR
             // dotcover enable
         }
     }
 }
+#endif

X10D/src/Text/StringExtensions.cs

@@ -2,7 +2,9 @@ using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 using System.Text;
+#if NET5_0_OR_GREATER
 using System.Text.Json;
+#endif
 using X10D.Collections;
 using X10D.CompilerServices;
 using X10D.Core;
@@ -573,6 +575,7 @@ public static class StringExtensions
         return Enum.Parse<T>(value, ignoreCase);
     }
 
+#if NET5_0_OR_GREATER
     /// <summary>
     ///     Returns an object from the specified JSON string.
     /// </summary>
@@ -587,6 +590,7 @@ public static class StringExtensions
     {
         return JsonSerializer.Deserialize<T>(value, options);
     }
+#endif
 
     /// <summary>
     ///     Gets a <see cref="byte" />[] representing the value the <see cref="string" /> with
@@ -684,6 +688,7 @@ public static class StringExtensions
 
         for (var index = 0; index < value.Length; index++)
         {
+#if NETCOREAPP3_0_OR_GREATER
             var rune = new Rune(value[index]);
 
             if (!Rune.IsLetter(rune))
@@ -695,6 +700,19 @@ public static class StringExtensions
             {
                 return false;
             }
+#else
+            char current = value[index];
+
+            if (!char.IsLetter(current))
+            {
+                continue;
+            }
+
+            if (!char.IsLower(current))
+            {
+                return false;
+            }
+#endif
         }
 
         return true;
@@ -759,6 +777,7 @@ public static class StringExtensions
 
         for (int index = 0, endIndex = value.Length - 1; index < value.Length; index++, endIndex--)
         {
+#if NETCOREAPP3_0_OR_GREATER
             Rune startRune = new Rune(value[index]);
             Rune endRune = new Rune(value[endIndex]);
 
@@ -778,6 +797,27 @@ public static class StringExtensions
             {
                 return false;
             }
+#else
+            char startChar = value[index];
+            char endChar = value[endIndex];
+
+            if (!char.IsLetter(startChar) && !char.IsNumber(startChar))
+            {
+                endIndex++;
+                continue;
+            }
+
+            if (!char.IsLetter(endChar) && !char.IsNumber(endChar))
+            {
+                index--;
+                continue;
+            }
+
+            if (char.ToUpperInvariant(startChar) != char.ToUpperInvariant(endChar))
+            {
+                return false;
+            }
+#endif
         }
 
         return true;
@@ -801,6 +841,7 @@ public static class StringExtensions
 
         for (var index = 0; index < value.Length; index++)
         {
+#if NETCOREAPP3_0_OR_GREATER
             var rune = new Rune(value[index]);
 
             if (!Rune.IsLetter(rune))
@@ -812,6 +853,19 @@ public static class StringExtensions
             {
                 return false;
             }
+#else
+            char current = value[index];
+
+            if (!char.IsLetter(current))
+            {
+                continue;
+            }
+
+            if (!char.IsUpper(current))
+            {
+                return false;
+            }
+#endif
         }
 
         return true;

X10D/src/Time/DateOnlyExtensions.cs

@@ -1,3 +1,4 @@
+#if NET6_0_OR_GREATER
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
@@ -197,3 +198,4 @@ public static class DateOnlyExtensions
         return value.ToDateTime(time).ToUnixTimeSeconds();
     }
 }
+#endif

X10D/src/Time/HalfExtensions.cs

@@ -1,3 +1,4 @@
+#if NET5_0_OR_GREATER
 using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 using X10D.CompilerServices;
@@ -91,3 +92,4 @@ public static class HalfExtensions
         return TimeSpan.FromDays((float)value * 7);
     }
 }
+#endif

tools/Benchmarks/Benchmarks.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net7.0;net6.0</TargetFrameworks>
+        <TargetFrameworks>net7.0;net6.0;netcoreapp3.1</TargetFrameworks>
         <Configuration>Release</Configuration>
         <OutputType>Exe</OutputType>
         <Optimize>true</Optimize>