diff --git a/QRCoder/Attributes/NotNullWhenAttribute.cs b/QRCoder/Attributes/NotNullWhenAttribute.cs
index 3cc3ce1c..258826ac 100644
--- a/QRCoder/Attributes/NotNullWhenAttribute.cs
+++ b/QRCoder/Attributes/NotNullWhenAttribute.cs
@@ -1,4 +1,4 @@
-#if !NETCOREAPP
+#if !NETCOREAPP && !NETSTANDARD2_1
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
diff --git a/QRCoder/PngByteQRCode.cs b/QRCoder/PngByteQRCode.cs
index 2ad015b7..297712e5 100644
--- a/QRCoder/PngByteQRCode.cs
+++ b/QRCoder/PngByteQRCode.cs
@@ -1,4 +1,4 @@
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
 using System.Buffers;
 #endif
 using System;
@@ -41,7 +41,7 @@ public byte[] GetGraphic(int pixelsPerModule, bool drawQuietZones = true)
         png.WriteHeader(size, size, 1, PngBuilder.ColorType.Greyscale);
         var scanLines = DrawScanlines(pixelsPerModule, drawQuietZones);
         png.WriteScanlines(scanLines);
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
         ArrayPool<byte>.Shared.Return(scanLines.Array!);
 #endif
         png.WriteEnd();
@@ -77,7 +77,7 @@ public byte[] GetGraphic(int pixelsPerModule, byte[] darkColorRgba, byte[] light
         png.WritePalette(darkColorRgba, lightColorRgba);
         var scanLines = DrawScanlines(pixelsPerModule, drawQuietZones);
         png.WriteScanlines(scanLines);
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
         ArrayPool<byte>.Shared.Return(scanLines.Array!);
 #endif
         png.WriteEnd();
@@ -97,7 +97,7 @@ private ArraySegment<byte> DrawScanlines(int pixelsPerModule, bool drawQuietZone
         var quietZoneOffset = (drawQuietZones ? 0 : 4);
         var bytesPerScanline = (matrixSize * pixelsPerModule + 7) / 8 + 1; // A monochrome scanline is one byte for filter type then one bit per pixel.
         var scanLinesLength = bytesPerScanline * matrixSize * pixelsPerModule;
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
         var scanlines = ArrayPool<byte>.Shared.Rent(scanLinesLength);
         Array.Clear(scanlines, 0, scanLinesLength);
 #else
diff --git a/QRCoder/QRCodeData.cs b/QRCoder/QRCodeData.cs
index ffd2ca8b..10a7a67b 100644
--- a/QRCoder/QRCodeData.cs
+++ b/QRCoder/QRCodeData.cs
@@ -152,7 +152,7 @@ public byte[] GetRawData(Compression compressMode)
         try
         {
             //Add header - signature ("QRR")
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
             targetStream.Write([0x51, 0x52, 0x52, 0x00]);
 #else
             targetStream.Write(new byte[] { 0x51, 0x52, 0x52, 0x00 }, 0, 4);
diff --git a/QRCoder/QRCodeGenerator.cs b/QRCoder/QRCodeGenerator.cs
index 3c598f8e..40890d5e 100644
--- a/QRCoder/QRCodeGenerator.cs
+++ b/QRCoder/QRCodeGenerator.cs
@@ -1,5 +1,5 @@
 using System;
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
 using System.Buffers;
 #endif
 using System.Collections;
@@ -586,7 +586,7 @@ private static void TrimLeadingZeros(BitArray fStrEcc, ref int index, ref int co
 
     private static void ShiftTowardsBit0(BitArray fStrEcc, int num)
     {
-#if NETCOREAPP
+#if HAS_SPAN
         fStrEcc.RightShift(num); // Shift towards bit 0
 #else
         for (var i = 0; i < fStrEcc.Length - num; i++)
@@ -598,7 +598,7 @@ private static void ShiftTowardsBit0(BitArray fStrEcc, int num)
 
     private static void ShiftAwayFromBit0(BitArray fStrEcc, int num)
     {
-#if NETCOREAPP
+#if HAS_SPAN
         fStrEcc.LeftShift(num); // Shift away from bit 0
 #else
         for (var i = fStrEcc.Length - 1; i >= num; i--)
@@ -697,7 +697,7 @@ private static ArraySegment<byte> CalculateECCWords(BitArray bitArray, int offse
         generatorPolynom.Dispose();
 
         // Convert the resulting polynomial into a byte array representing the ECC codewords.
-#if NETCOREAPP
+#if HAS_SPAN
         var array = ArrayPool<byte>.Shared.Rent(leadTermSource.Count);
         var ret = new ArraySegment<byte>(array, 0, leadTermSource.Count);
 #else
@@ -1009,7 +1009,7 @@ private static BitArray PlainTextToBinaryNumeric(string plainText)
         for (int i = 0; i < plainText.Length - 2; i += 3)
         {
             // Parse the next three characters as a decimal integer.
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
             var dec = int.Parse(plainText.AsSpan(i, 3), NumberStyles.None, CultureInfo.InvariantCulture);
 #else
             var dec = int.Parse(plainText.Substring(i, 3), NumberStyles.None, CultureInfo.InvariantCulture);
@@ -1021,7 +1021,7 @@ private static BitArray PlainTextToBinaryNumeric(string plainText)
         // Handle any remaining digits if the total number is not a multiple of three.
         if (plainText.Length % 3 == 2)  // Two remaining digits are encoded in 7 bits.
         {
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
             var dec = int.Parse(plainText.AsSpan(plainText.Length / 3 * 3, 2), NumberStyles.None, CultureInfo.InvariantCulture);
 #else
             var dec = int.Parse(plainText.Substring(plainText.Length / 3 * 3, 2), NumberStyles.None, CultureInfo.InvariantCulture);
@@ -1030,7 +1030,7 @@ private static BitArray PlainTextToBinaryNumeric(string plainText)
         }
         else if (plainText.Length % 3 == 1)  // One remaining digit is encoded in 4 bits.
         {
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
             var dec = int.Parse(plainText.AsSpan(plainText.Length / 3 * 3, 1), NumberStyles.None, CultureInfo.InvariantCulture);
 #else
             var dec = int.Parse(plainText.Substring(plainText.Length / 3 * 3, 1), NumberStyles.None, CultureInfo.InvariantCulture);
@@ -1101,7 +1101,7 @@ private static BitArray PlainTextToBinaryByte(string plainText, EciMode eciMode,
             }
         }
 
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
         // We can use stackalloc for small arrays to prevent heap allocations
         const int MAX_STACK_SIZE_IN_BYTES = 512;
 
@@ -1132,7 +1132,7 @@ private static BitArray PlainTextToBinaryByte(string plainText, EciMode eciMode,
             bitArray = ToBitArray(codeBytes);
         }
 
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
         if (bufferFromPool != null)
             ArrayPool<byte>.Shared.Return(bufferFromPool);
 #endif
@@ -1148,7 +1148,7 @@ private static BitArray PlainTextToBinaryByte(string plainText, EciMode eciMode,
     /// <param name="prefixZeros">The number of leading zeros to prepend to the resulting BitArray.</param>
     /// <returns>A BitArray representing the bits of the input byteArray, with optional leading zeros.</returns>
     private static BitArray ToBitArray(
-#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1
+#if HAS_SPAN
         ReadOnlySpan<byte> byteArray, // byte[] has an implicit cast to ReadOnlySpan<byte>
 #else
         byte[] byteArray,
@@ -1250,7 +1250,7 @@ private static Polynom MultiplyAlphaPolynoms(Polynom polynomBase, Polynom polyno
         }
 
         // Identify and merge terms with the same exponent.
-#if NETCOREAPP
+#if NET5_0_OR_GREATER
         var toGlue = GetNotUniqueExponents(resultPolynom, resultPolynom.Count <= 128 ? stackalloc int[128].Slice(0, resultPolynom.Count) : new int[resultPolynom.Count]);
         var gluedPolynoms = toGlue.Length <= 128
             ? stackalloc PolynomItem[128].Slice(0, toGlue.Length)
@@ -1276,7 +1276,7 @@ private static Polynom MultiplyAlphaPolynoms(Polynom polynomBase, Polynom polyno
 
         // Remove duplicated exponents and add the corrected ones back.
         for (int i = resultPolynom.Count - 1; i >= 0; i--)
-#if NETCOREAPP
+#if NET5_0_OR_GREATER
             if (toGlue.Contains(resultPolynom[i].Exponent))
 #else
             if (Array.IndexOf(toGlue, resultPolynom[i].Exponent) >= 0)
@@ -1290,7 +1290,7 @@ private static Polynom MultiplyAlphaPolynoms(Polynom polynomBase, Polynom polyno
         return resultPolynom;
 
         // Auxiliary function to identify exponents that appear more than once in the polynomial.
-#if NETCOREAPP
+#if NET5_0_OR_GREATER
         static ReadOnlySpan<int> GetNotUniqueExponents(Polynom list, Span<int> buffer)
         {
             // It works as follows:
diff --git a/QRCoder/QRCodeGenerator/CodewordBlock.cs b/QRCoder/QRCodeGenerator/CodewordBlock.cs
index 63c52115..7cf78577 100644
--- a/QRCoder/QRCodeGenerator/CodewordBlock.cs
+++ b/QRCoder/QRCodeGenerator/CodewordBlock.cs
@@ -2,7 +2,7 @@
 using System.Collections.Generic;
 using System.Threading;
 
-#if NETCOREAPP
+#if HAS_SPAN
 using System.Buffers;
 #endif
 
@@ -51,7 +51,7 @@ public static List<CodewordBlock> GetList(int capacity)
 
         public static void ReturnList(List<CodewordBlock> list)
         {
-#if NETCOREAPP
+#if HAS_SPAN
             foreach (var item in list)
             {
                 ArrayPool<byte>.Shared.Return(item.ECCWords.Array!);
diff --git a/QRCoder/QRCodeGenerator/Polynom.cs b/QRCoder/QRCodeGenerator/Polynom.cs
index 6cb6115d..cc904c7f 100644
--- a/QRCoder/QRCodeGenerator/Polynom.cs
+++ b/QRCoder/QRCodeGenerator/Polynom.cs
@@ -197,7 +197,7 @@ private void AssertCapacity(int min)
             void ThrowNotSupportedException() => throw new NotSupportedException("The polynomial capacity is fixed and cannot be increased.");
         }
 
-#if NETCOREAPP
+#if HAS_SPAN
         /// <summary>
         /// Rents memory for the polynomial terms from the shared memory pool.
         /// </summary>
diff --git a/QRCoder/QRCoder.csproj b/QRCoder/QRCoder.csproj
index cae73ca4..ed8fc4d6 100644
--- a/QRCoder/QRCoder.csproj
+++ b/QRCoder/QRCoder.csproj
@@ -1,11 +1,12 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net35;net40;netstandard1.3;netstandard2.0;net5.0;net5.0-windows;net6.0;net6.0-windows</TargetFrameworks>
+    <TargetFrameworks>net35;net40;netstandard1.3;netstandard2.0;netstandard2.1;net5.0;net5.0-windows;net6.0;net6.0-windows</TargetFrameworks>
     <PackageRequireLicenseAcceptance>false</PackageRequireLicenseAcceptance>
     <DefineConstants Condition="'$(TargetFramework)' != 'net6.0' AND '$(TargetFramework)' != 'netstandard1.3'">$(DefineConstants);SYSTEM_DRAWING</DefineConstants>
     <DefineConstants Condition="'$(TargetFramework)' == 'net5.0-windows'">$(DefineConstants);NET5_0_WINDOWS</DefineConstants>
     <DefineConstants Condition="'$(TargetFramework)' == 'net6.0-windows'">$(DefineConstants);NET6_0_WINDOWS</DefineConstants>
+    <DefineConstants Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netcoreapp2.1')) OR '$(TargetFramework)' == 'netstandard2.1'">$(DefineConstants);HAS_SPAN</DefineConstants>
     <DisableImplicitNuGetFallbackFolder>true</DisableImplicitNuGetFallbackFolder>
     <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
     <WarningsAsErrors>$(WarningsAsErrors);CS1591</WarningsAsErrors>
@@ -38,17 +39,17 @@
     <None Include="..\README.md" Pack="true" PackagePath="\" />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'net35' or '$(TargetFramework)' == 'net40' ">
+  <ItemGroup Condition=" '$(TargetFramework)' == 'net35' OR '$(TargetFramework)' == 'net40' ">
     <Reference Include="PresentationCore" />
     <Reference Include="PresentationFramework" />
     <Reference Include="WindowsBase" />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.3' or '$(TargetFramework)' == 'netstandard2.0' ">
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.3' OR '$(TargetFramework)' == 'netstandard2.0' OR '$(TargetFramework)' == 'netstandard2.1' ">
     <PackageReference Include="System.Text.Encoding.CodePages" Version="5.0.0" />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'net5.0' or '$(TargetFramework)' == 'net5.0-windows' ">
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' OR '$(TargetFramework)' == 'netstandard2.1' OR '$(TargetFramework)' == 'net5.0' OR '$(TargetFramework)' == 'net5.0-windows' ">
     <PackageReference Include="System.Drawing.Common" Version="5.0.3" />
   </ItemGroup>
 
diff --git a/QRCoderApiTests/net35+net40+net50+net50-windows+netstandard20/QRCoder.approved.txt b/QRCoderApiTests/net35+net40+net50+net50-windows+netstandard20+netstandard21/QRCoder.approved.txt
similarity index 100%
rename from QRCoderApiTests/net35+net40+net50+net50-windows+netstandard20/QRCoder.approved.txt
rename to QRCoderApiTests/net35+net40+net50+net50-windows+netstandard20+netstandard21/QRCoder.approved.txt