Added support back for netstandard 2.1

Author: synercoder

Directory.Build.props

@@ -50,6 +50,7 @@
   <PropertyGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)','net5.0'))">
     <DefineConstants>$(DefineConstants);SUPPORTS_MEMBERNOTNULL</DefineConstants>
     <DefineConstants>$(DefineConstants);SUPPORTS_INIT</DefineConstants>
+    <DefineConstants>$(DefineConstants);SUPPORTS_TYPED_ENUM_ISDEFINED</DefineConstants>
   </PropertyGroup>
 
-</Project>
+</Project>

src/Directory.Build.props

@@ -2,7 +2,7 @@
 <Project>
 
   <PropertyGroup>
-    <TargetFrameworks>net7.0;net6.0</TargetFrameworks>
+    <TargetFrameworks>net7.0;net6.0;netstandard2.1</TargetFrameworks>
     <MSBuildAllProjects>$(MSBuildAllProjects);$(MSBuildThisFileDirectory)..\Directory.Build.props</MSBuildAllProjects>
     <SynercodingProjectCategory>src</SynercodingProjectCategory>
   </PropertyGroup>

src/Synercoding.FileFormats.Pdf/PdfPage.cs

@@ -60,8 +60,14 @@ public PageRotation? Rotation
         get => _rotation;
         set
         {
+            const string ARGUMENT_OUT_OF_RANGE_MESSAGE = "The provided value can only be increments of 90.";
+#if SUPPORTS_TYPED_ENUM_ISDEFINED
             if (value is not null && !Enum.IsDefined(value.Value))
-                throw new ArgumentOutOfRangeException(nameof(Rotation), value, "The provided value can only be increments of 90.");
+                throw new ArgumentOutOfRangeException(nameof(Rotation), value, ARGUMENT_OUT_OF_RANGE_MESSAGE);
+#else
+            if (value is not null && !Enum.IsDefined(typeof(PageRotation), value.Value))
+                throw new ArgumentOutOfRangeException(nameof(Rotation), value, ARGUMENT_OUT_OF_RANGE_MESSAGE);
+#endif
 
             _rotation = value;
         }