Use benefits of FrozenDictionary only for net8, no cumbersome net462 with an additional package

Author: onizet

src/Html2OpenXml/Expressions/Numbering/NumberingExpressionBase.cs

@@ -9,7 +9,9 @@
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
  * PARTICULAR PURPOSE.
  */
+#if NET5_0_OR_GREATER
 using System.Collections.Frozen;
+#endif
 using System.Collections.Generic;
 using System.Linq;
 using AngleSharp.Html.Dom;
@@ -293,6 +295,10 @@ private static IReadOnlyDictionary<string, AbstractNum> InitKnownLists()
             knownAbstractNums.Add(listName, abstractNum);
         }
 
+#if NET5_0_OR_GREATER
         return knownAbstractNums.ToFrozenDictionary();
+#else
+        return knownAbstractNums;
+#endif
     }
 }

src/Html2OpenXml/HtmlToOpenXml.csproj

@@ -37,11 +37,6 @@
   <ItemGroup Condition=" '$(TargetFramework)' == 'net462' ">
     <Reference Include="System.Net.Http" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
-    <PackageReference Include="System.Collections.Immutable" Version="9.0.0" />
-  </ItemGroup>
-
-  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
-    <PackageReference Include="System.Collections.Immutable" Version="9.0.0" />
   </ItemGroup>
 
   <PropertyGroup Condition=" '$(TargetFramework)' == 'net8.0' ">

src/Html2OpenXml/IO/ImageHeader.cs

@@ -14,7 +14,6 @@
  * EMF Specifications: https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-emf/ae7e7437-cfe5-485e-84ea-c74b51b000be
  */
 using System;
-using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -36,22 +35,17 @@ public enum FileType { Unrecognized, Bitmap, Gif, Png, Jpeg, Emf, Xml }
 
     private static readonly byte[] pngSignatureBytes = [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A];
 
-    private static readonly IReadOnlyDictionary<byte[], FileType> imageFormatDecoders = InitKnownFormatDecoders();
-
-    private static IReadOnlyDictionary<byte[], FileType> InitKnownFormatDecoders()
+    private static readonly Dictionary<byte[], FileType> imageFormatDecoders = new()
     {
-        var decoders = new Dictionary<byte[], FileType>() {
-            { new byte[] { 0x42, 0x4D }, FileType.Bitmap },
-            { Encoding.UTF8.GetBytes("GIF87a"), FileType.Gif },
-            { Encoding.UTF8.GetBytes("GIF89a"), FileType.Gif }, // animated gif
-            { pngSignatureBytes, FileType.Png },
-            { new byte[] { 0xff, 0xd8 }, FileType.Jpeg },
-            { new byte[] { 0x1, 0, 0, 0 }, FileType.Emf },
-            { Encoding.UTF8.GetBytes("<?xml "), FileType.Xml }, // Xml so potentially Svg
-        };
-
-        return decoders.ToFrozenDictionary();
-    }
+        { new byte[] { 0x42, 0x4D }, FileType.Bitmap },
+        { Encoding.UTF8.GetBytes("GIF87a"), FileType.Gif },
+        { Encoding.UTF8.GetBytes("GIF89a"), FileType.Gif }, // animated gif
+        { pngSignatureBytes, FileType.Png },
+        { new byte[] { 0xff, 0xd8 }, FileType.Jpeg },
+        { new byte[] { 0x1, 0, 0, 0 }, FileType.Emf },
+        { Encoding.UTF8.GetBytes("<?xml "), FileType.Xml }, // Xml so potentially Svg
+    };
+
 
     private static readonly int MaxMagicBytesLength = imageFormatDecoders
         .Keys.OrderByDescending(x => x.Length).First().Length;

src/Html2OpenXml/Primitives/HtmlColor.Named.cs

@@ -1,5 +1,7 @@
 using System;
+#if NET5_0_OR_GREATER
 using System.Collections.Frozen;
+#endif
 using System.Collections.Generic;
 
 namespace HtmlToOpenXml;
@@ -169,6 +171,10 @@ private static IReadOnlyDictionary<string, HtmlColor> InitKnownColors()
             { "transparent", FromArgb(0, 0, 0, 0) }
         };
 
+#if NET5_0_OR_GREATER
         return colors.ToFrozenDictionary();
+#else
+        return colors;
+#endif
     }
 }