Use FrozenDictionary for better performance (thanks Graham!)

examples/Benchmark/Benchmarks.cs

@@ -18,7 +18,7 @@ public async Task ParseWithSpan()
         using (MemoryStream generatedDocument = new MemoryStream())
         using (WordprocessingDocument package = WordprocessingDocument.Create(generatedDocument, WordprocessingDocumentType.Document))
         {
-            MainDocumentPart mainPart = package.MainDocumentPart;
+            MainDocumentPart? mainPart = package.MainDocumentPart;
             if (mainPart == null)
             {
                 mainPart = package.AddMainDocumentPart();

src/Html2OpenXml/Expressions/Numbering/NumberingExpressionBase.cs

@@ -9,6 +9,7 @@
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
  * PARTICULAR PURPOSE.
  */
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Linq;
 using AngleSharp.Html.Dom;
@@ -26,7 +27,7 @@ abstract class NumberingExpressionBase(IHtmlElement node) : BlockElementExpressi
     public const int MaxLevel = 8;
     protected const int Indentation = 360;
     public const string HeadingNumberingName = "decimal-heading-multi";
-    private static readonly IDictionary<string, AbstractNum> predefinedNumberingLists = InitKnownLists();
+    private static readonly IReadOnlyDictionary<string, AbstractNum> predefinedNumberingLists = InitKnownLists();
     /// <summary>Contains the list of templated list along with the AbstractNumbId</summary>
     private Dictionary<string, int>? knownAbsNumIds;
     /// <summary>Contains the list of numbering instance.</summary>
@@ -218,7 +219,7 @@ private void InitNumberingIds(ParsingContext context)
     /// <summary>
     /// Predefined template of lists.
     /// </summary>
-    private static Dictionary<string, AbstractNum> InitKnownLists()
+    private static IReadOnlyDictionary<string, AbstractNum> InitKnownLists()
     {
         var knownAbstractNums = new Dictionary<string, AbstractNum>();
 
@@ -292,6 +293,6 @@ private static Dictionary<string, AbstractNum> InitKnownLists()
             knownAbstractNums.Add(listName, abstractNum);
         }
 
-        return knownAbstractNums;
+        return knownAbstractNums.ToFrozenDictionary();
     }
 }

src/Html2OpenXml/HtmlToOpenXml.csproj

@@ -37,6 +37,11 @@
   <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

@@ -14,6 +14,7 @@
  * 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;
@@ -35,16 +36,22 @@ 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 Dictionary<byte[], FileType> imageFormatDecoders = new()
+    private static readonly IReadOnlyDictionary<byte[], FileType> imageFormatDecoders = InitKnownFormatDecoders();
+
+    private static IReadOnlyDictionary<byte[], FileType> InitKnownFormatDecoders()
     {
-        { 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
-    };
+        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();
+    }
 
     private static readonly int MaxMagicBytesLength = imageFormatDecoders
         .Keys.OrderByDescending(x => x.Length).First().Length;

src/Html2OpenXml/Primitives/HtmlColor.Named.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 
 namespace HtmlToOpenXml;
@@ -8,7 +9,7 @@ namespace HtmlToOpenXml;
 /// </summary>
 partial struct HtmlColor
 {
-    private static readonly Dictionary<string, HtmlColor> namedColors = InitKnownColors();
+    private static readonly IReadOnlyDictionary<string, HtmlColor> namedColors = InitKnownColors();
 
     private static HtmlColor GetNamedColor (ReadOnlySpan<char> name)
     {
@@ -21,7 +22,7 @@ private static HtmlColor GetNamedColor (ReadOnlySpan<char> name)
         return color;
     }
 
-    private static Dictionary<string, HtmlColor> InitKnownColors()
+    private static IReadOnlyDictionary<string, HtmlColor> InitKnownColors()
     {
         var colors = new Dictionary<string, HtmlColor>()
         {
@@ -168,6 +169,6 @@ private static Dictionary<string, HtmlColor> InitKnownColors()
             { "transparent", FromArgb(0, 0, 0, 0) }
         };
 
-        return colors;
+        return colors.ToFrozenDictionary();
     }
 }