Use Dictionary.TryAdd instead of ContainsKey and indexer by reducing lookups.

src/Markdig/Helpers/CharacterMap.cs

@@ -51,10 +51,7 @@ public CharacterMap(IEnumerable<KeyValuePair<char, T>> maps)
             {
                 nonAsciiMap ??= [];
 
-                if (!nonAsciiMap.ContainsKey(openingChar))
-                {
-                    nonAsciiMap[openingChar] = state.Value;
-                }
+                nonAsciiMap.TryAdd(openingChar, state.Value);
             }
         }
 

src/Markdig/Parsers/ParserList.cs

@@ -38,10 +38,7 @@ protected ParserList(IEnumerable<T> parsersArg) : base(parsersArg)
             {
                 foreach (var openingChar in parser.OpeningCharacters)
                 {
-                    if (!charCounter.ContainsKey(openingChar))
-                    {
-                        charCounter[openingChar] = 0;
-                    }
+                    charCounter.TryAdd(openingChar, 0);
                     charCounter[openingChar]++;
                 }
             }

src/Markdig/Polyfills/DictionaryExtensions.cs

@@ -0,0 +1,23 @@
+// Copyright (c) Alexandre Mutel. All rights reserved.
+// This file is licensed under the BSD-Clause 2 license.
+// See the license.txt file in the project root for more information.
+
+#if !(NETSTANDARD2_1_OR_GREATER || NET)
+
+namespace System.Collections.Generic;
+
+internal static class DictionaryExtensions
+{
+    public static bool TryAdd<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key, TValue value) where TKey : notnull
+    {
+        if (!dictionary.ContainsKey(key))
+        {
+            dictionary[key] = value;
+            return true;
+        }
+
+        return false;
+    }
+}
+
+#endif

src/Markdig/Syntax/LinkReferenceDefinitionGroup.cs

@@ -40,10 +40,7 @@ public void Set(string label, LinkReferenceDefinition link)
         if (!Contains(link))
         {
             Add(link);
-            if (!Links.ContainsKey(label))
-            {
-                Links[label] = link;
-            }
+            Links.TryAdd(label, link);
         }
     }