Reduce need to call string.ToLower with proper comparisons

src/HtmlAgilityPack.Shared/HtmlDocument.cs

@@ -681,7 +681,7 @@ public HtmlNode GetElementbyId(string id)
                 throw new Exception(HtmlExceptionUseIdAttributeFalse);
             }
 
-            return Nodesid.ContainsKey(id) ? Nodesid[id] : null;
+            return Nodesid.TryGetValue(id, out var value) ? value : null;
         }
 
         /// <summary>
@@ -1769,7 +1769,7 @@ private void Parse()
 									// https://www.w3schools.com/jsref/prop_node_innertext.asp
 									// textContent returns the text content of all elements, while innerText returns the content of all elements, except for <script> and <style> elements.
 									// innerText will not return the text of elements that are hidden with CSS (textContent will). ==> The parser do not support that.
-									if (_currentnode.Name.ToLowerInvariant().Equals("script")  || _currentnode.Name.ToLowerInvariant().Equals("style"))
+									if (_currentnode.Name.Equals("script", StringComparison.InvariantCultureIgnoreCase)  || _currentnode.Name.Equals("style", StringComparison.InvariantCultureIgnoreCase))
                                     {
 	                                    _currentnode._isHideInnerText = true;
 									}
@@ -2115,7 +2115,7 @@ private bool PushNodeEnd(int index, bool close)
             if ((close) || (!_currentnode._starttag))
             {
                 if ((OptionStopperNodeName != null) && (_remainder == null) &&
-                    (string.Compare(_currentnode.Name, OptionStopperNodeName, StringComparison.OrdinalIgnoreCase) == 0))
+                    string.Equals(_currentnode.Name, OptionStopperNodeName, StringComparison.OrdinalIgnoreCase))
                 {
                     _remainderOffset = index;
                     _remainder = Text.Substring(_remainderOffset);
@@ -2168,7 +2168,7 @@ private void ReadDocumentEncoding(HtmlNode node)
             HtmlAttribute att = node.Attributes["http-equiv"];
             if (att != null)
             {
-                if (string.Compare(att.Value, "content-type", StringComparison.OrdinalIgnoreCase) != 0)
+                if (!string.Equals(att.Value, "content-type", StringComparison.OrdinalIgnoreCase))
                     return;
                 HtmlAttribute content = node.Attributes["content"];
                 if (content != null)

src/HtmlAgilityPack.Shared/HtmlNameTable.cs

@@ -9,7 +9,7 @@
 
 namespace HtmlAgilityPack
 {
-    internal class HtmlNameTable : XmlNameTable
+    internal sealed class HtmlNameTable : XmlNameTable
     {
         #region Fields
 

src/HtmlAgilityPack.Shared/HtmlNode.cs

@@ -382,13 +382,13 @@ public virtual string InnerText
 				var sb = new StringBuilder();
                 int depthLevel = 0;
 				string name = this.Name;
-	 
+
 				if (name != null)
 				{
-					name = name.ToLowerInvariant();
+					bool isDisplayScriptingText = string.Equals(name, "head", StringComparison.InvariantCultureIgnoreCase)
+					                              || string.Equals(name, "script", StringComparison.InvariantCultureIgnoreCase)
+					                              || string.Equals(name, "style", StringComparison.InvariantCultureIgnoreCase);
 
-					bool isDisplayScriptingText = (name == "head" || name == "script" || name == "style"); 
-
 					InternalInnerText(sb, isDisplayScriptingText, depthLevel);
 				}
 				else
@@ -1934,7 +1934,7 @@ public virtual void WriteTo(TextWriter outText, int level = 0)
 					if (_ownerdocument.OptionOutputAsXml)
 					{
 						var commentNode = (HtmlCommentNode) this;
-						if (!_ownerdocument.BackwardCompatibility && commentNode.Comment.ToLowerInvariant().StartsWith("<!doctype"))
+						if (!_ownerdocument.BackwardCompatibility && commentNode.Comment.StartsWith("<!doctype", StringComparison.InvariantCultureIgnoreCase))
 						{
 							outText.Write(commentNode.Comment);
 						}
@@ -2512,6 +2512,8 @@ public void AddClass(string name)
 			AddClass(name, false);
 		}
 
+		private static readonly char[] spaceSeparator = { ' ' };
+
 		/// <summary>
 		/// Adds one or more classes to this node.
 		/// </summary>
@@ -2520,26 +2522,26 @@ public void AddClass(string name)
 		public void AddClass(string name, bool throwError)
 		{
 			var classAttributes = Attributes.AttributesWithName("class");
+			var isEmpty = true;
 
-			if (!IsEmpty(classAttributes))
+			foreach (HtmlAttribute att in classAttributes)
 			{
-				foreach (HtmlAttribute att in classAttributes)
-				{ 
-					// Check class solo, check class in First with other class, check Class no first.
-					if (att.Value != null && att.Value.Split(' ').ToList().Any(x => x.Equals(name)))
-					{
-						if (throwError)
-						{
-							throw new Exception(HtmlDocument.HtmlExceptionClassExists);
-						}
-					}
-					else
+				isEmpty = false;
+				// Check class solo, check class in First with other class, check Class no first.
+				if (att.Value != null && Array.IndexOf(att.Value.Split(spaceSeparator), name) != -1)
+				{
+					if (throwError)
 					{
-						SetAttributeValue(att.Name, att.Value + " " + name);
+						throw new Exception(HtmlDocument.HtmlExceptionClassExists);
 					}
 				}
+				else
+				{
+					SetAttributeValue(att.Name, att.Value + " " + name);
+				}
 			}
-			else
+
+			if (isEmpty)
 			{
 				HtmlAttribute attribute = _ownerdocument.CreateAttribute("class", name);
 				Attributes.Append(attribute);
@@ -2607,7 +2609,7 @@ public void RemoveClass(string name, bool throwError)
 					{
 						Attributes.Remove(att);
 					}
-					else if (att.Value != null && att.Value.Split(' ').ToList().Any(x => x.Equals(name)))
+					else if (att.Value != null && att.Value.Split(' ').Contains(name))
 					{
 						string[] classNames = att.Value.Split(' ');
 

src/HtmlAgilityPack.Shared/HtmlWeb.cs

@@ -2128,14 +2128,14 @@ private bool IsCacheHtmlContent(string path)
         }
 #endif
 
-        private bool IsHtmlContent(string contentType)
+        private static bool IsHtmlContent(string contentType)
         {
-            return contentType.ToLowerInvariant().StartsWith("text/html");
+            return contentType.StartsWith("text/html", StringComparison.InvariantCultureIgnoreCase);
         }
 
-        private bool IsGZipEncoding(string contentEncoding)
+        private static bool IsGZipEncoding(string contentEncoding)
         {
-            return contentEncoding.ToLowerInvariant().StartsWith("gzip");
+            return contentEncoding.StartsWith("gzip", StringComparison.InvariantCultureIgnoreCase);
         }
 
 #if !(NETSTANDARD1_3 || NETSTANDARD1_6)

src/HtmlAgilityPack.Shared/MixedCodeDocumentFragmentList.cs

@@ -21,7 +21,7 @@ public class MixedCodeDocumentFragmentList : IEnumerable
         #region Fields
 
         private MixedCodeDocument _doc;
-        private IList<MixedCodeDocumentFragment> _items = new List<MixedCodeDocumentFragment>();
+        private List<MixedCodeDocumentFragment> _items = new List<MixedCodeDocumentFragment>();
 
         #endregion
 

src/HtmlAgilityPack.Shared/NameValuePairList.cs

@@ -50,7 +50,7 @@ internal List<KeyValuePair<string, string>> GetNameValuePairs(string name)
         {
             if (name == null)
                 return _allPairs;
-            return _pairsWithName.ContainsKey(name) ? _pairsWithName[name] : new List<KeyValuePair<string, string>>();
+            return _pairsWithName.TryGetValue(name, out var value) ? value : new List<KeyValuePair<string, string>>();
         }
 
         internal string GetNameValuePairValue(string name)