fix nullable return for .NET8

Author: JonathanMagnan

src/HtmlAgilityPack.Shared/HtmlEntity.cs

@@ -591,7 +591,13 @@ private HtmlEntity()
         /// </summary>
         /// <param name="text">The source text.</param>
         /// <returns>The result text.</returns>
-        public static string DeEntitize(string text)
+        public static
+#if NET8_0
+        string?
+#else
+        string
+#endif
+        DeEntitize(string text)
         {
             if (text == null)
                 return null;
@@ -770,7 +776,13 @@ public static string Entitize(string text, bool useNames)
         /// <param name="useNames">If set to false, the function will not use known entities name. Default is true.</param>
         /// <param name="entitizeQuotAmpAndLtGt">If set to true, the [quote], [ampersand], [lower than] and [greather than] characters will be entitized.</param>
         /// <returns>The result text</returns>
-        public static string Entitize(string text, bool useNames, bool entitizeQuotAmpAndLtGt)
+        public static
+#if NET8_0
+        string?
+#else
+        string
+#endif
+        Entitize(string text, bool useNames, bool entitizeQuotAmpAndLtGt)
 //        _entityValue.Add("quot", 34);    // quotation mark = APL quote, U+0022 ISOnum 
 //        _entityName.Add(34, "quot");
 //        _entityValue.Add("amp", 38);    // ampersand, U+0026 ISOnum 

src/HtmlAgilityPack.Shared/HtmlNode.Xpath.cs

@@ -37,7 +37,13 @@ public XPathNavigator CreateRootNavigator()
         /// </summary>
         /// <param name="xpath">The XPath expression.</param>
         /// <returns>An <see cref="HtmlNodeCollection"/> containing a collection of nodes matching the <see cref="XPath"/> query, or <c>null</c> if no node matched the XPath expression.</returns>
-        public HtmlNodeCollection SelectNodes(string xpath)
+        public
+#if NET8_0
+        HtmlNodeCollection?
+#else
+        HtmlNodeCollection
+#endif
+        SelectNodes(string xpath)
         {
             HtmlNodeCollection list = new HtmlNodeCollection(null);
 
@@ -62,7 +68,13 @@ public HtmlNodeCollection SelectNodes(string xpath)
         /// </summary>
         /// <param name="xpath">The XPath expression.</param>
         /// <returns>An <see cref="HtmlNodeCollection"/> containing a collection of nodes matching the <see cref="XPath"/> query, or <c>null</c> if no node matched the XPath expression.</returns>
-        public HtmlNodeCollection SelectNodes(XPathExpression xpath)
+        public
+#if NET8_0
+        HtmlNodeCollection?
+#else
+        HtmlNodeCollection
+#endif
+        SelectNodes(XPathExpression xpath)
         { 
             HtmlNodeCollection list = new HtmlNodeCollection(null);
 
@@ -87,7 +99,13 @@ public HtmlNodeCollection SelectNodes(XPathExpression xpath)
         /// </summary>
         /// <param name="xpath">The XPath expression. May not be null.</param>
         /// <returns>The first <see cref="HtmlNode"/> that matches the XPath query or a null reference if no matching node was found.</returns>
-        public HtmlNode SelectSingleNode(string xpath)
+        public
+#if NET8_0
+        HtmlNode?
+#else
+        HtmlNode
+#endif
+        SelectSingleNode(string xpath)
         {
             if (xpath == null)
             {
@@ -110,7 +128,13 @@ public HtmlNode SelectSingleNode(string xpath)
         /// </summary>
         /// <param name="xpath">The XPath expression.</param>
         /// <returns>An <see cref="HtmlNodeCollection"/> containing a collection of nodes matching the <see cref="XPath"/> query, or <c>null</c> if no node matched the XPath expression.</returns>
-        public HtmlNode SelectSingleNode(XPathExpression xpath)
+        public
+#if NET8_0
+        HtmlNode?
+#else
+        HtmlNode
+#endif
+        SelectSingleNode(XPathExpression xpath)
         {
             if (xpath == null)
             {

src/HtmlAgilityPack.Shared/HtmlNode.cs

@@ -1307,7 +1307,13 @@ public IEnumerable<HtmlNode> DescendantsAndSelf(string name)
 		/// </summary>
 		/// <param name="name"></param>
 		/// <returns></returns>
-		public HtmlNode Element(string name)
+		public
+#if NET8_0
+        HtmlNode?
+#else
+        HtmlNode
+#endif
+        Element(string name)
 		{
 			foreach (HtmlNode node in ChildNodes)
 				if (node.Name == name)

src/HtmlAgilityPack.Shared/HtmlNodeCollection.cs

@@ -68,7 +68,13 @@ public int this[HtmlNode node]
         /// </summary>
         /// <param name="nodeName"></param>
         /// <returns></returns>
-        public HtmlNode this[string nodeName]
+        public
+#if NET8_0
+        HtmlNode?
+#else
+        HtmlNode
+#endif
+        this[string nodeName]
         {
             get
             {
@@ -296,7 +302,13 @@ public void RemoveAt(int index)
         /// <param name="items"></param>
         /// <param name="name"></param>
         /// <returns></returns>
-        public static HtmlNode FindFirst(HtmlNodeCollection items, string name)
+        public static
+#if NET8_0
+        HtmlNode?
+#else
+        HtmlNode
+#endif
+        FindFirst(HtmlNodeCollection items, string name)
         {
             foreach (HtmlNode node in items)
             {

src/HtmlAgilityPack.Shared/HtmlNodeNavigator.cs

@@ -528,7 +528,13 @@ public override XPathNavigator Clone()
 		/// <param name="localName">The local name of the HTML attribute.</param>
 		/// <param name="namespaceURI">The namespace URI of the attribute. Unsupported with the HtmlNavigator implementation.</param>
 		/// <returns>The value of the specified HTML attribute. String.Empty or null if a matching attribute is not found or if the navigator is not positioned on an element node.</returns>
-		public override string GetAttribute(string localName, string namespaceURI)
+		public override
+#if NET8_0
+        string?
+#else
+        string
+#endif
+        GetAttribute(string localName, string namespaceURI)
 		{
 #if TRACE_NAVIGATOR
             InternalTrace("localName=" + localName + ", namespaceURI=" + namespaceURI);

src/HtmlAgilityPack.Shared/Utilities.cs

@@ -13,7 +13,13 @@ namespace HtmlAgilityPack
 {
     internal static class Utilities
     {
-        public static TValue GetDictionaryValueOrDefault<TKey, TValue>(Dictionary<TKey, TValue> dict, TKey key, TValue defaultValue = default(TValue)) where TKey : class
+        public static
+#if NET8_0
+        TValue?
+#else
+        TValue
+#endif
+        GetDictionaryValueOrDefault<TKey, TValue>(Dictionary<TKey, TValue> dict, TKey key, TValue defaultValue = default(TValue)) where TKey : class
         {
             TValue value;
             if (!dict.TryGetValue(key, out value))