Use Dictionary.TryGetValue to avoid double lookup

Author: gpetrou

src/HtmlAgilityPack.Shared/HtmlDocument.cs

@@ -694,7 +694,9 @@ public HtmlNode GetElementbyId(string id)
                 throw new Exception(HtmlExceptionUseIdAttributeFalse);
             }
 
-            return Nodesid.ContainsKey(id) ? Nodesid[id] : null;
+            return Nodesid.TryGetValue(id, out HtmlNode value)
+                ? value
+                : null;
         }
 
         /// <summary>

src/HtmlAgilityPack.Shared/NameValuePairList.cs

@@ -50,7 +50,10 @@ 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 List<KeyValuePair<string, string>> value) 
+                ? value 
+                : new List<KeyValuePair<string, string>>();
         }
 
         internal string GetNameValuePairValue(string name)