SelectNodes and GetElementsByTagName

Question

what are the main differences between SelectNodes and GetElementsByTagName.

Answer 1

SelectNodes is a .NET / MSXML-specific method that retrieves a list of matching nodes for XPath . XPaths can select items by tag name, but can also follow many other, more complex selection rules.

getElementByTagName is a standard standard level 1 DOM method, available in many languages ​​(but written using capital G in .NET). It selects items only by tag name; you cannot ask him to select elements with a specific attribute or elements with the tag name a inside other elements with the tag name b or any smart one. It is older, simpler, and faster in some environments.

Answer 2

SelectNodes takes an XPath expression as a parameter and returns all nodes matching that expression.

GetElementsByTagName takes a tag name as a parameter and returns all tags that have that name.

SelectNodes is more expressive as you can write any GetElementsByTagName call as a SelectNodes call, but not vice versa. XPath is a very reliable way to express sets of XML nodes that offer more filtering methods than just a name. For example, XPath can filter by tag name, attribute names, internal content, and various aggregate functions on child tags.

Answer 3

SelectNodes () is a Microsoft extension for the Document Object Model (DOM) ( msdn ). SelectNodes, as Welbog mentions, and others accept an XPath expression. I would like to mention the difference with GetElementsByTagName () when xml node removal is required.

Answer and provided chilberto user code in msdn forum

The following test illustrates the difference by performing the same function (deleting human nodes), but using the GetElementByTagName () method to select the nodes. Although the same type of object is returned, its design is different. SelectNodes () is a collection of links to an xml document. This means that we can remove from the document in foreach without affecting the list of links. This is evidenced by the calculation of the nodelist, which is not affected. GetElementByTagName () is a collection that directly reflects the nodes in the document. This means that when deleting elements in the parent element, we actually affect the set of nodes. This is why the nodelist cannot be manipulated in foreach, but it needs to be changed to a while loop.

.NET SelectNodes ()

  [TestMethod] public void TestSelectNodesBehavior() { XmlDocument doc = new XmlDocument(); doc.LoadXml(@"<root> <person> <id>1</id> <name>j</name> </person> <person> <id>2</id> <name>j</name> </person> <person> <id>1</id> <name>j</name> </person> <person> <id>3</id> <name>j</name> </person> <business></business> </root>"); XmlNodeList nodeList = doc.SelectNodes("/root/person"); Assert.AreEqual(5, doc.FirstChild.ChildNodes.Count, "There should have been a total of 5 nodes: 4 person nodes and 1 business node"); Assert.AreEqual(4, nodeList.Count, "There should have been a total of 4 nodes"); foreach (XmlNode n in nodeList) n.ParentNode.RemoveChild(n); Assert.AreEqual(1, doc.FirstChild.ChildNodes.Count, "There should have been only 1 business node left in the document"); Assert.AreEqual(4, nodeList.Count, "There should have been a total of 4 nodes"); } 

.NET GetElementsByTagName ()

  [TestMethod] public void TestGetElementsByTagNameBehavior() { XmlDocument doc = new XmlDocument(); doc.LoadXml(@"<root> <person> <id>1</id> <name>j</name> </person> <person> <id>2</id> <name>j</name> </person> <person> <id>1</id> <name>j</name> </person> <person> <id>3</id> <name>j</name> </person> <business></business> </root>");; XmlNodeList nodeList = doc.GetElementsByTagName("person"); Assert.AreEqual(5, doc.FirstChild.ChildNodes.Count, "There should have been a total of 5 nodes: 4 person nodes and 1 business node"); Assert.AreEqual(4, nodeList.Count, "There should have been a total of 4 nodes"); while (nodeList.Count > 0) nodeList[0].ParentNode.RemoveChild(nodeList[0]); Assert.AreEqual(1, doc.FirstChild.ChildNodes.Count, "There should have been only 1 business node left in the document"); Assert.AreEqual(0, nodeList.Count, "All the nodes have been removed"); } 

Using SelectNodes () we get a collection / list of links to the nodes of the XML document. We can manipulate these links. If we remove the node, the change will be visible for the xml document, but the collection / list of links will be the same (although the node that was deleted now points to null -> System.NullReferenceException) Although I do not know how this is implemented. I suppose if we use XmlNodeList nodeList = GetElementsByTagName () and remove the node with nodeList [i] .ParentNode.RemoveChild (nodeList [i]) frees / removes the link in the nodeList variable.