Get node XML in C#

Question

I'm having some problems getting a node of XML document in C#

I need to make a program that enables the user to edit the <LimitValue> of each <ControlItem>.

I have already tried to get a node with GetElementById but it always return null and I can't understand why.

This is my xml document:

<?xml version="1.0" encoding="utf-8"?>
<GradeLimits GradeName="1.0305" GradeDescription="P235G1TH (St35.8)" Norm="Trafilspec" BaseElement="Fe">
  <ControlItems>
    <ControlItem Name="C" DisplayUnit="%" Impurity="False">
      <LimitValue Type="UpperWarningLimit" Kind="Absolute" Unit="%">0.17000000178813934</LimitValue>
      <LimitValue Type="UpperAcceptanceLimit" Kind="Absolute" Unit="%">0.18000000715255737</LimitValue>
    </ControlItem>
    <ControlItem Name="Si" DisplayUnit="%" Impurity="False">
      <LimitValue Type="UpperWarningLimit" Kind="Absolute" Unit="%">0.34999999403953552</LimitValue>
      <LimitValue Type="UpperAcceptanceLimit" Kind="Absolute" Unit="%">0.37000000476837158</LimitValue>
    </ControlItem>
    <ControlItem Name="Mn" DisplayUnit="%" Impurity="False">
      <LimitValue Type="LowerWarningLimit" Kind="Absolute" Unit="%">0.40000000596046448</LimitValue>
      <LimitValue Type="UpperWarningLimit" Kind="Absolute" Unit="%">0.800000011920929</LimitValue>
      <LimitValue Type="LowerAcceptanceLimit" Kind="Absolute" Unit="%">0.37999999523162842</LimitValue>
      <LimitValue Type="UpperAcceptanceLimit" Kind="Absolute" Unit="%">0.8399999737739563</LimitValue>
    </ControlItem>
    <ControlItem Name="P" DisplayUnit="%" Impurity="False">
      <LimitValue Type="UpperWarningLimit" Kind="Absolute" Unit="%">0.039999999105930328</LimitValue>
      <LimitValue Type="UpperAcceptanceLimit" Kind="Absolute" Unit="%">0.05000000074505806</LimitValue>
    </ControlItem>
    <ControlItem Name="S" DisplayUnit="%" Impurity="False">
      <LimitValue Type="UpperWarningLimit" Kind="Absolute" Unit="%">0.039999999105930328</LimitValue>
      <LimitValue Type="UpperAcceptanceLimit" Kind="Absolute" Unit="%">0.05000000074505806</LimitValue>
    </ControlItem>
  </ControlItems>
</GradeLimits>

I've tried to do this too, but it doesn't work:

     foreach (FileInfo file in Files)
            {
                count1++;
                if (count1 == select) {         
                    namefile = Files[select].Name;
                    doc.Load(@"C:\Users\lab\Desktop\copy\spectro\"+namefile);

                    var node = doc.SelectSingleNode("//*[@Name='C']");
                    Console.WriteLine(node);

                }
            }

For example in this case the <ControlItem Name="C"> has <LimitValue Type="UpperWarningLimit" ...>0.17000</limitvalue>, and I want to give the user the possibility of setting the value that they want.

I can't read the node and I don't know how to set this value either.

Answer 1

Here is a code sample that will allow you to get the child node:

        XmlDocument doc = new XmlDocument();
        doc.Load(@"C:\Source\Testing\XML\doc.xml");

        var nodeControlItem = doc.SelectSingleNode("//*[@Name='C']");

        var nodeLimitValue = nodeControlItem.SelectSingleNode("//*[@Type='UpperWarningLimit']");

        Console.Write(nodeLimitValue.InnerText);

Your program can then use the same "nodeLimitValue.InnerText" property to update the value in the XML. (Remember to save your XML document when you are done making changes.

You were on the right track with your code, but you were only getting the ControlItem node, and not its child node (LimitValue). From your ControlItem node, you can use SelectSingleNode again to select his children nodes, thus being able to extract or manipulate the value of the LimitValue node.

Hope this helps.

EDIT:

Here is an expanded code sample to help you a bit more:

        XmlDocument doc = new XmlDocument();
        doc.Load(@"C:\Source\Testing\XML\doc.xml");

        // This line will select "the first node" in the XML Document with attribute Name="C"
        XmlNode nodeControlItem = doc.SelectSingleNode("//*[@Name='C']");

        // Alternatively, you can accomplish the same thing by iterating through all of the ControlItem nodes to find the one you want:
        foreach (XmlNode node in doc.SelectNodes("/GradeLimits/ControlItems/ControlItem"))
        {
            if (node.Attributes.GetNamedItem("Name").Value == "C")
            {
                nodeControlItem = node;
                break;
            }
        }

        //Now that you have your "C" ControlItem, you can find a child node with Type="UpperWarningLimit"
        XmlNode nodeLimitValue = nodeControlItem.SelectSingleNode("//*[@Type='UpperWarningLimit']");

        //Alternatively, you can accomplish the same thing by iterating through all of the ChildNodes of the ControlItem to find the one you want:
        foreach (XmlNode childNode in nodeControlItem.ChildNodes)
        {
            if (childNode.Attributes.GetNamedItem("Type").Value == "UpperWarningLimit")
            {
                nodeLimitValue = childNode;
                break;
            }
        }

        //Or, another is to iterating through all of the LimitValue child nodes of the ControlItem to find the one you want:
        foreach (XmlNode childNode in nodeControlItem.SelectNodes("./LimitValue"))
        {
            if (childNode.Attributes.GetNamedItem("Type").Value == "UpperWarningLimit")
            {
                nodeLimitValue = childNode;
                break;
            }
        }

        Console.Write(nodeLimitValue.Value);

        // Modify the value of the node
        nodeLimitValue.Value = "0.00000";

        Console.Write(nodeLimitValue.Value);

        // Save the XML document back to disk
        doc.Save(@"C:\Source\Testing\XML\doc.xml");

For more details, you should read about XPath syntax.