Question

I am overriding a method which has an XmlReader being passed in, I need to find a specific element, add an attribute and then either create a new XmlReader or just replace the existing one with the modified content. I am using C#4.0

I have investigated using XElement (Linq) but I can’t seem to manipulate an existing element and add an attribute and value.

I know that the XmlWriter has WriteAttributeString which would be fantastic but again I am not sure how it all fits together

I would like to be able to do something like — This is pseudo-code!

public XmlReader DoSomethingWonderful(XmlReader reader)
{
   Element element = reader.GetElement("Test");
   element.SetAttribute("TestAttribute","This is a test");
   reader.UpdateElement(element);
   return reader;
}

Answer 1

XmlReader/Writer are sequential access streams. You will have to read in on one end, process the stream how you want, and write it out the other end. The advantage is that you don’t need to read the whole thing into memory and build a DOM, which is what you’d get with any XmlDocument-based approach.

This method should get you started:

private static void PostProcess(Stream inStream, Stream outStream)
{
    var settings = new XmlWriterSettings() { Indent = true, IndentChars = " " };

    using (var reader = XmlReader.Create(inStream))
    using (var writer = XmlWriter.Create(outStream, settings)) {
        while (reader.Read()) {
            switch (reader.NodeType) {
                case XmlNodeType.Element:
                    writer.WriteStartElement(reader.Prefix, reader.Name, reader.NamespaceURI);
                    writer.WriteAttributes(reader, true);

                    //
                    // check if this is the node you want, inject attributes here.
                    //

                    if (reader.IsEmptyElement) {
                        writer.WriteEndElement();
                    }
                    break;

                case XmlNodeType.Text:
                    writer.WriteString(reader.Value);
                    break;

                case XmlNodeType.EndElement:
                    writer.WriteFullEndElement();
                    break;

                case XmlNodeType.XmlDeclaration:
                case XmlNodeType.ProcessingInstruction:
                    writer.WriteProcessingInstruction(reader.Name, reader.Value);
                    break;

                case XmlNodeType.SignificantWhitespace:
                    writer.WriteWhitespace(reader.Value);
                    break;
            }
        }
    }
}

This is not quite as clean as deriving your own XmlWriter, but I find that it’s much easier.

[EDIT]

An example of how you would open two streams at once might be something like this:

using (FileStream readStream = new FileStream(@"c:\myFile.xml", FileMode.OpenOrCreate, FileAccess.Read, FileShare.Write)) {
  using (FileStream writeStream = new FileStream(@"c:\myFile.xml", FileMode.OpenOrCreate, FileAccess.Write)) {
    PostProcess(readStream, writeStream);
  }
}