XML conversion to normal text

Please see my comments to the question. Tags themselves cannot be "invalid", your XML could be not well-formed, or well-formed XML might not comply with the schema, but this is a different issue. You did not explain your problem with tags made by yourself, but normally, you should make your code valid instead of looking for some compromise.

Nevertheless, making a text out of an XML file is way too easy, but it comes at the cost of lost some structural information. The simplest way is using the class System.Xml.XmlReader to read XML and the class System.IO.StreamWriter to write text:

http://msdn.microsoft.com/en-us/library/system.xml.xmlreader.aspx[^],
http://msdn.microsoft.com/en-us/library/system.io.streamwriter.aspx[^].

If could be something like this:

string XmlAttributeToText(int depth, string attributeName, string value) {
        //...
        return //... whatever you want
} //XmlAttributeToText

string XmlNodeToText(int depth, System.Xml.XmlNodeType nodeType, string name, string value) {
    //...
    return //... whatever you want
} //XmlNodeToText

string textFileName = //...
string xmlFileName = //...

using (System.IO.StreamWriter writer = new System.IO.StreamWriter(textFileName, false, System.Text.Encoding.UTF8)) {
    using (System.Xml.XmlReader reader = System.Xml.XmlReader.Create(xmlFileName)) {
        while (reader.Read()) {
            string nodeContent = XmlNodeToText(reader.Depth, reader.NodeType, reader.Name, reader.Value);
            writer.WriteLine(nodeContent); //or writer.Write, whatever is required
            if (reader.HasAttributes) {
                reader.MoveToNextAttribute();
                string attributeContent = XmlAttributeToText(reader.Depth, reader.Name, reader.Value);
                writer.WriteLine(attributeContent); //or writer.Write, whatever is required
            } //if
        } //loop
    } //using XmlReader // reader is disposed here
} //using StreamWriter // writer is disposed here, file buffer is flashed and file is closed, which is even more important

Also you can use XML reader options to ignore comment, processing instructions; you can ignore node types you are not interested in the loop shown above; in other words — program the mapping rules for mapping from XML to text the ways suitable for your goal.

—SA