using System;

using System.IO;

using System.Xml;

​

public class Program

{

    public static void Main(string[] args)

    {

        var xml = @"<a>

<b></b>

<e>

   <b></b>

   <c></c>

</e>

<i>

  <e>

    <b></b>

    <c></c>

  </e>

</i> 

</a>";

​

        var rs = new XmlReaderSettings

        {

            IgnoreWhitespace = true

        };

​

        var ws = new XmlWriterSettings

        {

            OmitXmlDeclaration = true,

            Indent = true

        };

​

​

        using (var reader = XmlReader.Create(new StringReader(xml), rs))

        using (var writer = XmlWriter.Create(Console.Out, ws))

        {

            while (reader.Read())

            {

                switch (reader.NodeType)

                {

                    case XmlNodeType.Element:

                        var subTreeReader = reader.ReadSubtree();

                        if (HandleElement(reader, writer))

                        {

                            ReadToEnd(subTreeReader);

                        }

                        else

                        {

                            writer.WriteStartElement(reader.Prefix, reader.LocalName, reader.NamespaceURI);

                            writer.WriteAttributes(reader, true);

                            if (reader.IsEmptyElement)

                            {

                                writer.WriteEndElement();

                            }

                        }

                        break;

                    case XmlNodeType.Text:

                        writer.WriteString(reader.Value);

                        break;

                    case XmlNodeType.Whitespace:

                    case XmlNodeType.SignificantWhitespace:

                        writer.WriteWhitespace(reader.Value);

                        break;

                    case XmlNodeType.CDATA:

                        writer.WriteCData(reader.Value);

                        break;

                    case XmlNodeType.EntityReference:

                        writer.WriteEntityRef(reader.Name);

                        break;

                    case XmlNodeType.XmlDeclaration:

                    case XmlNodeType.ProcessingInstruction:

                        writer.WriteProcessingInstruction(reader.Name, reader.Value);

                        break;

                    case XmlNodeType.DocumentType:

                        writer.WriteDocType(reader.Name, reader.GetAttribute("PUBLIC"), reader.GetAttribute("SYSTEM"), reader.Value);

                        break;

                    case XmlNodeType.Comment:

                        writer.WriteComment(reader.Value);

                        break;

                    case XmlNodeType.EndElement:

                        writer.WriteFullEndElement();

                        break;

                }

            }

        }

    }

​

    private static void ReadToEnd(XmlReader reader)

    {

        while (!reader.EOF)

        {

            reader.Read();

        }

    }

​

    private static bool HandleElement(XmlReader reader, XmlWriter writer)

    {

        if (reader.Name == "e")

        {

            writer.WriteElementString("element", "val1");

            return true;

        }

​

        return false;

    }

}

​