How can I remove the BOM from XmlTextWriter using C#?
You're saving the file twice - once with XmlTextWriter and once with xmlDoc.Save. Saving from the XmlTextWriter isn't adding a BOM - saving with xmlDoc.Save is.
Just save to a TextWriter instead, so that you can specify the encoding again:
using (TextWriter writer = new StreamWriter(filename, false, new UTF8Encoding(false)){ xmlDoc.Save(writer);}
I'd write the XML to a string(builder) instead and then write that string to file.