Trying to write to an existing XML file for C#

Question

Thank you very much in advance

This is the original XML File

<my:Incident>
       <my:Category>This is for Category</my:Category>
              <my:Status>`Status is Close`</my:Status>
       <my:Description>`This is the description part</my:Description>
</my:Incident>

and I would like to add other fields under my:Incident

This is an example of it:

<my:Incident>
       <my:Category>This is for Category</my:Category>
              <my:Status>`Status is Close`</my:Status>
       <my:SummaryDescription>This is the summary</my:SummaryDescription>
       <my:Description>`This is the description part</my:Description>
</my:Incident>

I tried to implemented but I got this error message:

The ':' character, hexadecimal value 0x3A, cannot be included in a name.

public void writerXMLTest(string fileName)
        {

            if (!File.Exists(fileName))
            {
                XmlTextWriter writer = new XmlTextWriter(fileName, null);
                writer.WriteStartElement("my:Incident");
                writer.WriteEndElement();
                writer.Close();
            }

            XDocument doc = XDocument.Load(fileName);
            XElement demoNode = new XElement("my:Incident");
            demoNode.Add(new XElement("my:SummaryDescription", "Test Test"));


            Console.WriteLine("I write it!!!!!"); 
        }

I would appreciate if anyone can guide me where I did wrong in my code.

---

I modified the code a little. But now I'm not able to write it to the existing XML File

This is my code:

public void writerXMLTest(string fileName)
        {

            if (!File.Exists(fileName))
            {
                XmlTextWriter writer = new XmlTextWriter(fileName, null);
                writer.WriteStartElement("Incident", "my");
                writer.WriteEndElement();
                writer.Close();
            }

            XDocument doc = XDocument.Load(fileName);
            XElement demoNode = new XElement("SummaryDescription", "Test Test");


            Console.WriteLine("I write it!!!!!"); 
        }

Answer 1

This is wrong:

 writer.WriteStartElement("my:Incident");

This is right:

 writer.WriteStartElement("Incident", "blablablaSpace:my");

Edit:

 writer.WriteStartElement("Incident", "http://schemas.microsoft.com/office/infopath/2003/myXSD/2005-09-22T20:42:56:my");