如何使用c#在现有的XML文件中删除声明和添加根元素

我对XML和c#相当陌生，所以如果这个问题太容易问，请理解。

我正在使用c# win-form应用程序转换XML格式。应用程序使用"OpenFileDialog"打开一个xml文件，然后转换将被执行(这已经完成，但我仍然需要添加或删除一些更像下面)。转换后，应用程序将使用'SaveFileDialog'保存修改后的xml文件。

原始XML格式

<?xml version="1.0" encoding="utf-8" ?> 
   <DataList>
    <Data>
     <ID>1</ID>
     <Name>Mike</Name>
     <Age>23</Age>
    </Data> 
    <Data>
     <ID>1</ID>
     <Name>Mike</Name>
     <Age>23</Age>
    </Data>
    <Data>
     <ID>1</ID>
     <Name>Mike</Name>
     <Age>23</Age>
    </Data>
     ..<Data></Data> continued...
   </DataList>

我想编辑XML文件如下

<?xml version="1.0" encoding="utf-8" ?> **→ Remove this delaration!**
 <MainInterface> **→ Add 'root element' before existing nodes**
   <DataList>
    <Data>
     <ID>1</ID>
     <Name>Mike</Name>
     <Age>23</Age>
    </Data> 
    <Data>
     <ID>1</ID>
     <Name>Mike</Name>
     <Age>23</Age>
    </Data>
    <Data>
     <ID>1</ID>
     <Name>Mike</Name>
     <Age>23</Age>
    </Data>
     ..<Data></Data> continued...
   </DataList>
 </MainInterface> **→ close newly added root element**

我试过下面的代码，但似乎不工作

            OpenFileDialog openFileDialogue = new OpenFileDialog();           
            openFileDialog1.DefaultExt = "xml";
            openFileDialog1.Filter = "xml files (*.xml)|*.xml";
            openFileDialog1.Title = "Select a xml File";
            openFileDialog1.ShowDialog();
            XDocument xmlFile = XDocument.Load(openFileDialog1.FileName);
            **// Remove Declaration**
            XDocument doc = new XDocument(new XDeclaration(null, null, null));
            **// Add Root Element**
            XElement doc1 = XElement.Parse(openFileDialog1.FileName);
            XElement root = new XElement("MainInterface", doc1);
            //doc.Save(_data)
            openFileDialog1.FileName = root.ToString();
-----------------------------------------------------------------------------------
            Do something for conversion ~~~
-----------------------------------------------------------------------------------
            SaveFileDialog saveFileDialogue1 = new SaveFileDialog();
            saveFileDialog1.Filter = "xml File |*.xml";
            saveFileDialog1.Title = "Conversion Completed! Save a XML file";
            saveFileDialog1.FileName = "XML Converted.xml";            
            saveFileDialog1.ShowDialog();
            xmlFile.Save(saveFileDialog1.FileName);

我应该使用XML Writer吗?是否有更简单的方法来删除声明并在现有xml文件上添加根元素?

var xDoc    = XDocument.Load(openFileDialog1.FileName);
//Use code below if you'll use string to Load XDocument
/*var xmlString = @"<?xml version=""1.0"" encoding=""utf-8"" ?> 
<DataList>
    <Data>
        <ID>1</ID>
        <Name>Mike</Name>
        <Age>23</Age>
    </Data> 
    <Data>
        <ID>1</ID>
        <Name>Mike</Name>
        <Age>23</Age>
    </Data>
    <Data>
        <ID>1</ID>
        <Name>Mike</Name>
        <Age>23</Age>
    </Data>
    <Data></Data> continued...
</DataList>
";
var xDoc     = XDocument.Parse(xmlString);*/
var dataList = xDoc.Descendants(@"Data");   
var newXDoc  = new XDocument(new XDeclaration(null, null, null),
                   new XElement("MainInterface",
                       new XElement("DataList",
                           dataList.Select(data =>
                               new XElement("Data",
                                   data.Element("ID"),
                                   data.Element("Name"),
                                   data.Element("Age")
                               )
                           ) 
                       )                              
                   )
               );