I have list of objects that loaded from csv file, my goal is write it to xml and download it.

here is original csv file data like this:

```
A  1  str1 1001
A  1  str2 1002
A  1  str2 1003
B  2  str3 1004
B  2  str3 1005

```

And output xml dta should look like this:

```
<MyHeader>
   <columnHeader1 foo='A' order='1'>
      <columnHeader2 title="str1">
         <columnHeader3 id="1001"/>
      </columnHeader2>
      <columnHeader2 title="str2">
          <columnHeader3 id="1002" />
          <columnHeader3 id="1003" />
      </columnHeader2>
   </columnHeader1>
<columnHeader1 foo='B' order='2'>
      <columnHeader2 title="str3">
         <columnHeader3 id="1004"/>
         <columnHeader3 id="1005"/>
      </columnHeader2>
   </columnHeader1>
...

```

here is model defination

```
public class foo
    {
        public string letter { get; set; }
        public string order { get; set; }
        public string title { get; set; }
        public string titleid { get; set; }
    }

```

how can I prepeare this xml file with list of models above ?