the process cannot access the file because it is being used by another process. c# xmltextwriter

I am currently trying to create some code to append to my XML file, it will first check if the file is created and if it is; it will then open the file ready for appending.

But the problem is, is that I am getting the above error message on the last line. If I put existingFile.Close() before this line, the error goes away; but then the data does not append.

Could someone please help me.

Thanks!

I can give the entire code if anyone wishes, but I just though I would give the bit that is giving my an error for now.

Code

if (System.IO.File.Exists(FullPath))
{
    found = true;
    System.IO.FileStream existingFile = System.IO.File.OpenWrite(FullPath);                    
    existingFile.Seek(-endtag.Length, System.IO.SeekOrigin.End);                   
    writer = new System.Xml.XmlTextWriter(FullPath, System.Text.Encoding.UTF8);
}