VB.NET Writing to a certain file (essentially creating a new XML element)

Question

Hi I'm making my own ToDo program, and I need to add the ability for the user to create a ToDo item. When they do this, I need to be able to add in an XML element/section in a list file.

My XML file looks like this:

<?xml version="1.0" encoding="utf-8"?>
<list>
<item>
    <name>Item 1</name>
    <due>Tomorrow</due>
</item>
<item>
    <name>Item 2</name>
    <due>Thursday</due>
</item>
</list>

Say if a user created an item with the name 'papers' and due date 'Tomorrow', I need to be able to make another list element like above and insert it before the </list> tag.

Basically I'm asking how can I write to my XML file at a specific position (at the end before the </list> tag)? Is it possible just to go to the end of the file and go back a line?

vcsjones · Accepted Answer · 2012-05-07 14:25:56Z

1

Is it possible just to go to the end of the file and go back a line?

Sure it is, but it would be very unsafe to do so. It makes an assumption about how the XML file is written, this is also equally valid XML:

<?xml version="1.0" encoding="utf-8"?>
<list><item><name>Item 1</name><due>Tomorrow</due></item><item><name>Item 2</name><due>Thursday</due></item></list>

Now it would be getting a little more complicated. Instead, you should use an XML parser, parse the XML file, manipulate the XML's Document Object Model (DOM), and save it back. You could do that like so:

Dim document As XDocument = XDocument.Load("pathtoxmlfile.xml")
Dim root = document.Root
root.Add(
    New XElement("item",
                 New XElement("name", "Item 3"),
                 New XElement("due", "Friday"))
             )
document.Save("pathtoxmlfile.xml")

This makes use of the Framework's XDocument class, which is a good tool to easily work with an XML file's object model.

How can I select and delete a specific node?

You can use LINQ-to-XML to write queries for deleting. Simply write a LINQ query to select the elements you want, and use Remove(). Here is an example that removes the item we just added:

Dim document As XDocument = XDocument.Load("pathtoxmlfile.xml")
Dim root = document.Root
Dim deleteThese = From e In root.Elements("item") Where e.Element("name").Value = "Item 3"
If deleteThese.Any() Then
    deleteThese.Remove()
    document.Save("pathtoxmlfile.xml")
End If

edited May 7, 2012 at 14:25

answered May 7, 2012 at 13:19

vcsjones

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2 Comments

ToshNeox Over a year ago

Thanks for the answer! It works now. How can I select and delete a specific node? Would I also have to use a 'for each' loop to delete multiple ones? I have looked around but can't find much on this.

ToshNeox Over a year ago

Thanks so, so, so much for your help! It's a very satisfying feeling to get it working. I used my previous VB.NET knowledge to make what you wrote, delete multiple items according to a list view: pastebin.com/VMC2A3sP

Guffa · Accepted Answer · 2012-05-07 13:14:15Z

0

You can write at a specific position in a file, but you can't insert data, you will be overwriting the rest of the file. Also, the position is not a line or even a character, but a byte position. As UTF-8 can use more than one byte for some characters, it would not be trivial to find the byte position for a character.

Simply load the file into an XmlDocument object, add the element, and write the document to the file.

answered May 7, 2012 at 13:14

Guffa

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Comments

Thorsten Dittmar · Accepted Answer · 2012-05-07 13:14:22Z

0

No, you don't write the XML file yourself. You'll use the XmlDocument class to create a DOM structure, which you can easily read from or write to XML files using the respective XmlWriter classes.

Alternatively, you could use a typed dataset, which can be written to an XML file with or without schema.

No way should you write the XML file manually when there are classes in the framework that do things for you!

answered May 7, 2012 at 13:14

Thorsten Dittmar

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