How to reopen a TextWriter object? Ist this posible?

Question

I have a new project that I'm working on that uses a TextWriter object to serialize a class. Something like this:

TextWriter txtStream = new StreamWriter("xmlStreamFile.xml");
xmlSel.Serialize(txtStream, gp); // gp is the class I want to serialize
txtStream.Flush();
txtStream.Close(); 

This code works the first time I use it - the file is created and the data is written ok - but the second time I try to use it I get a excception: "Cannot write to ca closed TextWriter". I just wanted to ask how can I reopen the TextWriter - or what should I do not to get this type of exception and be able to rewrite the txtStream Object. Thanks!

Answer 1

No, you cannot re-open something like a TextWriter, because in the general case it could be writing to something like a NetworkStream, or a GZipStream - and in these cases, when you close the writer (and stream) it has side-effects (terminating a network connection, writing the final gzip blocks, etc). So no: you can't do that.

Since you are writing to a file, either:

• don't close it until you are ready
• re-open the file (for append) when needed

Answer 2

The most straightforward answer would be to re-open a new TextWriter each time.

public static void SerializeAsXml(object gp, string target)
{
    XmlSerializer xmlSel = new XmlSerializer(gp.GetType());
    using (TextWriter txtStream = new StreamWriter(target))
        xmlSel.Serialize(txtStream, gp);
}

The using statement implicitly causes Flush and Close to be called on your txtStream.

Answer 3

If you want to append to the same file, just use the same TextWriter and close it when you're done:

XmlSerializer xmlSel = new XmlSerializer(typeof(Foo));
TextWriter txtStream = new StreamWriter("xmlStreamFile.xml");
for (int i = 1; i <= 3; i++)
{
    Foo foo = new Foo(i * 100);
    xmlSel.Serialize(txtStream, foo);
    Console.WriteLine("Serialize done #" + i);
    txtStream.Flush();
}
txtStream.Close();
txtStream.Dispose();

In this sample Foo is just a simple struct, but it would work with your class as well.

Edit: assuming you're using WinForms you can have the XmlSerializer and StreamWriter members of the class itself:

class Form1 : Form
{
    private XmlSerializer xmlSel = null;
    TextWriter txtStream = null;
    //....
}

Then in the button click method initialize them once if null and serialize what you need:

if (xmlSel == null)
{
    xmlSel = new XmlSerializer(typeof(Foo));
    txtStream = new StreamWriter("xmlStreamFile.xml");
}
xmlSel.Serialize(txtStream, foo);
txtStream.Flush();

And finally in the Form close event, close and dispose the writer:

txtStream.Close();
txtStream.Dispose();