Writing file to web server - ASP.NET

Question

I simply want to write the contents of a TextBox control to a file in the root of the web server directory... how do I specify it?

Bear in mind, I'm testing this locally... it keeps writing the file to my program files\visual studio\Common\IDE directory rather than my project directory (which is where I assume root is when the web server fires off).

Does my problem have something to do with specifying the right location in my web.config? I tried that and still no go...

Thanks much...

protected void TestSubmit_ServerClick(object sender, EventArgs e)
    {
        StreamWriter _testData = new StreamWriter("data.txt", true);
        _testData.WriteLine(TextBox1.Text); // Write the file.
        _testData.Close(); // Close the instance of StreamWriter.
        _testData.Dispose(); // Dispose from memory.       
    }

Answer 1

protected void TestSubmit_ServerClick(object sender, EventArgs e)
{
  using (StreamWriter _testData = new StreamWriter(Server.MapPath("~/data.txt"), true))
 {
  _testData.WriteLine(TextBox1.Text); // Write the file.
 }         
}

Server.MapPath takes a virtual path and returns an absolute one. "~" is used to resolve to the application root.