C# Empty path name is not legal
<p>I'm a complete beginner to this language and am currently trying to create a simple text editing application, but am getting the above error when trying to open a file.
The code's below, any help would be much appreciated.</p>

<pre><code>private void mnuOpen_Click(object sender, EventArgs e)
    {
        string chosenFile = "";

        openFD.Title = "Select a text file";
        openFD.InitialDirectory = System.Environment.GetFolderPath(Environment.SpecialFolder.Personal);
        openFD.FileName = "Choose a file..";
        openFD.Filter = "Text Files|*.txt|All Files|*.*";

        if (openFD.ShowDialog() != DialogResult.Cancel)
        {
            contentBox.LoadFile(chosenFile, RichTextBoxStreamType.PlainText);
        }
    }
</code></pre>

<p>The exception itself occurs here:</p>

<pre><code>contentBox.LoadFile(chosenFile, RichTextBoxStreamType.PlainText);
</code></pre>
