How to do an do file openor create

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C#

private void OnFileSave(object sender, EventArgs e)
{
    SaveFileDialog saveFileDialog1 = new SaveFileDialog();
    saveFileDialog1.Title = "Save a Binary File";
    saveFileDialog1.Filter = "Binary Files (*.log)|*.log|All files (*.*)|*.*";
    saveFileDialog1.DefaultExt = ".log";
    saveFileDialog1.FilterIndex = 1;

    string str;
    double flt;

    DialogResult dr;
    dr = saveFileDialog1.ShowDialog();

    if (dr == DialogResult.OK)
    {
        string savefile = saveFileDialog1.FileName;

        try
        {
             FileStream fs = new FileStream(savefile, FileMode.OpenOrCreate);
             BinaryWriter bw = new BinaryWriter(fs);
            // Write data
        }
        catch
        {
            MessageBox.Show("File save error.");
        }
    }
}

What I have tried:

I have several data grid data structures that I am using. The data is written to structures comprised of strings and doubles. Able to save the initial file OK which most of the structures are not populated. Then I add some additional data into my structures and try and save again which generates the error that it fails to open. In debug when it encounters this line it fails to open:

FileStream fs = new FileStream(savefile, FileMode.OpenOrCreate);

In my reading this lie above is suppose to create a new file is one does not exits or replace the file. The first case works. The second case files.

Writing in C++ using windows forms with Visual Studio 10