FileMode.Open and FileMode.OpenOrCreate difference when file exists? c# bug?

Question

I have wrote that code:

public void Save()
{
    using (FileStream fs = new FileStream(Properties.Settings.Default.settings_file_path, FileMode.Open))
    {
        XmlSerializer ser = new XmlSerializer(typeof(MySettings));
        ser.Serialize(fs, this);
    }
}

When I am using FileMode.Open everything is good, and output is e.x. like this:

<?xml version="1.0"?>
<MySettings xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <settingsList>
        <Setting>
            <Value>12</Value>
            <Name>A0</Name>
            <Type>MEASUREMENT</Type>
        </Setting>
        <Setting>
            <Value>5000</Value>
            <Name>C0</Name>
            <Type>MEASUREMENT</Type>
        </Setting>
    </settingsList>
</MySettings>

but when I change it to FileMode.OpenOrCreate output will change to:

<?xml version="1.0"?>
<MySettings xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <settingsList>
        <Setting>
            <Value>12</Value>
            <Name>A0</Name>
            <Type>MEASUREMENT</Type>
        </Setting>
        <Setting>
            <Value>5000</Value>
            <Name>C0</Name>
            <Type>MEASUREMENT</Type>
        </Setting>
    </settingsList>
</MySettings>>

what makes whole xml file corrupted because of additional > sign at the end.

Is this explanable or its c# bug?

Answer 1

I have just reproduced that issue. As I wrote in comment.

FileMode.Open erases contents of the file while FileMode.OpenOrCreate does not.

It seems that new content of the file is one char shorter than previous that's why you see ">" at the end.

If you are writing the file use FileMode.Create that should do for you.