使用XmlReader.Create时，不可捕获的FileNotFoundException

我目前正在设计一个程序，其中字符串数组保存每次按下按钮并在程序开始时加载。我有两个问题:一个，当试图加载文件时，它击中using语句，它给了我System.IO.FileNotFoundException的例外。后来在按下第三个按钮后，它给了我System.UnauthorizedAccessException的例外和System.Exception的崩溃。有什么办法可以解决这个问题吗?注意:这在Windows 8上运行。

public static void excecuteload()
{
    try
    {
        XmlSerializer ser = new XmlSerializer(typeof(string[]));
        string[] loadednames;
        using (XmlReader reader = XmlReader.Create("filenamestr")) //Causes an exception: System.IO.FileNotFoundException
        {
            loadednames = (string[])ser.Deserialize(reader);
        }
        names = (string[])loadednames.Clone();
    }
    catch (Exception)
    {
        ;
    }
}
public async static void excecutesave()
{
    try
    {
        StorageFolder folder = ApplicationData.Current.RoamingFolder;
        XmlWriterSettings setstr = new XmlWriterSettings();
        setstr.Encoding = Encoding.Unicode;
        XmlObjectSerializer serializerstr = new DataContractSerializer(typeof(string[])); //Gives error of System.UnauthorizedAccessException on the third go through
        Stream streamstr = await folder.OpenStreamForWriteAsync("filenamestr", CreationCollisionOption.ReplaceExisting);
        XmlWriter writerstr = XmlWriter.Create(streamstr, setstr);
        serializerstr.WriteObject(writerstr, names);
    }
    catch (Exception)
    {
        MessageDialog oops = new MessageDialog("Oops... we couldn't save your score :(");
        oops.ShowAsync();
     }
}