隔离存储读取数据：System.Xml.XmlException：意外的XML声明

Question

I am writing the data and reading it, i am getting the exception as "System.Xml.XmlException: Unexpected XML declaration" ,i am unable to figure it out whats the issue.

I have also added the exception that its printing.Please help me to solve the issue.

Here my code:

public static void WriteTopicState(Topic topic)
        {
            try
            {
                using (var store = IsolatedStorageFile.GetUserStoreForApplication())
                {
                    using (StreamWriter sw = new StreamWriter(store.OpenFile("Stats.xml", FileMode.Append, FileAccess.Write)))
                    {
                        XmlSerializer serializer = new XmlSerializer(typeof(Topic));
                        serializer.Serialize(sw, topic);
                        serializer = null;
                    }

                }
            }
            catch (Exception)
            {
                throw;
            }
        }


        public static Topic ReadMockTestTopicState()
        {
            Topic topic = null;
            try
            {
                using (IsolatedStorageFile isoStore = IsolatedStorageFile.GetUserStoreForApplication())
                {
                    // Read application settings. 
                    if (isoStore.FileExists("Stats.xml"))
                    {
                        using (var store = IsolatedStorageFile.GetUserStoreForApplication())
                        {
                            using (StreamReader SR = new StreamReader(store.OpenFile("Stats.xml", FileMode.Open, FileAccess.Read)))
                            {
                                XmlSerializer serializer = new XmlSerializer(typeof(Topic));
                                topic = (Topic)serializer.Deserialize(SR);
                                serializer = null;
                            }
                        }
                    }
                    else
                    {
                        // If setting does not exists return default setting.
                        topic = new Topic();
                    }
                }
            }
            catch (Exception)
            {
                throw;
            }
            return topic;
        }

Exception :

{System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> 
System.InvalidOperationException: There is an error in XML document (9, 19). ---> 
System.Xml.XmlException: Unexpected XML declaration. The XML declaration must be the first node in the document, and no white space characters are allowed to appear before it. Line 9, position 19.

EDIT

If there is any other way that i can save the data to a txt file also is fine for me, but only thing is i need to append the data to the existing document and read it get back the data.

Answer 1

Your problem is because you are appending to your Stats.xml document, as a result it will contain multiple root elements once it has been written to more than once.

If you wish to only store the latest stats, you should use FileMode.Create :

using (StreamWriter sw = new StreamWriter(store.OpenFile("Stats.xml", FileMode.Create, FileAccess.Write)))

Valid XmlDocuments can only contain one root element, if you wish to store multiple 'stats' a different strategy is required:

• If writes are only creates (eg not updates) write out each topic to a different file and combine them when reading
• Create a container element that will store multiple topics and then parse this from disk, add to it, then subsequently overwrite the file on disk (you'll need to be careful with concurrency if you choose this option)
• Use a different storage medium than the file system (eg a document database, a SQL database, etc)
• Many other options