[c#] Using StringWriter for XML Serialization

I'm currently searching for an easy way to serialize objects (in C# 3).

I googled some examples and came up with something like:

MemoryStream memoryStream = new MemoryStream ( );
XmlSerializer xs = new XmlSerializer ( typeof ( MyObject) );
XmlTextWriter xmlTextWriter = new XmlTextWriter ( memoryStream, Encoding.UTF8 );
xs.Serialize ( xmlTextWriter, myObject);
string result = Encoding.UTF8.GetString(memoryStream .ToArray());

After reading this question I asked myself, why not using StringWriter? It seems much easier.

XmlSerializer ser = new XmlSerializer(typeof(MyObject));
StringWriter writer = new StringWriter();
ser.Serialize(writer, myObject);
serializedValue = writer.ToString();

Another Problem was, that the first example generated XML I could not just write into an XML column of SQL Server 2005 DB.

The first question is: Is there a reason why I shouldn't use StringWriter to serialize an Object when I need it as a string afterwards? I never found a result using StringWriter when googling.

The second is, of course: If you should not do it with StringWriter (for whatever reasons), which would be a good and correct way?

Addition:

As it was already mentioned by both answers, I'll further go into the XML to DB problem.

When writing to the Database I got the following exception:

System.Data.SqlClient.SqlException: XML parsing: line 1, character 38, unable to switch the encoding

For string

<?xml version="1.0" encoding="utf-8"?><test/>

I took the string created from the XmlTextWriter and just put as xml there. This one did not work (neither with manual insertion into the DB).

Afterwards I tried manual insertion (just writing INSERT INTO ... ) with encoding="utf-16" which also failed. Removing the encoding totally worked then. After that result I switched back to the StringWriter code and voila - it worked.

Problem: I don't really understand why.

at Christian Hayter: With those tests I'm not sure that I have to use utf-16 to write to the DB. Wouldn't setting the encoding to UTF-16 (in the xml tag) work then?

It may have been covered elsewhere but simply changing the encoding line of the XML source to 'utf-16' allows the XML to be inserted into a SQL Server 'xml'data type.

using (DataSetTableAdapters.SQSTableAdapter tbl_SQS = new DataSetTableAdapters.SQSTableAdapter())
{
    try
    {
        bodyXML = @"<?xml version="1.0" encoding="UTF-8" standalone="yes"?><test></test>";
        bodyXMLutf16 = bodyXML.Replace("UTF-8", "UTF-16");
        tbl_SQS.Insert(messageID, receiptHandle, md5OfBody, bodyXMLutf16, sourceType);
    }
    catch (System.Data.SqlClient.SqlException ex)
    {
        Console.WriteLine(ex.Message);
        Console.ReadLine();
    }
}

The result is all of the XML text is inserted into the 'xml' data type field but the 'header' line is removed. What you see in the resulting record is just

<test></test>

Using the serialization method described in the "Answered" entry is a way of including the original header in the target field but the result is that the remaining XML text is enclosed in an XML <string></string> tag.

The table adapter in the code is a class automatically built using the Visual Studio 2013 "Add New Data Source: wizard. The five parameters to the Insert method map to fields in a SQL Server table.

One problem with StringWriter is that by default it doesn't let you set the encoding which it advertises - so you can end up with an XML document advertising its encoding as UTF-16, which means you need to encode it as UTF-16 if you write it to a file. I have a small class to help with that though:

public sealed class StringWriterWithEncoding : StringWriter
{
    public override Encoding Encoding { get; }

    public StringWriterWithEncoding (Encoding encoding)
    {
        Encoding = encoding;
    }    
}

Or if you only need UTF-8 (which is all I often need):

public sealed class Utf8StringWriter : StringWriter
{
    public override Encoding Encoding => Encoding.UTF8;
}

As for why you couldn't save your XML to the database - you'll have to give us more details about what happened when you tried, if you want us to be able to diagnose/fix it.

When serialising an XML document to a .NET string, the encoding must be set to UTF-16. Strings are stored as UTF-16 internally, so this is the only encoding that makes sense. If you want to store data in a different encoding, you use a byte array instead.

SQL Server works on a similar principle; any string passed into an xml column must be encoded as UTF-16. SQL Server will reject any string where the XML declaration does not specify UTF-16. If the XML declaration is not present, then the XML standard requires that it default to UTF-8, so SQL Server will reject that as well.

Bearing this in mind, here are some utility methods for doing the conversion.

public static string Serialize<T>(T value) {

    if(value == null) {
        return null;
    }

    XmlSerializer serializer = new XmlSerializer(typeof(T));

    XmlWriterSettings settings = new XmlWriterSettings()
    {
        Encoding = new UnicodeEncoding(false, false), // no BOM in a .NET string
        Indent = false,
        OmitXmlDeclaration = false
    };

    using(StringWriter textWriter = new StringWriter()) {
        using(XmlWriter xmlWriter = XmlWriter.Create(textWriter, settings)) {
            serializer.Serialize(xmlWriter, value);
        }
        return textWriter.ToString();
    }
}

public static T Deserialize<T>(string xml) {

    if(string.IsNullOrEmpty(xml)) {
        return default(T);
    }

    XmlSerializer serializer = new XmlSerializer(typeof(T));

    XmlReaderSettings settings = new XmlReaderSettings();
    // No settings need modifying here

    using(StringReader textReader = new StringReader(xml)) {
        using(XmlReader xmlReader = XmlReader.Create(textReader, settings)) {
            return (T) serializer.Deserialize(xmlReader);
        }
    }
}

First of all, beware of finding old examples. You've found one that uses XmlTextWriter, which is deprecated as of .NET 2.0. XmlWriter.Create should be used instead.

Here's an example of serializing an object into an XML column:

public void SerializeToXmlColumn(object obj)
{
    using (var outputStream = new MemoryStream())
    {
        using (var writer = XmlWriter.Create(outputStream))
        {
            var serializer = new XmlSerializer(obj.GetType());
            serializer.Serialize(writer, obj);
        }

        outputStream.Position = 0;
        using (var conn = new SqlConnection(Settings.Default.ConnectionString))
        {
            conn.Open();

            const string INSERT_COMMAND = @"INSERT INTO XmlStore (Data) VALUES (@Data)";
            using (var cmd = new SqlCommand(INSERT_COMMAND, conn))
            {
                using (var reader = XmlReader.Create(outputStream))
                {
                    var xml = new SqlXml(reader);

                    cmd.Parameters.Clear();
                    cmd.Parameters.AddWithValue("@Data", xml);
                    cmd.ExecuteNonQuery();
                }
            }
        }
    }
}

public static T DeserializeFromXml<T>(string xml)
{
    T result;
    XmlSerializerFactory serializerFactory = new XmlSerializerFactory();
    XmlSerializer serializer =serializerFactory.CreateSerializer(typeof(T));

    using (StringReader sr3 = new StringReader(xml))
    {
        XmlReaderSettings settings = new XmlReaderSettings()
        {
            CheckCharacters = false // default value is true;
        };

        using (XmlReader xr3 = XmlTextReader.Create(sr3, settings))
        {
            result = (T)serializer.Deserialize(xr3);
        }
    }

    return result;
}