XmlWriter encoding UTF-8 using StringWriter in C#

Your code does not compile -- StringWriterWithEncoding does not have a parameterless constructor. Or, if it does have a parameterless constructor, maybe it actually looks like this?

public sealed class StringWriterWithEncoding : StringWriter
{
    private readonly Encoding encoding;

    public StringWriterWithEncoding() { }

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

    public override Encoding Encoding
    {
        get { return encoding; }
    }
}

If so, that would explain your problem - the field encoding has been left at its default (null) value, meaning Encoding returns null, and so no encoding will appear in the XML file.

To fix it, eliminate the parameterless constructor, and do:

var sw = new StringWriterWithEncoding(Encoding.UTF8);

Or change the parameterless constructor to explicitly set Encoding.UTF8:

public sealed class StringWriterWithEncoding : StringWriter
{
    private readonly Encoding encoding;

    public StringWriterWithEncoding() : this(Encoding.UTF8) { }

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

    public override Encoding Encoding
    {
        get { return encoding; }
    }
}