所以基本上我想要这个

<?xml version="1.0" encoding="utf-16"?>
<element>
  a
</element>

但是使用下面的代码我得到了这个

<?xml version="1.0" encoding="utf-16"?>
<element>
a
</element>

这是我目前的测试工具

[Test]
public void XmlIndentingTest()
{
    var provider = new StringBuilder();
    var settings = new XmlWriterSettings
                       {
                           Indent = true,IndentChars = "  ",};
    using (var writer = new StringWriter(provider))
    {
        using (var xmlWriter = XmlTextWriter.Create(writer,settings))
        {
            xmlWriter.WriteStartElement("element");
            xmlWriter.WriteString("\r\na\r\n");
            xmlWriter.WriteEndElement();
        }
    }
    Debug.WriteLine(provider.ToString());
}