Return XML from a controller's action in as an ActionResult?

return this.Content(xmlString, "text/xml");

Use MVCContrib's XmlResult Action.

For reference here is their code:

public class XmlResult : ActionResult{    private object objectToSerialize;    /// <summary>    /// Initializes a new instance of the <see cref="XmlResult"/> class.    /// </summary>    /// <param name="objectToSerialize">The object to serialize to XML.</param>    public XmlResult(object objectToSerialize)    {        this.objectToSerialize = objectToSerialize;    }    /// <summary>    /// Gets the object to be serialized to XML.    /// </summary>    public object ObjectToSerialize    {        get { return this.objectToSerialize; }    }    /// <summary>    /// Serialises the object that was passed into the constructor to XML and writes the corresponding XML to the result stream.    /// </summary>    /// <param name="context">The controller context for the current request.</param>    public override void ExecuteResult(ControllerContext context)    {        if (this.objectToSerialize != null)        {            context.HttpContext.Response.Clear();            var xs = new System.Xml.Serialization.XmlSerializer(this.objectToSerialize.GetType());            context.HttpContext.Response.ContentType = "text/xml";            xs.Serialize(context.HttpContext.Response.Output, this.objectToSerialize);        }    }}

If you're building the XML using the excellent Linq-to-XML framework, then this approach will be helpful.

I create an XDocument in the action method.

public ActionResult MyXmlAction(){    // Create your own XDocument according to your requirements    var xml = new XDocument(        new XElement("root",            new XAttribute("version", "2.0"),            new XElement("child", "Hello World!")));    return new XmlActionResult(xml);}

This reusable, custom ActionResult serialises the XML for you.

public sealed class XmlActionResult : ActionResult{    private readonly XDocument _document;    public Formatting Formatting { get; set; }    public string MimeType { get; set; }    public XmlActionResult(XDocument document)    {        if (document == null)            throw new ArgumentNullException("document");        _document = document;        // Default values        MimeType = "text/xml";        Formatting = Formatting.None;    }    public override void ExecuteResult(ControllerContext context)    {        context.HttpContext.Response.Clear();        context.HttpContext.Response.ContentType = MimeType;        using (var writer = new XmlTextWriter(context.HttpContext.Response.OutputStream, Encoding.UTF8) { Formatting = Formatting })            _document.WriteTo(writer);    }}

You can specify a MIME type (such as application/rss+xml) and whether the output should be indented if you need to. Both properties have sensible defaults.

If you need an encoding other than UTF8, then it's simple to add a property for that too.