| Liquid Development Is What I Do

Seb Nilsson

NullableGuidConstraint for ASP.NET MVC & WebApi

Have you ever written a very usable Route-constraint in ASP.NET MVC or in WebAPI than you wanted to share between them both? For example a constraint that supports nullable Guids (Guid?) as route-parameter.

This can be done by implementing both System.Web.Routing.IRouteConstraint and System.Web.Http.Routing.IHttpRouteConstraint.

Hopefully this article will be obsolete with the release of ASP.NET 5, but until then, here's how you solve this problem:

public class NullableGuidConstraint : IRouteConstraint, IHttpRouteConstraint
    // ASP.NET MVC-signature
    public bool Match(
        HttpContextBase httpContext,
        Route route,
        string parameterName,
        RouteValueDictionary values,
        RouteDirection routeDirection)
        return MatchInternal(parameterName, values);

    // WebAPI-signature
    public bool Match(
        HttpRequestMessage request,
        IHttpRoute route,
        string parameterName,
        IDictionary<string, object> values,
        HttpRouteDirection routeDirection)
        return MatchInternal(parameterName, values);

    private static bool MatchInternal(string parameterName, IDictionary<string, object> values)
        object value;
        if (!values.TryGetValue(parameterName, out value))
            return false;

        if (value is Guid)
            return true;

        string stringValue = Convert.ToString(value, CultureInfo.InvariantCulture);

        Guid guid;
        bool isMatch = string.IsNullOrWhiteSpace(stringValue) || Guid.TryParse(stringValue, out guid);
        return isMatch;

Then you register this constraint through, for example, DefaultInlineConstraintResolver in System.Web.Mvc.Routing for ASP.NET MVC and System.Web.Http.Routing for WebAPI.

var constraintResolver = new DefaultInlineConstraintResolver();
constraintResolver.ConstraintMap.Add("guid?", typeof(NullableGuidConstraint));


// WebAPI

Now you can write attribute-routes like this:


PowerShell Profile with Permanent Aliases


If you're a PowerShell-user that manually run a script or bind an alias every time you open PowerShell, then there is a file you really should know about. It's the PowerShell profile-file (or actually, six profile-files).

I use the profile-file for Current User in the console located here:


When you've ensured that this file is created you can enter PowerShell-scripts that will run every time you're opening your PowerShell-console.

For example you can create an PowerShell-alias for the git-command to work with just using g.

set-alias g git

Now I can just type g status and save many, many seconds during a long week of Git-usage.


Render .ascx-Files in ASP.NET MVC Using Only RazorViewEngine

ASP.NET Band-Aid

If you're stuck in an environment where you're migrating from ASP.NET MVC to ASP.NET WebForms it's good to know that you can actually render your existing WebForms-Controls in you MVC-views. This might sound like a crazy thing to do (and it is in the long run!) but it might be useful if you're stuck between sprints and have perfectly working WebForms-Controls (.ascx-files) that you don't have time to migrate right now. All you have to do is use the HtmlHelper's helper-method .RenderPartial(string partialViewName) and pass it the path to the WebForms-Control.

// Write the content of a control inside a view:

// Or to get the content of a control as a MvcHtmlString, for further manipulation:

It's important that your controls inherits from System.Web.Mvc.ViewUserControl and NOT the old System.Web.UI.UserControl.

One performance-tip that is often mentioned around ASP.NET MVC is to deactivate the WebForms-View Engine for MVC Razor-views (which actually turns out to maybe not make such a big difference after all). This will not prevent .aspx, .ascx and other WebForms-files from working.

But you still want your .ascx-files to work inline in your MVC Razor-views. This can be achieved by implementing your own class that inherits RazorViewEngine, which only uses the WebFormViewEngine when actually needed.

Global.asax (or other config-class)

// Remove WebFormViewEngine (and RazorViewEngine)
ViewEngines.Engines.Add(new CustomRazorViewEngine());

CustomRazorViewEngine : System.Web.Mvc.RazorViewEngine

private static readonly WebFormViewEngine WebFormsEngine = new WebFormViewEngine();

public override ViewEngineResult FindPartialView(
    ControllerContext context, string name, bool useCache)
    if (name.EndsWith(".ascx"))
        return WebFormsEngine.FindPartialView(context, name, useCache);

    return base.FindPartialView(context, name, useCache);

If you need the actual class of the control to do some further analysis/manipulation, you can do the following anywhere in your code:

var viewPage = new ViewPage();
var control = viewPage.LoadControl("~/Controls/ControlVirtualPath.ascx") as ControlType;

Serialize HtmlString & MvcHtmlString in JSON.NET


The HtmlString-class (and MvcHtmlString) that is and has been used in the ASP.NET-platform, including WebPages, since the introduction of ASP.NET MVC is basically just a wrapped string, that doesn't gets automatically HTML-encoded when used in Razor-views. Despite this fact, if you want to serialize or deserialize this object in JSON.NET it will come back as null.

To be able to serialize an object containing a property of a type inheriting from IHtmlString, like HtmlString and MvcHtmlString, to then, for instance, cache the serialized object, you need to implement your own Newtonsoft.Json.JsonConverter that handles the serialization and deserialization.

HtmlStringConverter : Newtonsoft.Json.JsonConverter

public override bool CanConvert(Type objectType)
    return typeof(IHtmlString).IsAssignableFrom(objectType);

public override object ReadJson(
    JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    var value = reader.Value as string;
    // Specifically MvcHtmlString
    if (objectType == typeof(MvcHtmlString))
        return new MvcHtmlString(value);
    // Generally HtmlString
    if (objectType == typeof(HtmlString))
        return new HtmlString(value);

    // Fallback for other (future?) implementations of IHtmlString
    return Activator.CreateInstance(objectType, value);

public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    var htmlString = value as HtmlString;
    if (htmlString == null)


You then have to register this Converter in your JsonSerializerSettings like this:

CurrentConverter.Converters.Add(new HtmlStringConverter());

Rewrite Old URLs with Regex into RouteValueDictionary


Have you ever needed to rewrite/redirect old URLs that have very similar pattern as your new, modern URLs, but want to do it with simple, maintainable code?

In the following URL you very specifically can see an ID-value and a potential Category-value.

So you probably want to use Regex and point out named groups in above URL.


Then you want to get those named values into a RouteValueDictionary, with above named Keys, like <id> and <category> with the Regex-matched Values in the URL. You can then, for example, send it to a UrlHelper. This is what the following method does:

public static RouteValueDictionary GetRegexRouteValues(string url, string urlPattern)
    if (url == null)
        throw new ArgumentNullException("url");
    if (urlPattern == null)
        throw new ArgumentNullException("urlPattern");

    var regex = new Regex(urlPattern);
    var match = regex.Match(url);
    if (!match.Success)
        return null;

    var namedGroupNames = regex.GetGroupNames().Where(x => x != null && !Regex.IsMatch(x, @"^[0-9]+$"));
    var groups = (from groupName in namedGroupNames
                  let groupItem = match.Groups[groupName]
                  where groupItem != null
                  select new KeyValuePair<string, string>(groupName, groupItem.Value)).ToList();

    var routeValues = new RouteValueDictionary();
    groups.ForEach(x => routeValues.Add(x.Key, x.Value));

    return routeValues;

Now you can write code that is more easy to read and maintain to redirect specific URLs:

private static void RedirectProductDetails(string requestUrl) {
    string urlRegex = @"^(?:.*)\/?products\/details\.aspx\?id=(?<id>[\d]*)(&category=(?<category>[^&]*))?";
    var routeValues = GetRegexRouteValues(requestUrl, urlRegex);

    routeValues["controller"] = "Products";
    routeValues["action"] = "Details";
    return this.Url.RouteUrl(routeValues);