Ensure that HttpConfiguration.EnsureInitialized()

If you do it at the end of Application_Start it will be too late, as WebApiConfig.Register has been called.

The best way to resolve this is to use new initialization method by replacing in Global.asax :

WebApiConfig.Register(GlobalConfiguration.Configuration);

by

GlobalConfiguration.Configure(WebApiConfig.Register);

See @gentiane's answer below for the correct way to handle this now.

At the end of the Application_Start method in Global.Asax.cs try adding:-

GlobalConfiguration.Configuration.EnsureInitialized(); 

I actually got this error when I was using Attribute Routing within my WebApi.

I had

[Route("webapi/siteTypes/{siteTypeId"]

instead of

[Route("webapi/siteTypes/{siteTypeId}"]

for my route and got this error. I had simply missed out the closing curly bracket. Once I added it back in, this error didn't occur again.

This is old, but is the first result on google when searching for this error. After quite a bit of digging I was able to figure out what was going on.

tldr:
All GlobalConfiguration.Configure does is invoke your action and call EnsureInitialized(). config.MapAttributeRoutes() must be called before EnsureInitialized() since EnsureInitialized only runs once.

Meaning: if you're coming from an existing Mvc project, all you have to do is:

1. Add GlobalConfiguration.Configuration.EnsureInitialized(); to the bottom of your Application_Start method.

OR

2. Move your entire configuration into a single call to GlobalConfiguration.Configure:

GlobalConfiguration.Configure(config => 
{
    WebApiConfig.Register(config);
    config.MapAttributeRoutes();
    ...
});

Digging Deeper

HttpConfiguration.Configuration has an "Initializer" property defined like this:

public Action<HttpConfiguration> Initializer;

HttpConfiguration.EnsureInitialized() runs this action and sets _initialized to true

public void EnsureInitialized()
{ 
    if (_initialized)
    {
        return;
    }
    _initialized = true;
    Initializer(this);            
}

HttpConfiguration.MapAttributeRoutes calls internal method AttributeRoutingMapper.MapAttributeRoutes which sets HttpConfiguration.Initializer

public static void MapAttributeRoutes(...)
{
    RouteCollectionRoute aggregateRoute = new RouteCollectionRoute();
    configuration.Routes.Add(AttributeRouteName, aggregateRoute);

    ...

    Action<HttpConfiguration> previousInitializer = configuration.Initializer;
    configuration.Initializer = config =>
    {
        previousInitializer(config);
        ...
    };
}

GlobalConfiguration.Configure runs EnsureInitialized immediately after invoking your action:

public static void Configure(Action<HttpConfiguration> configurationCallback)
{
    if (configurationCallback == null)
    {
        throw new ArgumentNullException("configurationCallback");
    }

    configurationCallback.Invoke(Configuration);
    Configuration.EnsureInitialized();
}

Don't forget, if you run in to a wall, the source for asp.net is available at http://aspnetwebstack.codeplex.com/SourceControl/latest

I've had a related issue. Sometimes calling GlobalConfiguration.Configure multiple times triggers this error. As a workaround, I've put all configuration initialization logic in one place.