ASP.NET MVC: No parameterless constructor defined for this object.

Question

Server Error in '/' Application.
--------------------------------------------------------------------------------

No parameterless constructor defined for this object. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.MissingMethodException: No parameterless constructor defined for this object.

Source Error: 


Line 16:             HttpContext.Current.RewritePath(Request.ApplicationPath, false);
Line 17:             IHttpHandler httpHandler = new MvcHttpHandler();
Line 18:             httpHandler.ProcessRequest(HttpContext.Current);
Line 19:             HttpContext.Current.RewritePath(originalPath, false);
Line 20:         }

1. I was following Steven Sanderson's 'Pro ASP.NET MVC Framework' book.
2. On page 132, in accordance with the author's recommendation, I downloaded the ASP.NET MVC Futures assembly, and added it to my MVC project. [Note: This could be a red herring.]
3. After this, I could no longer load my project. The above error stopped me cold.

My question is not, "Could you help me fix my code?"

Instead, I'd like to know more generally:

• How should I troubleshoot this issue?
  • What should I be looking for?
  • What might the root cause be?

• It seems like I should understand routing and controllers at a deeper level than I do now.

Answer 1

You need the action that corresponds to the controller to not have a parameter.

Looks like for the controller / action combination you have:

public ActionResult Action(int parameter)
{

}

but you need

public ActionResult Action()
{

}

Also, check out Phil Haack's Route Debugger to troubleshoot routes.

Answer 2

By default, MVC Controllers require a default constructor with no parameters. The simplest would be to make a default constructor that calls the one with parameters:

public MyController() : this(new Helper()) {
}

public MyController(IHelper helper) {
  this.helper = helper;
}

However, you can override this functionality by rolling your own ControllerFactory. This way you can tell MVC that when you are creating a MyController give it an instance of Helper.

This allows you to use Dependency Injection frameworks with MVC, and really decouple everything. A good example of this is over at the StructureMap website. The whole quickstart is good, and he gets specific to MVC towards the bottom at "Auto Wiring".

Answer 3

I just had a similar problem. The same exception occurs when a Model has no parameterless constructor.

The call stack was figuring a method responsible for creating a new instance of a model.

System.Web.Mvc.DefaultModelBinder.CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)

---

Here is a sample:

public class MyController : Controller
{
    public ActionResult Action(MyModel model)
    {

    }
}

public class MyModel
{
    public MyModel(IHelper helper)
    {
        // ...
    }
}

Answer 4

I got the same error when:

Using a custom ModelView, both Actions (GET and POST) were passing the ModelView that contained two objects:

public ActionResult Add(int? categoryID)
{
    ...
    ProductViewModel productViewModel = new ProductViewModel(
            product,
            rootCategories
            );
    return View(productViewModel); 
}

And the POST also accepting the same model view:

[HttpPost]
[ValidateInput(false)]
public ActionResult Add(ProductModelView productModelView)
{...}

Problem was the View received the ModelView (needed both product and list of categories info), but after submitted was returning only the Product object, but as the POST Add expected a ProductModelView it passed a NULL but then the ProductModelView only constructor needed two parameters(Product, RootCategories), then it tried to find another constructor with no parameters for this NULL case then fails with "no parameterles..."

So, fixing the POST Add as follows correct the problem:

[HttpPost]
[ValidateInput(false)]
public ActionResult Add(Product product)
{...}

Hope this can help somebody (I spent almost half day to find this out!).