ASP.NET MVC - passing parameters to the controller

I have a controller with an action method as follows:

public class InventoryController : Controller
{
    public ActionResult ViewStockNext(int firstItem)
    {
        // Do some stuff
    }
}

And when I run it I get an error stating:

> The parameters dictionary does not contain a valid value of type 'System.Int32' for parameter 'firstItem'. To make a parameter optional its type should either be a reference type or a Nullable type.

I had it working at one point and I decided to try the function without parameters. Finding out that the controller was not persistant I put the parameter back in, now it refuses to recognise the parameter when I call the method.

I'm using this url syntax to call the action:

http://localhost:2316/Inventory/ViewStockNext/11

Any ideas why I would get this error and what I need to do to fix it?

I've tried adding another method that takes an integer to the class it it also fails with the same reason. I've tried adding one that takes a string, and the string is set to null. I've tried adding one without parameters and that works fine, but of course it won't suit my needs.

Your routing needs to be set up along the lines of {controller}/{action}/{firstItem}. If you left the routing as the default {controller}/{action}/{id} in your global.asax.cs file, then you will need to pass in id.

routes.MapRoute(
    "Inventory",
    "Inventory/{action}/{firstItem}",
    new { controller = "Inventory", action = "ListAll", firstItem = "" }
);

... or something close to that.

you can change firstItem to id and it will work

you can change the routing on global.asax (i do not recommed that)

and, can't believe no one mentioned this, you can call :

http://localhost:2316/Inventory/ViewStockNext?firstItem=11

In a @Url.Action would be :

@Url.Action("ViewStockNext", "Inventory", new {firstItem=11});

depending on the type of what you are doing, the last will be more suitable. Also you should consider not doing ViewStockNext action and instead a ViewStock action with index. (my 2cents)

To rephrase Jarret Meyer's answer, you need to change the parameter name to 'id' or add a route like this:

routes.MapRoute(
        "ViewStockNext", // Route name
        "Inventory/ViewStockNext/{firstItem}",  // URL with parameters
        new { controller = "Inventory", action = "ViewStockNext" }  // Parameter defaults
    );

The reason is the default route only looks for actions with no parameter or a parameter called 'id'.

Edit: Heh, nevermind Jarret added a route example after posting.

or do it with Route Attribute:

public class InventoryController : Controller
{
    [Route("whatever/{firstItem}")]
    public ActionResult ViewStockNext(int firstItem)
    {
        int yourNewVariable = firstItem;
        // ...
    }
}

Headspring created a nice library that allows you to add aliases to your parameters in attributes on the action. This looks like this:

[ParameterAlias("firstItem", "id", Order = 3)]
public ActionResult ViewStockNext(int firstItem)
{
    // Do some stuff
}

With this you don't have to alter your routing just to handle a different parameter name. The library also supports applying it multiple times so you can map several parameter spellings (handy when refactoring without breaking your public interface).

You can get it from Nuget and read Jeffrey Palermo's article on it here

public ActionResult ViewNextItem(int? id) makes the id integer a nullable type, no need for string<->int conversions.

Using the ASP.NET Core Tag Helper feature:

<a asp-controller="Home" asp-action="SetLanguage" asp-route-yourparam1="@item.Value">@item.Text</a>

There is another way to accomplish that (described in more details in Stephen Walther's Pager example

Essentially, you create a link in the view:

Html.ActionLink("Next page", "Index", routeData)

In routeData you can specify name/value pairs (e.g., routeData["page"] = 5), and in the controller Index function corresponding parameters receive the value. That is,

public ViewResult Index(int? page)

will have page passed as 5. I have to admit, it's quite unusual that string ("page") automagically becomes a variable - but that's how MVC works in other languages as well...

Or, you could try changing the parameter type to string, then convert the string to an integer in the method. I am new to MVC, but I believe you need nullable objects in your parameter list, how else will the controller indicate that no such parameter was provided? So...

public ActionResult ViewNextItem(string id)...

The reason for the special treatment of "id" is that it is added to the default route mapping. To change this, go to Global.asax.cs, and you will find the following line:

routes.MapRoute ("Default", "{controller}/{action}/{id}", 
                 new { controller = "Home", action = "Index", id = "" });

Change it to:

routes.MapRoute ("Default", "{controller}/{action}", 
                 new { controller = "Home", action = "Index" });