Question

I’m learning ASP.net MVC 3 with the music store tutorial–but translating the C# into VB. http://www.asp.net/mvc/tutorials/mvc-music-store/mvc-music-store-part-2

Jon Galloway says that “ASP.NET MVC’s default routing convention is to treat the segment of a URL after the action method name as a parameter named “ID”.

I have the following code in my StoreController

  Function Details(id As Integer) As String
        Return "Store Details, ID= " & i.ToString
    End Function

But when I put in the url http://localhost:53867/store/details/5 I get the following server error

The parameters dictionary contains a null entry for parameter ‘i’ of
non-nullable type ‘System.Int32’ for method ‘System.String
Details(Int32)’ in ‘MvcMusicStore.MvcMusicStore.StoreController’. An
optional parameter must be a reference type, a nullable type, or be
declared as an optional parameter. Parameter name: parameters

Based on the Galloway description of default ID paramaters above, I would expect this method to take the 5 that comes at the end of the URL as the (default) parameter. Why is it coming out null? What am I missing?

Answer 1

A few things that I am noticing in your code that you may want to check.

1. In your function body, you are calling i.ToString, but passing in id as parameter
2. Doesn’t VB require the ByVal preface to parameters of functions?
3. Is this copy/paste from your project or did you type it? It sounds like 5 is being assigned to ID which is in the default route, but you are appearing to use i as your parameter, which is null, since it isn’t part of the route and you are not passing it as a querystring parameter