Question

The back end of my web page has to load a different class depending on a value it receives in the query string. I need a way to make sure that all the other necessary parameters are present so that the class can be instantiated.

Here’s a stripped down example:

Two classes, Car and Cycle both inherit from Vehicle.

Public Class Car
    Inherits Vehicle

    Private Const _wheels as Int = 4
    Private _transmission as string
    Private _mpg as int

    Public Sub New(Byval transmission as string, Byval mpg as int)
        _transmission = transmission
        _mpg = mpg
    End Sub
End Class

Public Class Cycle
    Inherits Vehicle

    Private Const _wheels as Int = 2
    Private _gears as Int

    Public Sub New(ByVal gears as Int)
        _gears = gears                
    End Sub
End Class

This is my query string: ?type=car&transmission=automatic

So I know what the type is, but I haven’t been provided with the mpg details, which I need to be able to instantiate the class.

The “manual” way to do this would be to parse the query string and write something like:

if not string.isNullOrEmpty(transmission) and not string.isNullOrEmpty(mpg) then
    dim c as new Car(transmission, mpg)
end if

But this could get messy if there are lots of parameters. It’s also harder to maintain when you want to edit a class object.

I think I want to get the name of each parameter that the class requires, and see if that is present in the query string. However, I would obviously need to do this before instantiating the class. I’m not sure if that’s even possible.

Is this the smart way to do this? Is there a smarter way than just writing out every parameter I need to check the existence of?

The code in this question has been written in vb.net but I’m happy to discuss answers in C#

Answer 1

var queryParams = HttpUtility.ParseQueryString("?type=car&transmission=automatic");

Type type = typeof(Car);

bool canCreate = type.GetConstructors()
                .Any(c => c.GetParameters().All(p => queryParams[p.Name] != null));

---

You can even create the vehicle object dynamically from the query string.

var queryParams = HttpUtility.ParseQueryString("?type=car&transmission=automatic&mpg=12");

Dictionary<string, Type> types = new Dictionary<string, Type>()
{
    {"car",typeof(Car)},
    {"cycle",typeof(Cycle)}

};

Type vehicleType = types[queryParams["type"]];

var cInfo = vehicleType.GetConstructors()
        .FirstOrDefault(c => c.GetParameters().All(p => queryParams[p.Name] != null));

if (cInfo != null)
{
    var cParams=  cInfo.GetParameters()
                        .Select(p => Convert.ChangeType(queryParams[p.Name], p.ParameterType))
                        .ToArray();

    object vehicle = Activator.CreateInstance(vehicleType, cParams);
}