MVC Custom ViewModel and auto binding

Question

I have a custom ViewModel defined as :

public class SampleFormViewModel
{
    public SampleFormViewModel(SelectList companies, Widget widget)
    {
        Compa         


        

Answer 1

MVC requires, on strongly typed views, that the view can create the class used on that view. This means a constructor without any parameters. And this makes sense. Folks new to MVC will see similar "huh?" issues when they forget/fail to make parameters public and all such related errors that popup when the view attempts to put itself together (as opposed to a compiler error).

But what is "interesting" in this class of parameterless constructor problems is when a property of your class also does NOT have a parameter-free constructor. I guess this is the pessimistic approach?

Having spent some learning time on the SelectList class - a class specific to MVC - I wanted to hopefully help some folks save a few minutes/hours.

This really important tool/class for dropdown list creation, has the following constructors:

public SelectList(IEnumerable items); public SelectList(IEnumerable items, object selectedValue); public SelectList(IEnumerable items, string dataValueField, string dataTextField); public SelectList(IEnumerable items, string dataValueField, string dataTextField, object selectedValue);

..and therefore, if these are properties on your class (the one used for the view), MVC will give you the elusive "No parameterless constructor" error.

BUT, if you create something like a helper class, cut-n-paste the exact code from your original class, and then make that helper class a parameter (NOT a get/set) on your original class; you're good to go.

And in this manner, you can use a single view for gets and posts. Which is more beautiful :)

Personnally, I'd have either created the compiler to recognize the associations and requirements of strong typed views, or let the dropdown (or other "customer" of the SelectList) just fail to work rather then wonder if there's a specific level of recursive checking on paramerterless constructors.

Thankfully, the current version seems to only be top-level. Feels like a hack and I hope it's by design.

HTH.