ASP.NET MVC: DropDownList validation

Question

Note: The following is just an example.

I'm pretty new to ASP.NET MVC and I'm trying to get my head around how validation of dropdown lists work. I have the following property in my ProfileViewModel class:

[DisplayName("Gender")]
public bool? Gender { get; set; }

null is meant to mean "unknown", true female and false male. In the view model constructor I

AllGenders = new List<SelectListItem>(2)
             {
                 new SelectListItem {Text = "Unknown", Value = "null"},
                 new SelectListItem {Text = "Male", Value = "false"},
                 new SelectListItem {Text = "Female", Value = "true"}
             };

First of all, it seems that I have to use strings when populating a List<SelectListItem>, which feels kinda weird. Is this really how it's done?

Secondly, when I choose "Unknown" in the list the validation fails telling me:

The value 'null' is not valid for Gender.

Why is that? When I remove the "null" option and change Gender to a simple bool, everything seems fine.

This is the ASPX:

<%= Html.DropDownList("Gender", Model.AllGenders) %>

(I can't get DropDownListFor to work correctly and it seems that many others have the same problem.)

Any help appreciated!

John Farrell · Accepted Answer · 2010-09-02 14:29:08Z

4

new SelectListItem {Text = "Unknown", Value = "null"},

should be:

new SelectListItem {Text = "Unknown", Value = ""},

Posting "" will result in null being bound.

answered Sep 2, 2010 at 14:29

John Farrell

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2 Comments

Deniz Dogan Over a year ago

Thanks a lot, I never would have been able to figure that out. Did you learn this by trial and error or can I find that information somewhere? It seems that "go-to resources" are pretty scarce when it comes to ASP.NET MVC.

John Farrell Over a year ago

Nope, never read about that, made sense to me. I think I first noticed it leaving input type=texboxs empty. The model binder always bound the string fields to null.

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