Error assigning null to a nullable int - "The value 'null' is not valid for property"

Question

I have this property in my view model:

[DisplayName("Region")]
public int? RegionId { get; set; }

I pass my view model to my controller, and it fails at ModelState.IsValid if RegionId is null. If I pass an integer to it, it works fine.

The error message is:

The value 'null' is not valid for Region

I've also tried calling this before I check ModelState.IsValid, but I get the same error:

if (viewModel.RegionId == null)
    viewModel.RegionId = (int?)null;

What's the problem here? Why can't I assign null to something that is nullable?

Answer 1

Here's another possible explanation.

I am AJAX POSTing to an MVC action method using jQuery like so:

$.post('url/here',dataObject);

jQuery will turn the data object into a query string. I use the default model binder to push the values into my ViewModel. Model validation fails with "The value 'null' is not valid for field name". But the property is a nullable int. After inspecting the raw POST I find that the data object I am POSTing is serialized into a query string that looks like this:

Field1=null&Field2=value&Field3=value

and not this:

Field1=&Field2=value&Field3=value

So model validation is understandably complaining about assigning the string literal 'null' to my nullable int. One solution is to check for NULLs before POSTing. I wish I had time to dig in more but I just switched to sending a JSON representation of the data object which correctly handles the null values.