Error assigning null to a nullable int – “The value ‘null’ is not valid for property”

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?

Answers:

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Method 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.

Method 2

In your Global.asax.cs file, in the Application_Start method, add the following line:

DataAnnotationsModelValidatorProvider.AddImplicitRequiredAttributeForValueTypes = false;

The issue is that, even for a nullable value type, by default, a “Required” attribute is added by the default ValidatorProvider.

Method 3

See here: ModelState validation fails for nullable types

The string “null” may be being passed. In my scenario I updated to an empty string “” and resolved this error.

Method 4

I think the issue is not assigning null, it’s that the consuming code does not except a null value. The assignment is perfectly valid (although your null check is inverted, and the cast is redundant).

viewModel.RegionId = null;

As an aside you can use HasValue to check for null on a nullable type. Although in reality this it’s no different to a null check, just looks a bit cleaner:

if (viewModel.RegionId.HasValue)
{
    // do something
}

Method 5

I had a problem with this.

My problem with this is that I was setting the offending property to null explicitly in the POST. So, akin to what you have, the request would’ve looked like:

RegionId = null

In my case, it was as simple as omitting the explicit declaration (e.g., don’t say it’s null, just don’t send any data)

Method 6

I have a feeling the problem lies somewhere out of the sample code you have shown. I have created a sample application that just creates a form and has a nullable field, but it works as expected. I would start eliminating other variables in your setup until you can to a working ViewModel with a nullable property, and you should be able to identify what is causing the problem.

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