ASP.NET Core MVC model binding and nullable reference types

A few weeks ago I blogged about an issue where my breakpoint inside an ASP.NET MVC controller was never hit. The problem was caused by the built-in ModelStateInvalidFilter.

What I didn’t share in that blog post is why the ModelState was invalid in the first place. Let’s see how a small change in my csproj file caused this…

This is the model I was using:

	public class EmailMessage
	{
	public string FromAddress {get;set;}
	public string ToAddress {get;set;}
	}

view raw EmailMessage.cs hosted with ❤ by GitHub

And this was an example I tried to send through my API:

	var message=new EmailMessage
	{
	ToAddress="to@test"
	};

view raw NewMessage.cs hosted with ❤ by GitHub

Everything worked until I made one small change. I updated my csproj file to start using nullable reference types:

<Nullable>Enable</Nullable>

view raw Example.csproj hosted with ❤ by GitHub

Now when I executed the same code it failed?! One look at the ModelState errors explained it:

The FromAddress field is required.

By enabling nullable reference types it handled both the ‘ToAddress’ and ‘FromAddress’ as required.

To fix it, I had to update the DTO and enable nullability:

	public class EmailMessage
	{
	public string? FromAddress {get;set;}
	public string ToAddress {get;set;}
	}

view raw EmailMessageFixed.cs hosted with ❤ by GitHub