I ran in to a problem to do with non nullable types in EF4 and ASP.Net MVC the other day and spent a while trying to solve it in a tidy fashion.

The issue was all my entity objects have data annotations on them for validation. This allows me in my controller methods call TryUpdateModel function when I want to convert post data into an object, if this then returns false I can return the errors to the user.

This has been working fine until I submitted a form with an empty text box that mapped to a non nullable string in the entity framework, what I expected to happen here was for it to check my data annotations see its a required field and for the TryUpdateModel to return false so I could return the error to the user.

However what actualy happened was TryUpdateModel threw an exception because it couldnt set this field to null. This still seems wrong to me but after some digging around it appears to be by design, it looks like TryUpdateModel first populates the model if any exceptions occur here they are thrown. It then runs your data annotations and if any of them fail it returns false else it returns true.

After a lot of head scratching I finally settled on the following solution.

  1. Extending the particular entity with a partial class.
  2. Adding a new property called NewSiteName
  3. Set the getter to return the failing Name Field
  4. Set the setter to set the value of the failing Name field if the value is not null else set it to "".

This now works fine, If you pass in Null the field gets set to “” and my required field data annotations can handle the error as they should. Here is the code I ended up using…

    public partial class Site
        //This property is here so if you try to create a new site with no name
        //an error is not thrown as SiteName is non nullable TryUpdateModel fails
        //when you pass in a sitename of null so here I convert null to "" then pass
        //it along to the real Name field
        public string NewSiteName
            get { return Name; }
                Name = value ?? "";