model showing invalid even after it contain values - asp.net-mvc

Question

I have a model in mvc as below
public class person
{
[Required(ErrorMessage = "Please enter First Name.")]
public string first_name {get;set;}
[Required(ErrorMessage = "Please enter last Name")]
public string last_name {get;set;}
[Required(ErrorMessage = "Please enter |DOB")]
public DateTime DOB {get;set;}
}
post method
[HttpPost]
public ActionResult save_person(person per)
{
if(per.first_name == null || per.first_name =="")
per.first_name ="demo_first";
if(per.lastname == null || per.lastname =="")
per.last_name ="demo_last";
if (ModelState.IsValid) //fails even assignment is done above
{
}
}
so using if condition I make sure the model elements will contain atleast some value but even after that
ModelState.IsValid is failing and returning back to the view saying first_name and last_name is requried
how can we achieve this logic??

Answer 1

It is quite unclear why are the first_name and last_name properties on your model decorated with the Required attribute if they clearly are not required.
This being said, if you want to update the value of some model property in your controller you might need to ensure that you also update it in the ModelState:
if (per.first_name == null || per.first_name == "")
{
per.first_name ="demo_first";
ModelState.Remove("first_name");
ModelState.SetModelValue("first_name", new ValueProviderResult(per.first_name, per.first_name, CultureInfo.InvariantCulture));
}
if (per.last_name == null || per.lastname == "")
{
per.last_name ="demo_last";
ModelState.Remove("last_name");
ModelState.SetModelValue("last_name", new ValueProviderResult(per.last_name, per.last_name, CultureInfo.InvariantCulture));
}
if (ModelState.IsValid)
{
...
}

Answer 2

I just want to add some reference to the actual documentation to clear things up for the OP on why changing properties in the method does not solve his problem.
From the docs:
Handling Model State Errors
Model validation occurs prior to each
controller action being invoked, and it is the action method’s
responsibility to inspect ModelState.IsValid and react appropriately.
In many cases, the appropriate reaction is to return some kind of
error response, ideally detailing the reason why model validation
failed.
That means, that validation has already happened when your ActionResult is invoked and the .IsValid property is set.
As per your comment: If the same model needs to be used in a different view, but with different validation requirements, it would be a good idea to create a new ViewModel with appropriate annotations:
public class AnotherPersonViewModel {
//no required attribute
public string last_name {get; set;}
//no required attribute
public string first_name {get; set;}
//... rest of attributes
}
After this you could always map your view model to your entity model or whatever you are currently doing. But you avoid cluttering your controller with unnecessary code to remedy mistakes in your architecture.