Asp.net MVC ModelState.Clear

Update:

• This is not a bug.
• Please stop returning View() from a POST action. Use PRG instead and redirect to a GET if the action is a success.
• If you are returning a View() from a POST action, do it for form validation, and do it the way MVC is designed using the built in helpers. If you do it this way then you shouldn't need to use .Clear()
• If you're using this action to return ajax for a SPA, use a web api controller and forget about ModelState since you shouldn't be using it anyway.

Old answer:

ModelState in MVC is used primarily to describe the state of a model object largely with relation to whether that object is valid or not. This tutorial should explain a lot.

Generally you should not need to clear the ModelState as it is maintained by the MVC engine for you. Clearing it manually might cause undesired results when trying to adhere to MVC validation best practises.

It seems that you are trying to set a default value for the title. This should be done when the model object is instantiated (domain layer somewhere or in the object itself - parameterless ctor), on the get action such that it goes down to the page the 1st time or completely on the client (via ajax or something) so that it appears as if the user entered it and it comes back with the posted forms collection. Some how your approach of adding this value on the receiving of a forms collection (in the POST action // Edit) is causing this bizarre behaviour that might result in a .Clear() appearing to work for you. Trust me - you don't want to be using the clear. Try one of the other ideas.

I think is a bug in MVC. I struggled with this issue for hours today.

Given this:

public ViewResult SomeAction(SomeModel model) 
{
    model.SomeString = "some value";
    return View(model); 
}

The view renders with the original model, ignoring the changes. So I thought, maybe it does not like me using the same model, so I tried like this:

public ViewResult SomeAction(SomeModel model) 
{
    var newModel = new SomeModel { SomeString = "some value" };
    return View(newModel); 
}

And still the view renders with the original model. What's odd is, when I put a breakpoint in the view and examine the model, it has the changed value. But the response stream has the old values.

Eventually I discovered the same work around that you did:

public ViewResult SomeAction(SomeModel model) 
{
    var newModel = new SomeModel { SomeString = "some value" };
    ModelState.Clear();
    return View(newModel); 
}

Works as expected.

I don't think this is a "feature," is it?