Question

As stated in the title: how does razor know when the page should be rendered in error or normal state?

I need to know this information so I can display a page in error mode with validation messages displayed etc. Unfortunately I cannot simply do View() because the code which finds the model in invalid state is in subview.

EDIT
I’m trying to do something like this to force razor to render a view in error mode:

// just for tests....
// model: a model which was marked as invalid in different controller
// state: state of the model from that controller
public ActionResult asdf(TModel model, ModelStateDictionary state) {
    var result = View(this.Partial, model);
    result.ViewData.ModelState.Clear();
    foreach (var x in state) {
        result.ViewData.ModelState.Add(x.Key, x.Value);
    }

    return result;
}

EDIT2
Final solution. In the previous attempt auto-deserialization from Json to c# types didn’t work (sic!) so I’ve decided to receive a plain json string and deserialize it with another library like so:

public ActionResult ErrorIndex(string jsonParamsString) {
    var param = Newtonsoft.Json.JsonConvert.DeserializeObject<ForceInvalidStateRequestArg<TModel>>(jsonParamsString);

    if (param != null && param.Errors != null) {
        this.ModelState.Clear();
        foreach (var s in param.Errors) {
            this.ModelState.AddModelError(s.PropertyName, s.ErrorMessage);
        }
    }
    var result = View(this.PartialName, param == null ? this.NewModel : param.Model);
    return result;
}

Answer 1

Information about model validity is stored in ModelState object.

Which is accessible in controller like:

this.ModelState.IsValid

In view:

this.ViewContext.ViewData.ModelState

To your EDIT:

You can add model validation errors like this:

this.ModelState.AddModelError("key", "an error message");

So if you want to force @razor to render validation error messages. It could look like:

public ActionResult asdf(TModel model, ModelStateDictionary state) 
{
    var result = View(this.Partial, model);
    result.ViewData.ModelState.Clear();
    foreach (var pair in state.Where(m=> m.Value != null && m.Value.Errors.Any()))
    {
        result.ViewData.ModelState.AddModelError(pair.Key, string.Join(",",pair.Value.Errors.Select(e=>e.ErrorMessage).ToArray()));
    }  
    return result;
}