Returning an error response in the case of a null model

Question

In my ASP.NET MVC API application, I can return a helpful `ErrorResponse` if a few of my `Required` fields are missing: ``` return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState); ``` ``` "Message": "The request is invalid.", "ModelState": { "myModel.FooA": [ "The FooA is required." ], "myModel.FooC": [ "The FooC property is required." ], "myModel.FooD": [ "The FooD property is required." ] } ``` However as this answer confirms, a NULL model will validate. As I don't allow this, how can I return an equally helpful error response stating all the values that are required? I know that I can manually add a ModelError for each property, but I suspect there may be a way that `CreateErrorResponse` can do this for me.

Accepted Answer

Are you using mvc3 or web-api? your tags indicate you're using mvc but your opening sentence implies web-api. If using mvc3 you can use the following:

In your controller before your call to ModelState.IsValid add:

```
if (modelObj == null)
{
    ModelState.Clear();
    var blankModel = new MyClass();
    TryValidateModel(blankModel);
    return View("About", blankModel);        
}
```

If you're using web-api and assuming you're using `System.ComponentModel.DataAnnotations` you can use the following:

```
ModelState.Clear();

var model = new MyClass();
var results = new List<ValidationResult>();
Validator.TryValidateObject(model, new ValidationContext(model, null, null), 
                                                                 results, true);

var modelState = new ModelStateDictionary();
foreach (var validationResult in results)
    modelState.AddModelError(validationResult.MemberNames.ToArray()[0],
                                                 validationResult.ErrorMessage);

return Request.CreateErrorResponse(HttpStatusCode.BadRequest, modelState);
```

Returning an error response in the case of a null model

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