Introduction
ASP.NET MVC has always had a robust request validation model that is easily harnessed. The model in question has to be decorated with validation attributes from a broad collection of possibilities, and the framework takes care of ensuring compliance and distilling it into a single boolean that can be referenced from ModelState.IsValid.
Nothing could be easier.
However, this approach requires that the programmer must remember to write the code to perform a validation check in each action method. This is tedious and error-prone, and results in duplicated code.
It is possible to implement this functionality in a more modular fashion by implementing it as a filter attribute and plugging it into the ASP.NET request pipeline.
Passive Attributes
Before we go into the implementation of the filter itself, there is a slight design idiosyncracy that has to be understood. Request filtering through a class that derives from ActionFilterAttribute is a common enough pattern. A request is filtered through this attribute if the method implementing its corresponding action is decorated with this attribute. But this approach imposes various technical restrictions and design compromises.
A more robust approach is to decorate the action method with a non-behavioural attribute that derives from Attribute, and adding the filter directly into the global filter collection.
Passive attributes are described by Mark Seemann at the link below.
Implementation
The passive attribute approach results in an attribute class called ValidateModelAttribute. Any method that requires model validation can be decorated with this attribute.
[HttpPost]
[ValidateModel]
public ActionResult Edit(PageViewModel<ExerciseEditViewModel> viewModel)
A RequestValidateModelFilter class implements the IActionFilter interface and is added into the global filters collection during the Application_Start event. When an incoming request arrives, it is passed through the RequestValidateModelFilter instance, which checks if the request requires validation, and if so, checks if the IsValid property of the model state is true.
If the model state is not valid, then the request pipeline is truncated and all validation errors are gathered into a JSON response. The application returns a HTTP status code 400 along with the list of errors in the body of the response.