ASP.NET MVC 4, how to access/modify the view model object (and change view and action method) before it is used as action method parameter?

问题

Is there any useful hook in ASP.NET MVC (MVC4) which can let you access the Action method parameter (View model) before the action method becomes invoked, and then also (e.g. depending on the value of something you checked in the action method parameter) let you prevent the action method from being invoked, i.e. instead either forward the view model object (action method parameter) to another action method or directly to some view (i.e. without any further processing in an action method) ?

If you do not understand the question, please see the code example below which should illustrate the kind of code I am looking for... (though I do not know if there actually exists such kind of interface and a possibility to hook an implementation into the MVC framework)

If this is indeed possible, I would like to see an answer with code example about how to do it (and not just a response with someone claiming that e.g. "try using method 'ActionFilterAttribute.OnActionExecuting' or 'IModelBinder.BindModel' " because I have already tried those and could not make it work). Also, please respect that I do not want this thread to become a discussion about WHY to do it, but want to see HOW to do it. (i.e. I am not interested in getting into discussions with responses such as "What are you actually trying to achieve?" or "There are probably better things of doing what you want to do...")

The question can be split into three subquestions/code examples as my own code samples below try to illustrate: (but would like them "refactored" into REAL code with usage of real existing types) (obviously, every type below which includes the substring "Some" is something I have made up, and I am looking for the corresponding real thing ...)

(1) Example of how to get access to (and potentially modify) view model objects (action method parameters) in a generic place before the actual action method is invoked with the view model object parameter.

The kind of code example I am looking for would probably be similar to below but do not know what kind of interface to use and how to register it to be able to do something like below:

public class SomeClass: ISomeInterface { // How to register this kind of hook in Application_Start ?
  public void SomeMethodSomewhere(SomeActionMethodContext actionMethodContext, object actionMethodParameterViewModel) {
    string nameOfTheControllerAboutToBeInvoked = actionMethodContext.ControllerName;
    string nameOfTheActionMethodAboutToBeInvoked = actionMethodContext.MethodName;
    // the above strings are not used below but just used for illustrating that the "context object" contains information about the action method to become invoked by the MVC framework
    if(typeof(IMyBaseInterfaceForAllMyViewModels).IsAssignableFrom(actionMethodParameterViewModel.GetType())) {
        IMyBaseInterfaceForAllMyViewModels viewModel = (IMyBaseInterfaceForAllMyViewModels) actionMethodParameterViewModel;
        // check something in the view model:
        if(viewModel.MyFirstGeneralPropertyInAllViewModels == "foo") {
            // modify something in the view model before it will be passed to the target action method
            viewModel.MySecondGeneralPropertyInAllViewModels = "bar";
        }
    }
  }
}

(2) Example of how to prevent the targeted action method from being executed and instead invoke another action method. The example might be an extension of the above example, with something like below:

    public void SomeMethodSomewhere(SomeActionMethodContext actionMethodContext, object actionMethodParameterViewModel) {
        ... same as above ...
        if(viewModel.MyFirstGeneralPropertyInAllViewModels == "foo") {
            actionMethodContext.ControllerName = "SomeOtherController";
            actionMethodContext.MethodName = "SomeOtherActionMethod";
            // The above is just one example of how I imagine this kind of thing could be implemented with changing properties, and below is another example of doing it with a method invocation:
            SomeHelper.PreventCurrentlyTargetedActionMethodFromBecomingExecutedAndInsteadExecuteActionMethod("SomeOtherController", "SomeOtherActionMethod", actionMethodParameterViewModel);
            // Note that I do _NOT_ want to trigger a new http request with something like the method "Controller.RedirectToAction"
        }           

(3) Example of how to prevent the normal action method from being executed and instead forward the view model object directly to a view without any further processing.

The example would be an extension of the first above example, with something like below:

    public void SomeMethodSomewhere(SomeActionMethodContext actionMethodContext, object actionMethodParameterViewModel) {
        ... same as the first example above ...
        if(viewModel.MyFirstGeneralPropertyInAllViewModels == "foo") {
            // the below used razor view must of course be implemented with a proper type for the model (e.g. interface 'IMyBaseInterfaceForAllMyViewModels' as used in first example above)
            SomeHelper.PreventCurrentlyTargetedActionMethodFromBecomingExecutedAndInsteadForwardViewModelToView("SomeViewName.cshtml", actionMethodParameterViewModel);
        }

回答1:

You could use an action filter and override the OnActionExecuting event:

public class MyActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        ...            
    }
}

Now let's see what useful information you could extract from this filterContext argument that is passed to this method. The property you should be looking for is called ActionParameters and represents an IDictionary<string, object>. As its name suggests this property contains all the parameters that are passed to the controller action by name and value.

So let's suppose that you have the following controller action:

[MyActionFilter]
public ActionResult Index(MyViewModel model)
{
    ...
}

Here's how you could retrieve the value of the view model after model binding:

public class MyActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        var model = filterContext.ActionParameters["model"] as MyViewModel;
        // do something with the model
        // You could change some of its properties here
    }
}

Now let's see the second part of your question. How to shortcircuit the controller action and redirect to another action?

This could be done by assigning a value to the Result property:

public class MyActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext) 
    {
        ... some processing here and you decide to redirect:

        var routeValues = new RouteValueDictionary(new
        {
            controller = "somecontroller",
            action = "someaction"
        });
        filterContext.Result = new RedirectToRouteResult(routeValues);
    }
}

or for example you decide to shortcircuit the execution of the controller action and directly render a view:

public class MyActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        var viewResult = new ViewResult
        {
            ViewName = "~/Views/FooBar/Baz.cshtml",
        };
        MyViewModel someModel = ... get the model you want to pass to the view
        viewResult.ViewData.Model = model;
        filterContext.Result = viewResult;
    }
}

or you might decide to render a JSON result:

public class MyActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        MyViewModel someModel = ... get the model you want to pass to the view
        filterContext.Result = new JsonResult
        {
            Data = model,
            JsonRequestBehavior = JsonRequestBehavior.AllowGet
        };
    }
}

So as you can see the possibilities are unlimited of what you can do.

回答2:

I have experimented with the code in the answer provided by the user Darin Dimitrov, and the first and third parts of the answer are correct.

(Though, for others who might find this thread and be interested, I can clarify that in the first answer the "model" does not seem to be a hardcoded keyword always used for the model but seems to have to correspond to the chosen name of the action method parameter. In other words, if you instead have the method signature

public ActionResult Index(MyViewModel myViewModel)

then in your action filter you have to use

var model = filterContext.ActionParameters["myViewModel"] as MyViewModel;

)

Regarding the second answer, the usage of 'RedirectToRouteResult' will trigger a new http request (which was not desired as I mentioned in the second code example of mine). I found another way of "changing" action method by actually invoking it explicitly:

var controller = new SomeController();
ActionResult result = controller.SomeAction(model);
filterContext.Result = result;

The above code actually seems to prevent the originally targeted action method from becoming invoked, i.e. when I put a breakpoint in the method annotated with '[MyActionFilter]' the execution never got into that method. Typically, it is probably not desired to hardcode a controller like above, but instead reflection might be used, for example as below with the thirdpart library "fasterflect":

string nameOfController = ... 
string nameOfActionMethod = ... 
// both above variables might for example be derived by using some naming convention   and parsing the refering url, depending on what you want to do ...
var theController = this.GetType().Assembly.CreateInstance(nameOfController);
ActionResult result = (ActionResult)theController.CallMethod(nameOfActionMethod, model);
filterContext.Result = result;

(for those who want to extract the names of the current target controller and action method, when implementing logic to determine the controller you want to invoke, you can use this code in the filter:

var routeValueDictionary = filterContext.RouteData.Values;
string nameOfTargetedController = routeValueDictionary["controller"].ToString();
string nameOfTargetedActionMethod = routeValueDictionary["action"].ToString();

)

I think it feels a bit awkward to instantiate and invoke controllers like above, and would prefer to change the target controller and action method in another way if possible ? So, the remaining question is if there is still (in MVC 4 final version) no way of redirecting/forwarding execution "internally" (without a new http request being fired as with 'RedirectToAction') at the server ?

Basically, I think I am here just looking for something like "Server.Transfer" which was used with ASP.NET Web Forms (and also the old classic ASP I believe could use the same thing).

I have seen older question/answers on this issue with people implementing this behaviour themselves with some "TransferResult" class of their own, but it seems to tend to become broken i different MVC versions. (for example, see here for MVC 4 beta: How to redirect MVC action without returning 301? (using MVC 4 beta) ).

Is there really still not a simple standard solution (implemented in MVC 4 final) about how to do an "internal redirect" without a new http request (as RedirectToAction does) ?

来源：https://stackoverflow.com/questions/14447297/asp-net-mvc-4-how-to-access-modify-the-view-model-object-and-change-view-and-a