Is it possible to pass a delegate to a controller action?

Question

I want to pass a custom filter to an action method in my controller. I try to define it like that.

public ActionResult GetResult(Func<Fault,bool> filter)
{
    List<Fault> faultList;
    using (var _context = new myDB())
    {
        faultList = 
            from f in _context.Faults
            where filter(f)
            select f;
    }
    return Json(faultList);
}

but when I run the action I get an error

No parameterless constructor defined for this object.

at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck) at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark) at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark) at System.Activator.CreateInstance(Type type, Boolean nonPublic) at System.Activator.CreateInstance(Type type) at System.Web.Mvc.DefaultModelBinder.CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) at System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) at System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) at System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) at System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor) at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) at System.Web.Mvc.Controller.ExecuteCore() at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) at System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) at System.Web.Mvc.MvcHandler.<>c_DisplayClass6.<>c_DisplayClassb.b_5() at System.Web.Mvc.Async.AsyncResultWrapper.<>c_DisplayClass1.b_0() at System.Web.Mvc.Async.AsyncResultWrapper.<>c_DisplayClass81.b__7(IAsyncResult _) at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult1.End() at System.Web.Mvc.MvcHandler.<>c_DisplayClasse.b_d() at System.Web.Mvc.SecurityUtil.b__0(Action f) at System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) at System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) at System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Is it event possible to do something like that ?

If not by passing a delegate could you suggest another way ?

Answer 1

As I understood your question I'd say that it is not possible.

ASP NET MVC Action method in Controller is a kind of entry point against mapping http requests (url routing) where Action method is fired after received http request. So input parameters of the method, in fact, are result of deserializing (or mapping) Post or Get parameters from a http request to strongly typed object. When MVC framework creates the object it looks for parameterless constructor. It relates to data containers which are of reference type.

Another thing is sending functionality (logic) through http request and mapping it to NET delegate. Since NET is compile framework - not interpreter - there is no way to send NET code as text (e.g. C# code) and mapping it to delegate (for example we can return javascript code from Action method).

Last thing if even it was possible to compile NET code in runtime (there are some methods) the possibility of injecting NET code to Action method would cause security problems.

Answer 2

I don't know if it's possible to pass a delegate to an Action, but I have a suggestion for an alternative. You could pass a parameter to the action and assign the appropriate function to the filter delegate based on that value.

Here's a sample console app to illustrate my idea:

public enum Cases { Case1, Case2, Case3 };

class Program
{
    private static bool Test1(int test) { return test == 1; }

    private static bool Test2(int test) { return test == 2; }

    private static bool Test3(int test) { return test == 3; }

    public static void Main()
    {
        RunTest(Cases.Case1);
        RunTest(Cases.Case2);
        RunTest(Cases.Case3);

        Console.ReadLine();
    }


    private static void RunTest(Cases testCase)
    {
        var list = new List<int> {1, 2, 3};
        Func<int, bool> del;

        switch (testCase)
        {
            case Cases.Case1:
                del = Test1;
                break;
            case Cases.Case2:
                del = Test2;
                break;
            case Cases.Case3:
                del = Test3;
                break;
            default:
                throw new InvalidDataException();
        }

        list.ForEach( i => Console.WriteLine(del(i) ? i.ToString() : "--")  );
    }
}

Answer 3

It is definitely possible to pass a delegate. lets say you have a command class like below

public class MyCommand<T>{

Action myAction;
Func<T, bool> canExecute;

public MyCommand(Action<T> actionToBeExecuted, Func<T, bool> canExecute)
{
this.myAction = actionToBeExecuted;
this.canExecute = canExecute;
}

public void ExecuteMyCommand<T>(T param)
{
if(this.canExecute(param))
   this.myAction(param);
}
}

I believe the above code gives you a example and you can proceed along in the direction.

Answer 4

While what you're attempting to do isn't strictly possible, there are ways to do something similar.

I'm not passing in the Func from the user on the HTTP Request, but rather, using the Func as a seam in my code for testing and other purposes. I ran into the same issue, but worked around it by overloading a method with different parameters.

Here's what I had:

public class SomeController : Controller
{
    [HttpGet]
    public ActionResult SomeAction(string q, Func<string, SomeResultType> fn = null) 
    {
        fn = fn ?? SomeDefaultFn;
        ...

        return View(...);
    }
}

This didn't work for the reasons as @Bronek stated in his answer. However, since I wanted to provide a default anyway it was just a simple matter of overloading my method signatures.

public class SomeController : Controller
{
   [HttpGet]
   public Action SomeAction(string q)
   {
       // mvc will execute this action and pass
       // my default off to the next method
       return SomeAction(q, SomeDefaultFn);
   }

   public ActionResult SomeAction(string q, Func<string, SomeResultType> fn)
   {
        ...
        return View(...);
   }
}

In another place I have something like the following:

public class SomeController : Controller
{ 
    [HttpGet]
    public ActionResult SomeAction(string q, string filterType)
    {
        return SomeAction(q, FilterFactory.Build(filterType));
    }

    ...
}