Altering the ASP.NET MVC model with an ActionFilter

Action filters are executed before and/or after the execution of an action method, they can be used to modify how an action executes or to perform a separate task, such as authorization or logging.

Action filters can be applied globally, at the controller level, or on an action method.

See here for more details.

In this post I’ll deal with how action filters allow you intercept an outgoing response to change a value in the returned model.

Action filter

Create a filter that inherits from ActionFilterAttribute and override the OnActionExecuted method.

public override void OnActionExecuted(ActionExecutedContext filterContext)

From the filterContext we can get model.

var model = filterContext.Controller.ViewData.Model as BaseModel;

Any value in the BaseModel can now be changed, in the example provided I change the MeetingDate property.

This action filter to needs to be applied to the appropriate action methods.
I’m going to register the filter globally, in this way it will run on all executed action methods.

Go to the FilterConfig class to register the filter.

public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
	filters.Add(new HandleErrorAttribute());
	filters.Add(new ModelAlterFilterAttribute());
}

That is all you need to do to create an action filter, register it and to alter the returned model.

As an aside, if you want to alter the value of an incoming parameter you override the OnActionExecuting method of the action filter.
In this example customerID is just an int but this approach will work for complex models too.

public override void OnActionExecuting(ActionExecutingContext filterContext)
{
	var actionParams = filterContext.ActionParameters;

	object customerID;
	actionParams.TryGetValue("customerid", out customerID);
	if (customerID != null)
	{
		customerID = 222;
	}

	base.OnActionExecuting(filterContext);
}

In the next post I show how you to selectively disable a global action filter on specific action methods or controllers.

Full source code here.

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