what is the usage of ExecuteResultAsync(ActionContext) method

What is the usage of ExecuteResultAsync(ActionContext) method?

1. IActionResult is the result (object) returned by your controller action method. ASP.NET Core executes this result by invoking the IActionResult::ExecuteResultAsync(ActionContext) method.
2. Action method might return different kinds of IActionResult, such as JsonResult (which sends a json to client when executed), BadRequestResult(which sends 400 response when executed) and so on.

3. You can also create your own implementation of IActionResult. Just be aware that the ExecuteResultAsync(ActionContext) method should write bytes to the HTTP Response . For example, I created a custom IActionResult to return CSV file for your reference. This is a simple implementation that doesn't contains too much complicated logic.

   Some action results are rather complicated that they introduce a new IActionResultExecutor<TResult> interface to deal with these process, for example, the ObjectResult employ an IActionResultExecutor<ObjectResult> to do that. When creating your own implementation, whether to use an IActionResultExecutor<ObjectResult> is up to you.

How the MVC will used the method?

1. WebApp developers don't need to invoke thisIActionResult::ExecuteResultAsync(ActionContext) method manually. This is a method that will be invoked by the MVC/RazorPage subsystem.
2. If you're interested, the whole process is:
   • a coming request received
   • match current request against the pre-defined routes (route table or graph). If matched :
     • we know the controller name, action name, and other route data.
     • Since we've known the controller name and action name, ASP.NET Core generates an instance of ActionDescriptor that describes the target C# action method, such as the parameters.
   • Since ASP.NET Core has known the Controller/Action and routes data, it creates an instance of IActionInvoker to invoke that action method pipline (including the filters, for more details, see official docs):
     • Action Method returns an instance of IActionResult
     • Before invoking the IActionResult::ExecuteResultAsync(ActionContext), invoke Result Filters OnResultExecuting() method.
     • invoke IActionResult::ExecuteResultAsync(ActionContext)
     • After that, invoke the Result Filters OnResultExecuted() method.

where is the ActionContext parameter from? from the Http request?

Firstly, HttpContext is built by the underlying server. It contains a Request property that mimics the HTTP Request.

Next, we'll get another two objects after selecting the action:

• RouteData: the route data, e.g. current area name, current page name, e.t.c.
• ActionDescriptor: a description about the current action that are matched with current route.

With the above three objects, ASP.NET Core creates the ActionContext by simply new it. For example, the IRouter-based Routing system creates an actionContext as below:( see source code)

// create action context
var actionContext = new ActionContext(context.HttpContext, routeData, actionDescriptor);

// create action invoker
var invoker = _actionInvokerFactory.CreateInvoker(actionContext);
if (invoker == null){ throw ...;}

// invoke the pipeline
return invoker.InvokeAsync();