Model validation in Web API - Exception is thrown with out a throw statement?

Question

I have seen the model validation from here (Under section: Handling Validation Errors).

The code-snippet is as below in Web API

public class ValidateModel : ActionFilterAttribute
    {
        public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
        {
            if (actionContext.ModelState.IsValid == false)
            {
                actionContext.Response = actionContext.Request.CreateErrorResponse(
                    HttpStatusCode.BadRequest, actionContext.ModelState);
            }

            base.OnActionExecuting(actionContext);
        }
    }

The problem is upon validation of model, if there were any errors, it assigns an model state invalid exception.

And after that, before going to the actual method (which is decorated with this [ValidateModel] attribute), WebAPI simply returns a 400 request.

But how ? Which function is returning a HTTP 400?

What happens after this method is done executing? Where does the control flow ?

EDIT:

The action that I am applying this attribute to is a normal one.

[ValidateModel]
public IHttpActionResult Post([FromBody]Request)
{
//do normal business logics here.
return Ok(SuccessMessage);
}

Answer 1

To understand the control flow you first need to visit this link here -

Action Attribute Class Reference

Check the method sections there. If clearly states that, OnActionExecuting is performed before that particular action, decorated with this attribute, is executed and OnActionExecuted is performed after the execution is complete. Since you are implementing OnActionExecuting like this -

public class ValidateModel : ActionFilterAttribute
{
    public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
    {
        if (actionContext.ModelState.IsValid == false)
        {
            actionContext.Response = actionContext.Request.CreateErrorResponse(
                HttpStatusCode.BadRequest, actionContext.ModelState);
        }

        base.OnActionExecuting(actionContext);
    }
}

and since the error is thrown inside this method, like this -

 if (actionContext.ModelState.IsValid == false)
        {
            actionContext.Response = actionContext.Request.CreateErrorResponse(
                HttpStatusCode.BadRequest, actionContext.ModelState);
        }

Your method will only execute if the OnActionExecuting method finds a valid model. And from the error 400, it seems that your current model state is not valid and thus it fails to succeed and your method will never be executed unless you provide a valid model. Set a debug point inside this method and you can find out why it fails.

By the way, the exception is not thrown its just a standard response that is handled by -

base.OnActionExecuting(actionContext);

http://msdn.microsoft.com/en-us/library/system.web.mvc.actionfilterattribute(v=vs.118).aspx