[c#] Why is JsonRequestBehavior needed?

Why is Json Request Behavior needed?

If I want to restrict the HttpGet requests to my action I can decorate the action with the [HttpPost] attribute

Example:

[HttpPost]
public JsonResult Foo()
{
    return Json("Secrets");
}

// Instead of:
public JsonResult Foo()
{
    return Json("Secrets", JsonRequestBehavior.AllowGet);
}

Why isn't [HttpPost]sufficient?
Why the framework "bugs" us with the JsonRequestBehavior.AllowGet for every JsonResult that we have. If I want to deny get requests I'll add the HttpPost attribute.

You do not need it.

If your action has the HttpPost attribute, then you do not need to bother with setting the JsonRequestBehavior and use the overload without it. There is an overload for each method without the JsonRequestBehavior enum. Here they are:

Without JsonRequestBehavior

protected internal JsonResult Json(object data);
protected internal JsonResult Json(object data, string contentType);
protected internal virtual JsonResult Json(object data, string contentType, Encoding contentEncoding);

With JsonRequestBehavior

protected internal JsonResult Json(object data, JsonRequestBehavior behavior);
protected internal JsonResult Json(object data, string contentType, 
                                   JsonRequestBehavior behavior);
protected internal virtual JsonResult Json(object data, string contentType, 
    Encoding contentEncoding, JsonRequestBehavior behavior);

Improving upon the answer of @Arjen de Mooij a bit by making the AllowJsonGetAttribute applicable to mvc-controllers (not just individual action-methods):

using System.Web.Mvc;
public sealed class AllowJsonGetAttribute : ActionFilterAttribute, IActionFilter
{
    void IActionFilter.OnActionExecuted(ActionExecutedContext context)
    {
        var jsonResult = context.Result as JsonResult;
        if (jsonResult == null) return;

        jsonResult.JsonRequestBehavior = JsonRequestBehavior.AllowGet;
    }

    public override void OnResultExecuting(ResultExecutingContext filterContext)
    {
        var jsonResult = filterContext.Result as JsonResult;
        if (jsonResult == null) return;

        jsonResult.JsonRequestBehavior = JsonRequestBehavior.AllowGet;
        base.OnResultExecuting(filterContext);
    }
}

To make it easier for yourself you could also create an actionfilterattribute

public class AllowJsonGetAttribute : ActionFilterAttribute
{
    public override void OnResultExecuting(ResultExecutingContext filterContext)
    {
        var jsonResult = filterContext.Result as JsonResult;

        if (jsonResult == null)
            throw new ArgumentException("Action does not return a JsonResult, 
                                                   attribute AllowJsonGet is not allowed");

        jsonResult.JsonRequestBehavior = JsonRequestBehavior.AllowGet;            

        base.OnResultExecuting(filterContext);
    }
}

and use it on your action

[AllowJsonGet]
public JsonResult MyAjaxAction()
{
    return Json("this is my test");
}

By default Jsonresult "Deny get"

Suppose if we have method like below

  [HttpPost]
 public JsonResult amc(){}

By default it "Deny Get".

In the below method

public JsonResult amc(){}

When you need to allowget or use get ,we have to use JsonRequestBehavior.AllowGet.

public JsonResult amc()
{
 return Json(new Modle.JsonResponseData { Status = flag, Message = msg, Html = html }, JsonRequestBehavior.AllowGet);
}