The type or namespace name 'HttpGet' could not be found when add 'System.Web.Http' namespace

The reason you are getting this exception is because there are 2 different HttpGetAttribute classes in 2 different namespaces:

• System.Web.Mvc.HttpGetAttribute
• System.Web.Http.HttpGetAttribute

The first is used in ASP.NET MVC controllers and the second is used in ASP.NET Web API controllers.

When you imported the second namespace the compiler is no longer able to disambiguate which of the 2 classes you are referring to because the 2 namespaces are in scope.

Basically Microsoft duplicated all the classes that existed in ASP.NET MVC for the Web API but placed them in different namespace. Basically you shouldn't be mixing those namespaces.

But i need to add another one Namespace using System.Web.Http; for httpsresponseexpection error handling in my controller

Why would you need to use this in an ASP.NET MVC controller? Normally that's something you should be doing in a Web API controller.

But if for some reason you need to mix the 2 you will have to explicitly specify which attribute you need to use by fully qualifying it:

[System.Web.Mvc.HttpGet]
public ActionResult About()
{
    ViewBag.Message = "Your app description page.";
    return View();
}