How do I get an HttpContext object from HttpContextBase in ASP.NET MVC 1?

The simplest way is to get the application, ApplicationInstance, and use its Context property:

// httpContextBase is of type HttpContextBase
HttpContext context = httpContextBase.ApplicationInstance.Context;

_{(thanks to Ishmael Smyrnow who noted this in the comments)}

Original answer:

You can, especially if the HttpContextBase instance you've been handed is of type HttpContextWrapper at run-time. The following example illustrates how you can do this. It supposes you have a method called Foo that accepts context as HttpContextBase but then needs to call a method in a third-party assembly (that you may not have the good fortune to modify) that is expecting the context to be typed as HttpContext.

void Foo(HttpContextBase context) 
{
    var app = (HttpApplication) context.GetService(typeof(HttpApplication));
    ThirdParty.Bar.Baz(app.Context);
}

// Somewhere in assembly and namespace ThirdParty,
// in a class called Bar, there is Baz expecting HttpContext:

static void Baz(HttpContext context) { /* ... */ }

HttpContextBase has a method called GetService as a result of supporting IServiceProvider. The GetService override of HttpContextWrapper delegates to the GetService implementation of the wrapped HttpContext instance. The GetService implementation of HttpContext allows you to query for usual suspects like HttpApplication, HttpRequest, HttpResponse and so on. It just so happens that HttpApplication has a property called Context and which returns an instance of HttpContext. So one gets at the wrapped HttpContext instance by asking HttpContextBase for HttpApplication via GetService followed by reading the Context property of the returned HttpApplication instance.

Unlike HttpContextBase, GetService does not appear as a public member of HttpContext but that is because HttpContext implements IServiceProvider.GetService explicity while HttpContextBase doesn't.

Bear in mind that Foo is no longer testable because it relies on being able to unwrap the underlying HttpContext during testing and which is next to impossible to fake/stub in the first place. The point of this answer, however, is to address the question, “How do I get an HttpContext object from HttpContextBase?”, literally. The illustrated technique is useful in those situations where you find yourself sandwiched between components you don't necessarily have the luxury to modify.

You can,

var abstractContext = new System.Web.HttpContextWrapper(System.Web.HttpContext.Current);