SystemWebdll

System.Web.dll is a core library of the Microsoft ASP.NET framework used by applications built on the .NET Framework to run Web Forms and other ASP.NET features. The assembly provides the runtime components for processing HTTP requests and generating responses, as well as many foundational types used across ASP.NET applications.

The library contains a wide range of namespaces, including System.Web, System.Web.UI, System.Web.Security, System.Web.Caching, System.Web.SessionState, and System.Web.Routing, among others. Core types include HttpContext, HttpRequest, HttpResponse, HttpApplication, Page, and HttpModule, which together implement the ASP.NET page lifecycle, server controls, state management, authentication, and caching. It also supports the HTTP pipeline through modules and handlers that respond to request events.

System.Web.dll is installed with the .NET Framework and is typically loaded from the Global Assembly Cache for compatible installations. Its version aligns with the framework version in use (for example, 4.x). It is not part of .NET Core or later cross-platform runtimes; modern ASP.NET functionality on those platforms is provided by a modular set of NuGet packages (such as Microsoft.AspNetCore.*) rather than System.Web.dll.

Usage and role: the assembly underpins Web Forms applications, the ASP.NET runtime, server-side controls, and related features such as authentication, session state, caching, and routing within the traditional ASP.NET pipeline. Modern development in the .NET ecosystem has shifted toward ASP.NET Core, which reorganizes and reimplements many of these capabilities in a different, cross-platform model. Web Forms remains supported only on the .NET Framework and is not part of the .NET Core family.