Added documentation for IHttpContextAccessor to describe its use in c… (#6489)

aspnetcore/fundamentals/http-context.md

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+---
+title: Access HttpContext in ASP.NET Core
+author: coderandhiker
+description: Learn how to access HttpContext in ASP.NET Core.
+ms.author: riande
+ms.custom: mvc
+ms.date: 07/20/2018
+uid: fundamentals/httpcontext
+---
+# Access HttpContext in ASP.NET Core
+
+ASP.NET Core apps access the `HttpContext` through the [IHttpContextAccessor](/dotnet/api/microsoft.aspnetcore.http.ihttpcontextaccessor) interface and its default implementation [HttpContextAccessor](/dotnet/api/microsoft.aspnetcore.http.httpcontextaccessor).
+
+::: moniker range=">= aspnetcore-2.0"
+
+## Use HttpContext from Razor Pages
+
+The Razor Pages [PageModel](/dotnet/api/microsoft.aspnetcore.mvc.razorpages.pagemodel) exposes the [HttpContext](/dotnet/api/microsoft.aspnetcore.mvc.razorpages.pagemodel.httpcontext) property:
+
+```csharp
+public class AboutModel : PageModel
+{
+    public string Message { get; set; }
+
+    public void OnGet()
+    {
+        Message = HttpContext.Request.PathBase;
+    }
+}
+```
+
+::: moniker-end
+
+## Use HttpContext from a controller
+
+Controllers expose the [ControllerBase.HttpContext](/dotnet/api/microsoft.aspnetcore.mvc.controllerbase.httpcontext) property:
+
+```csharp
+public class HomeController : Controller
+{
+    public IActionResult About()
+    {
+        var pathBase = HttpContext.Request.PathBase;
+        // Do something with the PathBase.
+
+        return View();
+    }
+}
+```
+
+## Use HttpContext from middleware
+
+When working with custom middleware components, `HttpContext` is passed into the `Invoke` or `InvokeAsync` method and can be accessed when the middleware is configured:
+
+```csharp
+public class MyCustomMiddleware
+{
+    public Task InvokeAsync(HttpContext context)
+    {
+        // Middleware initialization optionally using HttpContext
+    }
+}
+```
+
+## Use HttpContext from custom components
+
+For other framework and custom components that require access to `HttpContext`, the recommended approach is to register a dependency using the built-in [dependency injection](xref:fundamentals/dependency-injection) container. The dependency injection container supplies the `IHttpContextAccessor` to any classes that declare it as a dependency in their constructors.
+
+::: moniker range=">= aspnetcore-2.1"
+
+```csharp
+public void ConfigureServices(IServiceCollection services)
+{
+     services.AddMvc()
+         .SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
+     services.AddHttpContextAccessor();
+     services.AddTransient<IUserRepository, UserRepository>();
+}
+```
+
+::: moniker-end
+
+::: moniker range="<= aspnetcore-2.0"
+
+```csharp
+public void ConfigureServices(IServiceCollection services)
+{
+     services.AddMvc();
+     services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
+     services.AddTransient<IUserRepository, UserRepository>();
+}
+```
+
+::: moniker-end
+
+In the preceding example:
+
+* `UserRepository` declares its dependency on `IHttpContextAccessor`.
+* The dependency is supplied when dependency injection resolves the dependency chain and creates an instance of `UserRepository`.
+
+```csharp
+public class UserRepository : IUserRepository
+{
+    private readonly IHttpContextAccessor _httpContextAccessor;
+
+    public UserRepository(IHttpContextAccessor httpContextAccessor)
+    {
+        _httpContextAccessor = httpContextAccessor;
+    }
+
+    public void LogCurrentUser()
+    {
+        var username = _httpContextAccessor.HttpContext.User.Identity.Name;
+        service.LogAccessRequest(username);
+    }
+}
+```