App_Start Compilation Stubs

The BundleConfig and RouteConfig stubs allow your migrated App_Start/ directory to compile without modification. These are no-op shims that do nothing at runtime — they exist only to make Web Forms configuration files compile as-is during Phase 1 migrations.

Overview

What they are: - No-op (no-operation) stub classes in System.Web.Optimization and System.Web.Routing namespaces - Emulate the Web Forms bundle and routing APIs just enough to compile - Located in the BlazorWebFormsComponents library and auto-imported via global usings

Why they matter: When you migrate a Web Forms application, your App_Start/BundleConfig.cs and App_Start/RouteConfig.cs files contain code like:

BundleTable.Bundles.Add(new ScriptBundle("~/bundles/jquery")...);
RouteTable.Routes.MapPageRoute(...);

Without stubs, this code fails to compile. The stubs allow your App_Start/ directory to compile unchanged in Phase 1, giving you time to plan the Blazor-native alternatives in Phase 2+.

Before and After

Web Forms (Original App_Start)Blazor with BWFC Stubs (No Changes Needed)

// App_Start/BundleConfig.cs
using System.Web.Optimization;

public class BundleConfig
{
    public static void RegisterBundles(BundleCollection bundles)
    {
        bundles.Add(new ScriptBundle("~/bundles/jquery")
            .Include("~/Scripts/jquery-{version}.js"));

        bundles.Add(new StyleBundle("~/Content/css")
            .Include("~/Content/site.css"));
    }
}

// App_Start/RouteConfig.cs
using System.Web.Routing;

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapPageRoute("", "{controller}/{action}/{id}", "~/Pages/{controller}/{action}.aspx");
    }
}

// Global.asax
protected void Application_Start()
{
    BundleConfig.RegisterBundles(BundleTable.Bundles);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
}

// App_Start/BundleConfig.cs — compiles unchanged!
using System.Web.Optimization;

public class BundleConfig
{
    public static void RegisterBundles(BundleCollection bundles)
    {
        bundles.Add(new ScriptBundle("~/bundles/jquery")
            .Include("~/Scripts/jquery-{version}.js"));

        bundles.Add(new StyleBundle("~/Content/css")
            .Include("~/Content/site.css"));
    }
}

// App_Start/RouteConfig.cs — compiles unchanged!
using System.Web.Routing;

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
        routes.MapPageRoute("", "{controller}/{action}/{id}", 
            "~/Pages/{controller}/{action}.aspx");
    }
}

// Program.cs (Blazor)
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddBlazorWebFormsComponents();

var app = builder.Build();
app.UseStaticFiles();
app.UseRouting();
app.MapBlazorHub();
app.MapFallbackToPage("/_Host");
app.Run();

Key difference: The App_Start/ classes compile but do nothing. Blazor handles bundling and routing completely differently (see below).

The Stubs

The BWFC library provides minimal implementations of these classes:

namespace System.Web.Optimization
{
    public class Bundle
    {
        public Bundle(string virtualPath) { }
        public Bundle Include(params string[] virtualPaths) => this;
    }

    public class ScriptBundle : Bundle { }
    public class StyleBundle : Bundle { }

    public static class BundleTable
    {
        public static BundleCollection Bundles { get; } = new();
    }

    public class BundleCollection
    {
        public void Add(Bundle bundle) { }
    }
}

namespace System.Web.Routing
{
    public class RouteCollection
    {
        public void MapPageRoute(string routeName, string routeUrl, 
            string physicalFile) { }
        public void Ignore(string url) { }
        public void Ignore(string url, object constraints) { }
    }

    public static class RouteTable
    {
        public static RouteCollection Routes { get; } = new();
    }
}

These stub classes do nothing — they exist only to satisfy the compiler.

⚠️ Important: These Stubs Are No-Ops

The stubs allow your code to compile, but they have zero runtime effect:

Web Forms Feature	What Happens	Blazor Alternative
BundleConfig.RegisterBundles()	Called but does nothing	CSS/JS Isolation (see below)
Bundle minification	Disabled — stubs ignore it	Build tool or Vite/esbuild
RouteTable.Routes.MapPageRoute()	Called but does nothing	@page directives in Blazor
Route ignores ({resource}.axd)	Ignored	Not needed in Blazor

In other words, if your Web Forms app relied on bundling or routing, you must implement Blazor alternatives. The stubs just prevent compilation errors.

Blazor Alternatives

CSS and JavaScript Bundling

Instead of BundleConfig, Blazor uses CSS/JS Isolation:

Web Forms (BundleConfig)Blazor (CSS Isolation)

public class BundleConfig
{
    public static void RegisterBundles(BundleCollection bundles)
    {
        bundles.Add(new StyleBundle("~/Content/css")
            .Include("~/Content/bootstrap.css", "~/Content/site.css"));
    }
}

// In .aspx page:
// <link href="~/Content/css" rel="stylesheet" />

<!-- Components/Layout/MainLayout.razor -->
@inherits LayoutComponentBase

<link href="bootstrap.css" rel="stylesheet" />
<link href="site.css" rel="stylesheet" />

@Body

Or, for component-scoped styles:

<!-- MyComponent.razor -->
<button class="btn">Click me</button>

<!-- MyComponent.razor.css (automatically scoped to component) -->
button.btn {
    background-color: blue;
}

For JavaScript bundling, use standard npm/webpack tooling:

npm install webpack webpack-cli --save-dev
npm run build  # Bundles and minifies JS

Routing

Instead of RouteConfig.MapPageRoute(), Blazor uses @page directives:

Web Forms (RouteConfig)Blazor (@page Directives)

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.MapPageRoute("product_detail", 
            "products/{id}", 
            "~/Pages/ProductDetail.aspx");
    }
}

<!-- ProductDetail.aspx accessed via /products/123 -->

<!-- Pages/ProductDetail.razor -->
@page "/products/{id}"

@code {
    [Parameter]
    public string Id { get; set; }

    protected override void OnInitialized()
    {
        // Load product with ID
    }
}

Blazor's routing is declarative (@page) rather than centralized. This is simpler and more composable.

Phase 1 → Phase 2+ Migration

Here's a suggested timeline:

Phase 1 (Now): - Keep App_Start/BundleConfig.cs and RouteConfig.cs unchanged - They compile with BWFC stubs - Focus on UI migration

Phase 2 (After initial migration): 1. Delete App_Start/BundleConfig.cs (no longer used) 2. Add CSS/JS imports to Shared/MainLayout.razor 3. Add @page directives to your Blazor components 4. Delete App_Start/RouteConfig.cs

Example transition:

// Phase 1: Stub-based (works but ignored)
public class BundleConfig
{
    public static void RegisterBundles(BundleCollection bundles)
    {
        bundles.Add(new StyleBundle("~/Content/css")
            .Include("~/Content/bootstrap.css", "~/Content/site.css"));
    }
}

// Phase 2: Migrate to Blazor alternatives
// Delete BundleConfig.cs, add to MainLayout.razor:
<link href="bootstrap.css" rel="stylesheet" />
<link href="site.css" rel="stylesheet" />

Troubleshooting

"BundleConfig not found" during compilation

Ensure the BlazorWebFormsComponents NuGet package is installed and AddBlazorWebFormsComponents() is called in Program.cs:

dotnet add package Fritz.BlazorWebFormsComponents

// Program.cs
builder.Services.AddBlazorWebFormsComponents();

Global usings automatically import the stubs into every file.

Bundling or routing not working at runtime

This is expected — the stubs do nothing. You must implement Blazor alternatives:

• For CSS/JS: Use Blazor CSS/JS Isolation or standard build tools
• For routing: Use @page directives instead of RouteConfig

Summary

• ✅ BundleConfig and RouteConfig files compile with BWFC stubs
• ✅ No code changes needed in Phase 1
• ❌ Stubs do nothing at runtime — bundling and routing are disabled
• 🔄 Plan Phase 2 migration to Blazor alternatives (CSS Isolation, @page directives)

See the following for implementation details:

• CSS Isolation in Blazor
• Blazor Routing
• JavaScript Interoperability