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How to Modernise a Legacy ASP.NET WebForms Application in 2025 (A Practical Guide)

Published on 1 Sep 2025 by New Media Aid — bespoke SME app development since the year 2000

ASP.NET WebForms .NET Core Razor Pages SME Software Legacy Modernisation SQL Server App Upgrades

Published by New Media Aid — bespoke SME app development since 2000

If your business still relies on a legacy ASP.NET WebForms application, you're not alone. Thousands of UK SMEs continue running mission-critical systems originally built between 2005 and 2015 — job management platforms, CRM systems, quoting tools, staff scheduling systems, stock/warehouse apps, and internal workflow tools.

The problem is simple: WebForms is now a dead-end technology. It is no longer developed by Microsoft, it performs poorly on modern browsers, and it simply cannot deliver the speed, security, usability or integration that SMEs expect in 2025.

But here’s the good news: you don’t need to scrap everything. Modernising a WebForms application is completely achievable — and usually far more cost-effective than rewriting from scratch.

Why Modernise WebForms in 2025?

Let’s start with the business case. A modernisation project typically delivers:

  • Better performance — WebForms relies on ViewState and heavy server controls, often making page loads 2–4× slower than Razor Pages or APIs.
  • Lower hosting costs — .NET 8/9 apps run faster, use fewer resources, and can scale horizontally.
  • Improved security — WebForms lacks modern middleware, security headers, and hardened authentication flows.
  • Mobile-friendly UI — Bootstrap 5/6 or Tailwind can transform the UX instantly.
  • Access to modern tooling — email APIs, SMS, AI chatbots, cloud storage, webhooks, Stripe, etc.
  • Future-proof code — ditch outdated Web.config files, hardcoded SQL in code-behind, and brittle event-driven pages.

Most importantly, modernisation reduces your technical risk. If your WebForms system goes down, finding a developer who can fix it in 2025 is increasingly difficult.

Modernisation Path: The Three Approaches That Actually Work

1. A Straight Rebuild (Full Rewrite)

This is only suitable for small systems or systems with severe legacy issues (e.g. no source control, broken logic, heavy SQL technical debt). It involves re-implementing the entire app in:

  • .NET 8 Razor Pages (best for SME CRUD systems)
  • .NET APIs + React/Vue (best for large teams / large UX requirements)
  • Blazor Server or Blazor WASM (if you want a .NET-only stack)

A full rewrite is the most expensive option, but it gives you full freedom to redesign workflows, improve UX and remove years of accumulated tech debt.

2. The “Progressive Upgrade” (Most Popular)

This is the approach New Media Aid uses most often. It lets you modernise the system one page, one feature or one module at a time — without shutting down the live WebForms app.

The typical flow:

  1. Keep your WebForms app running as normal.
  2. Create a new .NET Razor Pages project alongside it.
  3. Start moving individual modules across (e.g. Quotes → Jobs → Invoicing → Reports).
  4. Use the same database during the transition.
  5. Gradually retire WebForms pages as the new versions go live.

This approach reduces risk dramatically because your business never experiences downtime — and you avoid the “big bang rewrite” nightmare where everything must be perfect before launch.

3. API Layer + UI Replacement

This approach is best when your WebForms system is full of code-behind SQL, business logic mixed with UI logic, or extremely complex workflows.

It involves:

  • Extracting your business logic into a .NET 8 Web API.
  • Keeping the WebForms UI temporarily.
  • Building a modern frontend (.NET Razor Pages or React) on top of the API.

This makes future integrations much easier because all business logic lives in a clean, documented API layer.

What Parts of WebForms Should Be Replaced First?

When I modernise an SME WebForms app, I usually start with the highest-impact areas:

1. SQL Queries & Data Access

Most legacy apps contain:

  • Huge inline SQL queries in code-behind
  • Inefficient SELECT *
  • No indexing strategy
  • Slow dynamic queries
  • Blocking table-level scans

Simply moving to Dapper or EF Core with clean SQL often produces 10–100× performance improvements.

2. Authentication & Security

Legacy membership providers, SHA1 hashing and Web.config authentication must be replaced with:

  • ASP.NET Core Identity
  • JWT tokens
  • Modern cookie middleware

This alone eliminates many vulnerabilities.

3. File Uploads, Emails, PDFs and Exports

These are the lowest-hanging fruit to modernise because the .NET Core equivalents are vastly simpler and more secure:

  • IFormFile uploads
  • SendGrid / SMTP for email
  • QuestPDF / Puppeteer / WKHTMLtoPDF for PDFs
  • CSV/Excel export libraries

4. UI & User Experience

A new UI can make a 15-year-old system feel brand new. I typically rebuild pages with:

  • Bootstrap 5/6
  • Responsive layouts
  • Fewer postbacks
  • AJAX endpoints
  • Cleaner navigation and menus

A polished UI dramatically increases employee buy-in to the new system.

Modernisation Mistakes to Avoid

Modernisation projects fail for the same handful of reasons:

  • Trying to rebuild the entire system at once
  • Rewriting business logic instead of reusing it
  • Not cleaning up slow SQL
  • Ignoring mobile/tablet usability
  • No migration plan for URLs, SEO or routing
  • Forgetting to remove WebForms dependencies

The key is to modernise a system incrementally with clear milestones.

What Does a Modernised System Look Like?

When finished, your SME system should:

  • Run on .NET 8 or .NET 9
  • Use Razor Pages or APIs + a frontend framework
  • Include clean, well-indexed SQL
  • Support mobile browsers
  • Have a modern UI and UX
  • Use proper logging, caching and error tracking
  • Be ready for cloud hosting or containerisation

This instantly extends the life of your system for the next decade.

How New Media Aid Helps SMEs Modernise Their WebForms Systems

Over the last 20+ years, I’ve modernised dozens of SME systems. My approach is always:

  • Zero disruption to day-to-day operations
  • Incremental upgrades so staff adapt naturally
  • Database performance tuning for instant gains
  • Modern UI without rewriting all business logic
  • Steady long-term roadmap for future improvements

If your business relies on a WebForms system and you're unsure where to begin, I can analyse your existing app and recommend the simplest low-risk upgrade path.

Ready to Modernise? Let’s Talk.

Whether you need a small module upgraded, a slow SQL query optimised, or a full WebForms-to-.NET-Core roadmap, just get in touch.

Email: ab@newma.co.uk
Phone: +44 7967 219288

I’m always happy to advise SMEs on the best approach — no obligation.