.NET Framework to .NET Migration: 3 Scenarios (And How to Choose the Right One)

It’s not breaking news that .NET Framework has gone legacy. Its last stable release saw the light in August 2022. Yes, Microsoft keeps rolling out security and reliability updates, but .NET Framework is no longer evolving with new features and capabilities.

Yet, you might be stalling with .NET Framework to .NET migration, and we get why. Changing a critical system means substantial risks, so you might be hesitant to modernize it. Downtime, for one, could hurt both your bottom line and your users’ experience.

Source: RedHat, Legacy application modernization challenges organizations face

Legacy .NET application modernization doesn’t always mean tearing down the whole codebase and writing a new one from scratch. Here’s how to pick the right modernization scenario for your system’s architecture, team, and business constraints.

Why It’s Time to Leave .NET Framework Behind

We’ve already mentioned that you shouldn’t expect any new features from .NET Framework. Microsoft has put it into maintenance mode, meaning you’ll get only security and reliability updates for its latest stable release.

But what exactly does that mean for your application and your business? Here are the five consequences of keeping the .NET Framework in place:

• Poor performance. Applications running on the .NET Framework are slower, plain and simple. For example, one Microsoft Partner saw a 7.4x improvement in throughput after switching out the .NET Framework for .NET 6.
• Declining NuGet ecosystem support. Modern libraries and APIs focus on .NET support. So, the share of up-to-date development tools compatible with the .NET Framework is declining.
• Windows lock-in. The .NET Framework runs only on Windows, but .NET can set you free: it supports Windows, Linux, and macOS. Linux environments can help you reduce infrastructure costs.
• Deployment limitations. Besides being stuck with Windows environments, you also can’t use containers or go the cloud-native deployment route. Cross-platform deployment isn’t an option with the .NET Framework, either.
• Growing talent gaps. New developers don’t waste their time learning how a framework on life support works. Instead, they focus on .NET 6+. So, the talent pool of .NET Framework professionals is slowly shrinking.

3 Scenarios for .NET Framework Migration

What comes to your mind when you imagine .NET Framework to .NET migration? Rewriting everything in one go? If so, that’s a misconception: most modernizations can be incremental.

That said, choosing the right migration strategy is crucial for mitigating risks. Here’s your .NET Framework migration approach comparison, based on Exoft’s experience.

1. Upgrade the Tech Stack

Upgrading the tech stack means migrating the application to .NET without making any changes to its architecture or business logic. It’s the fastest and most straightforward approach on the list. You probably won’t run into bad surprises along the way, so it’s the most predictable option, too. That said, it doesn’t address the tech debt your application has accumulated.

This path works best for mid-sized applications with a stable architecture, particularly for companies that have an in-house development team capable of executing the migration.

✅ What changes:

• Runtime (.NET replaces .NET Framework)
• Libraries and dependencies
• Development tools
• CI/CD pipelines and processes
• Hosting environment
• Data access libraries (optional)

❌ What stays intact:

• Architecture
• Business logic
• Database schema
• Integrations
• User interface

Pro tip from Exoft: Treat this .NET Framework upgrade as the first step in the overall migration. It will help you stabilize the system before addressing the tech debt with replatforming or modularization.2. Incrementally Rewrite

2. Incrementally Rewrite

Incremental rewriting involves migrating the system to .NET piece by piece, while the system itself remains live. This is the most common method for .NET Framework migration to .NET. It helps address the underlying technical debt, all while mitigating risks. Besides, the system stays live throughout migration, allowing you to migrate .NET Framework without downtime.

This approach is the go-to method for monolithic applications that require a transition to microservices, business-critical systems that have to be modernized with zero downtime, and legacy systems with complex business logic that are too risky to replace in one go.

For example, when Exoft migrated a two-decades-old gym management platform to a modern .NET tech stack, the incremental approach ensured zero downtime for its millions of users.

✅ What changes:

• Architecture
• Components (one by one)

❌ What stays intact:

• User interface
• Integrations
• Database

You can use multiple methods for incremental migration. Strangler pattern, for instance, involves layering new functionality on top of the old and routing traffic to those new layers. The old layers are systematically retired.

Source: Software Patterns Lexicon, How traffic is routed during legacy application modernization under the strangler pattern

In turn, vertical slice architecture deals with reorganizing the codebase by capability instead of keeping it structured as technical layers. You can build new capabilities alongside existing ones and run in parallel.Source: Dotdev, An example of a vertical slice architecture

Exoft’s advice: Prepare for a longer timeline and more hands-on management. Incremental rewriting typically requires 6+ months to complete (depending on complexity), and strong coordination is a must.

Case Study: Migrating a Marketing Campaign Management Solution

An international media company owner and operator turned to Exoft to modernize a marketing campaign management tool built on .NET Framework 4.8. When providing legacy application modernization services, Exoft’s team had to balance preserving critical features with introducing new technologies. Within a year, Exoft:

• Replaced .NET Framework with .NET 8
• Sped up backend processes
• Simplified maintenance
• Moved the application to a domain-driven architecture

Within less than a year, the company saw faster load times, fewer bugs and lower error rates, better responsiveness. There was an improved scalability across a platform serving over 1 million readers in 17 countries.

3. Rebuild Strategically (If Migration Isn’t Enough)

Strategic rebuilding involves starting fresh and building a new system from scratch using a modern architecture and tech stack. It’s the most time-consuming, expensive, and riskiest approach on the list. Still, it’s the best solution in rare cases when the system is too outdated to salvage incrementally.

This path works best for companies that can afford a full rewrite and only if the cost of carrying existing tech debt exceeds the cost of a full rebuild. In our experience, a full rebuild is rarely the right call. Most assessments and solution audits lead us toward incremental rewriting or a tech stack upgrade instead.

✅ What changes:

• Architecture
• Tech stack
• Business logic
• Database
• User interface
• Integrations

❌ What stays intact:

• General purpose and functionality

Exoft’s pro tip: Opt for this approach only if preserving legacy business logic would cost more than rebuilding the system. Do not choose it if there’s no real business need for a rebuild and you simply want a more modern tech stack.

«Every client who comes to us asking for a full rebuild has the same fear: what happens to the business while we’re rebuilding? Honestly, it’s a valid fear. A full rebuild takes months of work happening at the same time. The old system keeps running while the new one isn’t finished yet. That’s why we almost always advocate for incremental migration. This way, you’re not risking all your money and efforts on one launch day.»

How to Choose Your .NET Framework Migration Scenario: 6 Questions to Ask

Not sure how to migrate from .NET Framework to .NET Core in your specific case? Here are the six questions to consider before selecting the migration path:

1. How large and complex is your codebase? Large, complex codebases call for incremental rewriting. Small to medium ones give you more flexibility: incremental rewriting or a full rebuild can both work.
2. How coupled is it? Tightly coupled systems are best handled through incremental rewriting. Loosely coupled ones can go either way: a tech stack upgrade or incremental rewriting.
3. What is your current test coverage? If testing is mostly manual, prioritize automated test creation before touching anything else. If you already have automated tests, plan for a parallel rollout to catch bugs that slip through.
4. Can your system tolerate any downtime? If the answer is no, incremental rewriting is your only safe option. If some downtime is acceptable, a full rebuild becomes viable.
5. What is your timeline? If you need results fast, do the tech stack upgrade now and tackle tech debt later. Have a few months on hand? We recommend incremental rewriting.
6. How much tech debt does your system have? Little to none: go with a tech stack upgrade. Some but manageable: incremental rewriting. Too much to untangle: a full rebuild may be the only way forward (once again, very rare cases).

How Does the Migration Work in Practice?

The approach you pick will determine the project’s scope, costs, and timeline. That said, most .NET Framework migration projects unfold in five phases.

Assess First, Code Second

First, you need to know what you’re dealing with, and that calls for a system audit. During it, developers assess third-party dependencies, windows-specific APIs, integration points, and test coverage.

.NET Upgrade Assistant has been the go-to tool for automating the initial discovery. However, the tool has been deprecated and replaced with an AI-powered GitHub Copilot modernization agent:

Criteria	Legacy Upgrade Assistant	GitHub Copilot App
Cost	No charge	Paid Copilot subscription required
Strategy	Deterministic, rule-driven	Adaptive, AI-driven
Predictability	Consistent (identical inputs yield identical outputs)	Inconsistent (AI may produce varying solutions)
Ease of use	Step-by-step wizard UI	Conversational chat interface
Git integration	Commits handled manually	Each change triggers an automatic commit
Availability	Accessible via CLI or hidden settings	Built-in default starting VS 2026+
Maintenance status	No longer actively developed (maintenance mode)	Under active development

Source of the table: GAP Velocity, Legacy .NET Upgrade Assistant vs GitHub Copilot modernization agent: Comparison table

A thorough audit of dependencies early on can save you time and money down the road, as the team behind the Microsoft Commerce migration can attest. Before touching a single line of code, treat this like an IT rationalization exercise: make inventory of your apps and assess their value first.

Resolve the Hard Dependencies

Your application may rely on .NET Framework components that don’t have a cut-and-dried alternative in .NET. Those hard dependencies can block the migration. They include:

• Server-side WCF: Modern .NET doesn’t have a port of the legacy WCF. Replace it with gRPC or REST APIs.
• ASP.NET Web Forms: They also haven’t been ported to modern .NET. Rebuild the forms using Blazor or Razor Pages.
• System.Web: ASP.NET Core doesn’t have a System.Web namespace. Use a middleware pipeline to support it.

Migrate Small, Then Scale

Full rebuilds are rare and in most cases, you’ll start small and iterate. Here’s what it means depending on the approach:

• Tech stack upgrade. Update files to match .NET’s SDK-style project format and set the target framework to the desired version of .NET. Then, replace incompatible packages and update dependencies.
• Incremental rewriting. Migrate one module at a time. Extract it, retarget it for .NET, and connect the updated module with the rest of the legacy system.

Test, Test, Test

Subject the updated codebase to a battery of tests, including:

• Unit tests to validate each component
• Integration tests to verify the system components interact as expected
• Performance tests to quantify performance improvements
• Security tests to verify the system remains compliant with security standards

Important: Run both versions of the system in parallel for some time before cutting over to catch issues and compare performance in production.

Deploy

Unlike the .NET Framework, .NET offers multiple deployment options:

• Self-contained: Deploying a platform-specific executable together with the .NET runtime
• Framework-dependent: Deploying files without including the .NET runtime
• Single file: Bundling all application files in a single binary
• Containers: Publishing the app as a container image

Your choices here will impact your long-term infrastructure costs, application performance, and maintainability.

How to Prepare for .NET Framework Migration

Before you start racking your brain over how to migrate from .NET Framework to .NET 10, you’ll have to decide who will take care of the migration. Will it be your in-house team? An external partner? Both?

Each approach has its pros and cons:

Migration ownership	Pros	Cons	Best for
Internal	Full control Long-term capability building	Execution risk Opportunity costs	Simple migration projects Companies with strong internal .NET expertise
External	Reduced execution risk Access to narrow migration expertise	Knowledge transfer risks Vendor dependency risks	Companies with no or few in-house .NET developers Complex and risky migration projects
Hybrid	Combines the pros of both approaches	Coordination complexity Higher costs	Mission-critical systems Companies with in-house developers but no migration experience

Besides securing the expertise required, you will need to secure stakeholders’ approval and resources and retrieve all the app documentation you can find. If you partner up with an external vendor, knowledge transfer is non-negotiable. It goes both ways:

• You provide information on business processes and existing documentation
• The vendor provides you with documentation for the migrated system and trains your internal team, if necessary

Conclusion

Legacy .NET application modernization is by no means a straightforward, one-size-fits-all process. Your system’s complexity, dependencies, and tech debt will dictate which approach is the best in your specific case. So, audit the codebase before making a decision.

Need a hand selecting the right approach and executing it? Exoft has been helping companies identify the most effective migration path and leave legacy technology behind with minimal risks. Discover what we can do for your .NET Framework application.