Migrating a large application from .NET Core 2.1 → .NET 8 is a big jump (over 6 major versions), and teams normally face many challenges.
Here are the major, real-world challenges you can mention in interviews or documentation:
✅ 1. Breaking Changes Across Multiple Versions
Going from 2.1 to 8 means passing through .NET Core 3.1, .NET 5, 6, 7, and 8—each with its own breaking changes.
Typical issues:
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Removed APIs
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Updated package versions
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Framework changes (e.g., EF Core, JSON handling, authentication libraries)
✅ 2. Dependency and NuGet Package Compatibility
Many third-party libraries built for .NET Core 2.1 are:
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Outdated
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Unmaintained
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Not compatible with .NET 6+ or .NET 8
Examples:
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Old Swagger/Swashbuckle versions stop working
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Old JWT or Identity packages incompatible
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Outdated logging and configuration libraries
Fix often required:
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Upgrading to latest versions
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Replacing deprecated libraries with alternatives
✅ 3. Configuration & Hosting Model Changes
.NET Core 3.1 introduced a new generic host model.
Common issues:
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Startup.csincompatibilities -
Changes in
CreateHostBuilder,IHostingEnvironment,IApplicationLifetime -
Removal of
.UseJson()in config providers
.NET 8 encourages:
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Minimal hosting model
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Top-level statements
Migration often requires restructuring Program.cs.
✅ 4. JSON Serialization: Newtonsoft.Json → System.Text.Json
By default .NET switched to System.Text.Json which:
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Does not support all Newtonsoft features
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Behaves differently with casing, null handling, enums, custom converters
Challenges:
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API responses suddenly changing
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Serialization breaking
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Need to write custom converters
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Bring back Newtonsoft manually when needed
✅ 5. Entity Framework Core Major Changes
If your project uses EF Core:
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EF Core 2.1 → EF Core 8 has many breaking changes
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LINQ queries that previously worked may now break
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Query translation changes
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Async behaviors changes
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Lazy loading differences
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Updated annotations and Fluent APIs
Sometimes large refactoring required.
✅ 6. API Changes for Authentication & Authorization
Major issues when updating:
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AddAuthentication()changes -
JWT token handling changes
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Microsoft Identity Model packages updated
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Deprecated handlers and schemes
Common result:
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Users unable to authenticate until the pipeline is fixed.
✅ 7. ASP.NET Core Middleware Changes
Middleware signature changes cause:
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Old custom middleware failing
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Missing
UseRouting()/UseEndpoints()causing routing to break -
IHttpContextAccessorbehavior differences
✅ 8. Obsolete or Removed Features
From 2.1 to 8, Microsoft removed several APIs:
Examples:
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BinaryFormatter serialization
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Web API / MVC options renamed
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HttpClient factory changes
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Removed old Razor syntax constructs
You must replace or rewrite code.
✅ 9. C# Language & Compiler Changes
Going from C# 7 → C# 12:
Issues include:
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Nullable reference types (
?) suddenly creating warnings -
Async streams requiring refactors
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Refactoring around tuples, pattern matching, and null safety
✅ 10. Performance Impacts & Behavior Changes
While .NET 8 is faster, old logic may behave differently.
Examples:
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Threading behavior changes
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Memory usage patterns updated
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Garbage collector improved
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Kestrel server pipeline changed
These can cause:
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Latency issues
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Unexpected behavior under load
🧩 11. CI/CD & Runtime Environment Changes
Older Linux containers (2.1) don’t match .NET 8 images.
Issues:
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OS-level compatibility
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Docker image restructuring
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Alpine vs Debian changes
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SSL/cert changes
Deployment pipelines often require rework.
🎯 In Short — Key Points to Mention in Interviews
If answering concisely, you can say:
“The major challenges we faced migrating from .NET Core 2.1 to 8 were breaking API changes, third-party package incompatibility, changes in hosting and configuration model, JSON serializer differences, Entity Framework Core migration issues, authentication pipeline updates, and required refactoring in Program/Startup files. We also encountered deployment and Docker image changes as well as C# compiler/nullable reference warnings.”
Here are both versions — 30-second and 2-minute — that you can use in interviews.
⏱️ 30-Second Interview Answer (Short & Strong)
“Migrating from .NET Core 2.1 to 8 involved several major challenges. The biggest ones were breaking changes across versions, especially around the hosting model, Startup/Program refactoring, and dependency compatibility. Many NuGet packages were outdated and needed replacements. We also had to adjust JSON serialization differences between Newtonsoft and System.Text.Json, update our authentication pipeline, and resolve EF Core query-breaking changes. Additionally, deployment required new Docker images and environment adjustments. Overall, the migration required careful testing, refactoring, and library upgrades.”
⏱️ 2-Minute Interview Answer (Detailed & Confident)
“Our migration from .NET Core 2.1 to .NET 8 was a significant effort because we were jumping across six major versions. The first major challenge was breaking changes, especially around the hosting model—
Startup.cs,IHostingEnvironment, and the old WebHostBuilder were no longer compatible, so we refactored the entire startup pipeline into the new minimal hosting model.The next challenge was NuGet package compatibility. Many third-party libraries built for 2.1 were outdated or unsupported, especially Swagger, JWT libraries, and logging packages. We had to upgrade or replace several dependencies and fix API changes that came with them.
Another major area was JSON serialization. The default serializer changed from Newtonsoft.Json to System.Text.Json, which broke several custom converters, camel-case settings, and API response formats. We either added custom converters or explicitly re-added Newtonsoft where needed.
EF Core was another big task—LINQ queries behaved differently, some translations failed, and lazy loading changed. We had to rewrite a few complex queries and update annotations.
Authentication and authorization pipelines also changed significantly, requiring updates to JWT handling, authentication schemes, and middleware order.
Finally, deployment required adjustments because the .NET 8 runtime and Docker images are very different from 2.1. We updated CI/CD pipelines, switched container bases, and validated environment compatibility.
Overall, the migration involved code refactoring, package upgrades, fixing compatibility issues, updating config and hosting structure, and extensive testing to ensure functionality and performance improvements.”
If you want, I can also prepare a 10-second answer or a bullet format for your resume.
