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.NET 8 → .NET 9 Migration

Migrate a .NET 8 project or solution to .NET 9, systematically resolving all breaking changes. The outcome is a project targeting net9.0 that builds cleanly, passes tests, and accounts for every behavioral, source-incompatible, and binary-incompatible change introduced in the .NET 9 release.

When to Use

• Upgrading TargetFramework from net8.0 to net9.0
• Resolving build errors or new warnings after updating the .NET 9 SDK
• Adapting to behavioral changes in .NET 9 runtime, ASP.NET Core 9, or EF Core 9
• Replacing BinaryFormatter usage (now always throws at runtime)
• Updating CI/CD pipelines, Dockerfiles, or deployment scripts for .NET 9

When Not to Use

• The project already targets net9.0 and builds cleanly — migration is done. If the goal is to reach net10.0, use the migrate-dotnet9-to-dotnet10 skill as the next step.
• Upgrading from .NET 7 or earlier — address the prior version breaking changes first
• Migrating from .NET Framework — that is a separate, larger effort
• Greenfield projects that start on .NET 9 (no migration needed)

Inputs

Workflow

Answer directly from the loaded reference documents. Do not search the filesystem or fetch web pages for breaking change information — the references contain the authoritative details. Focus on identifying which breaking changes apply and providing concrete fixes.

Commit strategy: Commit at each logical boundary — after updating the TFM (Step 2), after resolving build errors (Step 3), after addressing behavioral changes (Step 4), and after updating infrastructure (Step 5). This keeps each commit focused and reviewable.

Step 1: Assess the project

1. Identify how the project is built and tested. Look for build scripts, .sln/.slnx files, or individual .csproj files.
2. Run dotnet --version to confirm the .NET 9 SDK is installed. If it is not, stop and inform the user.
3. Determine which technology areas the project uses by examining:
   • SDK attribute : Microsoft.NET.Sdk.Web → ASP.NET Core; Microsoft.NET.Sdk.WindowsDesktop with <UseWPF> or <UseWindowsForms> → WPF/WinForms
   • PackageReferences : Microsoft.EntityFrameworkCore.* → EF Core; → HttpClientFactory

Step 2: Update the Target Framework

1. In each .csproj (or Directory.Build.props if centralized), change:

   <TargetFramework>net8.0</TargetFramework>
   

to:

     <TargetFramework>net9.0</TargetFramework>
     

For multi-targeted projects, add net9.0 to <TargetFrameworks> or replace net8.0.

1. Update all Microsoft.Extensions.*, Microsoft.AspNetCore.*, Microsoft.EntityFrameworkCore.*, and other Microsoft package references to their 9.0.x versions. If using Central Package Management (Directory.Packages.props), update versions there.

2. Run dotnet restore. Watch for:

   • Version requirements : .NET 9 SDK requires Visual Studio 17.12+ to target net9.0 (17.11 for net8.0 and earlier).
   • New warnings for .NET Standard 1.x and .NET 7 targets — consider updating or removing outdated target frameworks.

3. Run a clean build. Collect all errors and new warnings. These will be addressed in Step 3.

Step 3: Resolve build errors and source-incompatible changes

Work through compilation errors and new warnings systematically. Load the appropriate reference documents based on the project type:

Common source-incompatible changes to check for:

1. params span overload resolution — New params ReadOnlySpan<T> overloads on String.Join, String.Concat, Path.Combine, Task.WhenAll, and many more now bind preferentially. Code calling these methods inside Expression lambdas will fail (CS8640/CS9226). See references/core-libraries-dotnet8to9.md.

2. StringValues ambiguous overload — The params Span<T> feature creates ambiguity with implicit operators on methods like , , . Fix by explicitly casting arguments. See .

Build again after each batch of fixes. Repeat until the build is clean.

Step 4: Address behavioral changes

Behavioral changes do not cause build errors but may change runtime behavior. Review each applicable item and determine whether the previous behavior was relied upon.

High-impact behavioral changes (check first):

1. Floating-point to integer conversions are now saturating — Conversions from float/double to integer types now saturate instead of wrapping on x86/x64. See references/deployment-runtime-dotnet8to9.md.

2. EF Core: Pending model changes exception — Migrate()/MigrateAsync() now throws if the model has pending changes. Search forDateTime.Now, DateTime.UtcNow, or Guid.NewGuid() in any HasData call — these must be replaced with fixed constants (e.g., ). See .

Other behavioral changes to review (may cause runtime exceptions ⚠️ or subtle behavioral differences):

• ⚠️ FromKeyedServicesAttribute no longer injects non-keyed service fallback — throws InvalidOperationException
• ⚠️ Container images no longer install zlib — apps depending on system zlib will fail
• ⚠️ Intel CET is now enabled by default — non-CET-compatible native libraries may cause process termination
• BigInteger now has a maximum length of (2^31) - 1 bits
• JsonDocument deserialization of JSON null now returns non-null JsonDocument with JsonValueKind.Null instead of C# null
• System.Text.Json metadata reader now unescapes metadata property names

Step 5: Update infrastructure

1. Dockerfiles : Update base images. Note that .NET 9 container images no longer install zlib. If your app depends on zlib, add RUN apt-get update && apt-get install -y zlib1g to your Dockerfile.

   # Before
   FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
   FROM mcr.microsoft.com/dotnet/aspnet:8.0
   # After
   FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
   FROM mcr.microsoft.com/dotnet/aspnet:9.0
   

2. CI/CD pipelines : Update SDK version references. If using global.json, update:

   {
     "sdk": {
       "version": "9.0.100",
       "rollForward": "latestFeature"
     }
   }
   

Review the rollForward policy — if set to "disable" or "latestPatch", the SDK may not resolve correctly after upgrading. "latestFeature" (recommended) allows the SDK to roll forward to the latest 9.0.x feature band.

1. Visual Studio version : .NET 9 SDK requires VS 17.12+ to target net9.0. VS 17.11 can only target net8.0 and earlier.

2. Terminal Logger : dotnet build now uses Terminal Logger by default in interactive terminals. CI scripts that parse MSBuild console output may need --tl:off or MSBUILDTERMINALLOGGER=off.

3. dotnet workload output: Output format has changed. Update any scripts that parse workload command output.

4. .NET Monitor images : Tags simplified to version-only (affects container orchestration referencing specific tags).

Step 6: Verify

1. Run a full clean build: dotnet build --no-incremental
2. Run all tests: dotnet test
3. If the application is containerized, build and test the container image
4. Smoke-test the application, paying special attention to:
   • BinaryFormatter usage (will throw at runtime)
   • Floating-point to integer conversion behavior
   • EF Core migration application
   • HttpClientFactory handler casting and logging
   • DI validation in development environment
   • Runtime configuration settings (environment variable precedence)
5. Review the diff and ensure no unintended behavioral changes were introduced

Reference Documents

The references/ folder contains detailed breaking change information organized by technology area. Load only the references relevant to the project being migrated:

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