.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

Input Required Description

Project or solution path Yes The .csproj , .sln , or .slnx entry point to migrate

Build command No How to build (e.g., dotnet build , a repo build script). Auto-detect if not provided

Test command No How to run tests (e.g., dotnet test ). Auto-detect if not provided

Project type hints No Whether the project uses ASP.NET Core, EF Core, WinForms, WPF, containers, etc. Auto-detect from PackageReferences and SDK attributes if not provided

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

• Identify how the project is built and tested. Look for build scripts, .sln /.slnx files, or individual .csproj files.

• Run dotnet --version to confirm the .NET 9 SDK is installed. If it is not, stop and inform the user.

• 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; Microsoft.Extensions.Http → HttpClientFactory

• Dockerfile presence → Container changes relevant

• P/Invoke or native interop usage → Interop changes relevant

• BinaryFormatter usage → Serialization migration needed

• System.Text.Json usage → Serialization changes relevant

• X509Certificate constructors → Cryptography changes relevant

• Record which reference documents are relevant (see the reference loading table in Step 3).

• Do a clean build (dotnet build --no-incremental or delete bin /obj ) on the current net8.0 target to establish a clean baseline. Record any pre-existing warnings.

Step 2: Update the Target Framework

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 .

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.

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.

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:

If the project uses… Load reference

Any .NET 9 project references/csharp-compiler-dotnet8to9.md

Any .NET 9 project references/core-libraries-dotnet8to9.md

Any .NET 9 project references/sdk-msbuild-dotnet8to9.md

ASP.NET Core references/aspnet-core-dotnet8to9.md

Entity Framework Core references/efcore-dotnet8to9.md

Cryptography APIs references/cryptography-dotnet8to9.md

System.Text.Json, HttpClient, networking references/serialization-networking-dotnet8to9.md

Windows Forms or WPF references/winforms-wpf-dotnet8to9.md

Docker containers, native interop references/containers-interop-dotnet8to9.md

Runtime configuration, deployment references/deployment-runtime-dotnet8to9.md

Common source-incompatible changes to check for:

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 .

StringValues ambiguous overload — The params Span<T> feature creates ambiguity with StringValues implicit operators on methods like String.Concat , String.Join , Path.Combine . Fix by explicitly casting arguments. See references/core-libraries-dotnet8to9.md .

New obsoletion warnings (SYSLIB0054–SYSLIB0057):

• SYSLIB0054 : Replace Thread.VolatileRead /VolatileWrite with Volatile.Read /Volatile.Write

• SYSLIB0057 : Replace X509Certificate2 /X509Certificate binary/file constructors with X509CertificateLoader methods

• Also SYSLIB0055 (ARM AdvSimd signed overloads) and SYSLIB0056 (Assembly.LoadFrom with hash algorithm) — see references/core-libraries-dotnet8to9.md

C# 13 InlineArray on record structs — [InlineArray] attribute on record struct types is now disallowed (CS9259). Change to a regular struct . See references/csharp-compiler-dotnet8to9.md .

C# 13 iterator safe context — Iterators now introduce a safe context in C# 13. Local functions inside iterators that used unsafe code inherited from an outer unsafe class will now error. Add unsafe modifier to the local function. See references/csharp-compiler-dotnet8to9.md .

C# 13 collection expression overload resolution — Empty collection expressions ([] ) no longer use span vs non-span to tiebreak overloads. Exact element type is now preferred. See references/csharp-compiler-dotnet8to9.md .

String.Trim(params ReadOnlySpan<char>) removed — Code compiled against .NET 9 previews that passes ReadOnlySpan<char> to Trim /TrimStart /TrimEnd must rebuild; the overload was removed in GA. See references/core-libraries-dotnet8to9.md .

BinaryFormatter always throws — If the project uses BinaryFormatter , stop and inform the user — this is a major decision. See references/serialization-networking-dotnet8to9.md .

HttpListenerRequest.UserAgent is nullable — The property is now string? . Add null checks. See references/serialization-networking-dotnet8to9.md .

Windows Forms nullability annotation changes — Some WinForms API parameters changed from nullable to non-nullable. Update call sites. See references/winforms-wpf-dotnet8to9.md .

Windows Forms security analyzers (WFO1000) — New analyzers produce errors for properties without explicit serialization configuration. See references/winforms-wpf-dotnet8to9.md .

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):

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 .

EF Core: Pending model changes exception — Migrate() /MigrateAsync() now throws if the model has pending changes. Search for DateTime.Now , DateTime.UtcNow , or Guid.NewGuid() in any HasData call — these must be replaced with fixed constants (e.g., new DateTime(2024, 1, 1, 0, 0, 0, DateTimeKind.Utc) ). See references/efcore-dotnet8to9.md .

EF Core: Explicit transaction exception — Migrate() inside a user transaction now throws. See references/efcore-dotnet8to9.md .

HttpClientFactory uses SocketsHttpHandler by default — Code that casts the primary handler to HttpClientHandler will get InvalidCastException . See references/serialization-networking-dotnet8to9.md .

HttpClientFactory header redaction by default — All header values in Trace -level logs are now redacted. See references/serialization-networking-dotnet8to9.md .

Environment variables take precedence over runtimeconfig.json — Runtime configuration settings from environment variables now override runtimeconfig.json . See references/deployment-runtime-dotnet8to9.md .

ASP.NET Core ValidateOnBuild /ValidateScopes in development — HostBuilder now enables DI validation in development by default. See references/aspnet-core-dotnet8to9.md .

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

• ZipArchiveEntry names/comments now respect the UTF-8 flag

• IncrementingPollingCounter initial callback is now asynchronous

• InMemoryDirectoryInfo prepends rootDir to files

• RuntimeHelpers.GetSubArray returns a different type

• PictureBox raises HttpRequestException instead of WebException

• StatusStrip uses a different default renderer

• IMsoComponent support is opt-in

• SafeEvpPKeyHandle.DuplicateHandle up-refs the handle

• HttpClient metrics report server.port unconditionally

• URI query strings redacted in HttpClient EventSource events and IHttpClientFactory logs

• dotnet watch is incompatible with Hot Reload for old frameworks

• WPF GetXmlNamespaceMaps returns Hashtable instead of String

Step 5: Update infrastructure

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

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.

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

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 .

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

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

Step 6: Verify

• Run a full clean build: dotnet build --no-incremental

• Run all tests: dotnet test

• If the application is containerized, build and test the container image

• 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)

• 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:

Reference file When to load

references/csharp-compiler-dotnet8to9.md

Always (C# 13 compiler breaking changes — InlineArray on records, iterator safe context, collection expression overloads)

references/core-libraries-dotnet8to9.md

Always (applies to all .NET 9 projects)

references/sdk-msbuild-dotnet8to9.md

Always (SDK and build tooling changes)

references/aspnet-core-dotnet8to9.md

Project uses ASP.NET Core

references/efcore-dotnet8to9.md

Project uses Entity Framework Core

references/cryptography-dotnet8to9.md

Project uses System.Security.Cryptography or X.509 certificates

references/serialization-networking-dotnet8to9.md

Project uses BinaryFormatter, System.Text.Json, HttpClient, or networking APIs

references/winforms-wpf-dotnet8to9.md

Project uses Windows Forms or WPF

references/containers-interop-dotnet8to9.md

Project uses Docker containers or native interop (P/Invoke)

references/deployment-runtime-dotnet8to9.md

Project uses runtime configuration, deployment, or has floating-point to integer conversions