/migrate-dotnet9-to-dotnet10

name: migrate-dotnet9-to-dotnet10
description: >
  Migrate a .NET 9 project or solution to .NET 10 and resolve all breaking changes.
  USE FOR: upgrading TargetFramework from net9.0 to net10.0, fixing build errors
  after updating the .NET 10 SDK, resolving source and behavioral changes in
  .NET 10 / C# 14 / ASP.NET Core 10 / EF Core 10, updating Dockerfiles for
  Debian-to-Ubuntu base images, resolving obsoletion warnings
  (SYSLIB0058-SYSLIB0062), adapting to SDK/NuGet changes (NU1510,
  PrunePackageReference), migrating System.Linq.Async to built-in
  AsyncEnumerable, fixing OpenApi v2 API changes, cryptography renames, and
  C# 14 compiler changes (field keyword, extension keyword, span overloads).
  DO NOT USE FOR: .NET Framework migrations, upgrading from .NET 8 or earlier
  (use migrate-dotnet8-to-dotnet9 first), greenfield .NET 10 projects, or
  cosmetic modernization.
  LOADS REFERENCES: csharp-compiler, core-libraries, sdk-msbuild (always);
  aspnet-core, efcore, cryptography, extensions-hosting,
  serialization-networking, winforms-wpf, containers-interop (selective).
license: MIT

.NET 9 → .NET 10 Migration

Migrate a .NET 9 project or solution to .NET 10, systematically resolving all breaking changes. The outcome is a project targeting `net10.0` that builds cleanly, passes tests, and accounts for every behavioral, source-incompatible, and binary-incompatible change introduced in the .NET 10 release.

When to Use

• Upgrading `TargetFramework` from `net9.0` to `net10.0`
• Resolving build errors or new warnings after updating the .NET 10 SDK
• Adapting to behavioral changes in .NET 10 runtime, ASP.NET Core 10, or EF Core 10
• Updating CI/CD pipelines, Dockerfiles, or deployment scripts for .NET 10
• Migrating from the community `System.Linq.Async` package to the built-in `System.Linq.AsyncEnumerable`

When Not to Use

• The project already targets `net10.0` and builds cleanly — migration is done
• Upgrading from .NET 8 or earlier — use the `migrate-dotnet8-to-dotnet9` skill first to reach `net9.0`, then return to this skill for the `net9.0` → `net10.0` migration
• Migrating from .NET Framework — that is a separate, larger effort
• Greenfield projects that start on .NET 10 (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. **Exception:** If you suspect a security vulnerability (CVE) may apply to the project's dependencies, check for published security advisories — the reference documents may not cover post-publication CVEs. > > **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 10 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; `Microsoft.Data.Sqlite` → Sqlite; `Microsoft.Extensions.Hosting` → Generic Host / BackgroundService
• **Dockerfile presence** → Container changes relevant
• **P/Invoke or native interop usage** → Interop changes relevant
• **`System.Linq.Async` package reference** → AsyncEnumerable migration needed
• **`System.Text.Json` usage with polymorphism** → Serialization changes relevant

4. Record which reference documents are relevant (see the reference loading table in Step 3). 5. Do a **clean build** (`dotnet build --no-incremental` or delete `bin`/`obj`) on the current `net9.0` target to establish a clean baseline. Record any pre-existing warnings.

Step 2: Update the Target Framework

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

   <TargetFramework>net9.0</TargetFramework>

to:

   <TargetFramework>net10.0</TargetFramework>

For multi-targeted projects, add `net10.0` to `<TargetFrameworks>` or replace `net9.0`.

2. Update all `Microsoft.Extensions.*`, `Microsoft.AspNetCore.*`, `Microsoft.EntityFrameworkCore.*`, and other Microsoft package references to their 10.0.x versions. If using Central Package Management (`Directory.Packages.props`), update versions there.

3. Run `dotnet restore`. Watch for:

• **NU1510**: Direct references pruned by NuGet — the package may be included in the shared framework now. Remove the explicit `<PackageReference>` if so.
• **PackageReference without a version now raises an error** — every `<PackageReference>` must have a `Version` (or use CPM).
• **NuGet auditing of transitive packages** (`dotnet restore` now audits transitive deps) — review any new vulnerability warnings.

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

Step 3: Resolve build errors and source-incomp