Migrate Dotnet9 To Dotnet10

.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

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-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. System.Linq.Async conflicts — Remove the System.Linq.Async package reference or upgrade to v7.0.0. If consumed transitively, add <ExcludeAssets>compile</ExcludeAssets>. Rename SelectAwait calls to Select where needed.

2. New obsoletion warnings (SYSLIB0058–SYSLIB0062):

   • SYSLIB0058: Replace SslStream.KeyExchangeAlgorithm/CipherAlgorithm/HashAlgorithm with NegotiatedCipherSuite — if the old properties were used to reject weak TLS ciphers, preserve equivalent validation logic using the new API
   • SYSLIB0059: Replace SystemEvents.EventsThreadShutdown with AppDomain.ProcessExit
   • SYSLIB0060: Replace Rfc2898DeriveBytes constructors with Rfc2898DeriveBytes.Pbkdf2
   • SYSLIB0061: Replace Queryable.MaxBy/MinBy overloads taking IComparer<TSource> with ones taking IComparer<TKey>
   • SYSLIB0062: Replace XsltSettings.EnableScript usage

3. C# 14 field keyword in property accessors — The identifier field is now a contextual keyword inside property get/set/init accessors. Local variables named field cause CS9272 (error). Class members named field referenced without this. cause CS9258 (warning). Fix by renaming (e.g., fieldValue) or escaping with @field. See references/csharp-compiler-dotnet9to10.md.

4. C# 14 extension contextual keyword — Types, aliases, or type parameters named extension are disallowed. Rename or escape with @extension.

5. C# 14 overload resolution with span parameters — Expression trees containing .Contains() on arrays may now bind to MemoryExtensions.Contains instead of Enumerable.Contains. Enumerable.Reverse on arrays may resolve to the in-place Span extension. Fix by casting to IEnumerable<T>, using .AsEnumerable(), or explicit static invocations. See references/csharp-compiler-dotnet9to10.md for full details.

6. ASP.NET Core obsoletions (if applicable):

   • WebHostBuilder, IWebHost, WebHost are obsolete — migrate to Host.CreateDefaultBuilder or WebApplication.CreateBuilder
   • IActionContextAccessor / ActionContextAccessor obsolete
   • WithOpenApi extension method deprecated
   • IncludeOpenAPIAnalyzers property deprecated
   • IPNetwork and ForwardedHeadersOptions.KnownNetworks obsolete
   • Razor runtime compilation is obsolete
   • Microsoft.Extensions.ApiDescription.Client package deprecated
   • Microsoft.OpenApi v2.x breaking changes — Microsoft.AspNetCore.OpenApi 10.0 pulls in Microsoft.OpenApi v2.x which restructures namespaces and models. OpenApiString/OpenApiAny types are removed (use JsonNode), OpenApiSecurityScheme.Reference replaced by OpenApiSecuritySchemeReference, collections on OpenAPI model objects may be null, and OpenApiSchema.Nullable is removed. See references/aspnet-core-dotnet9to10.md for migration patterns.

7. SDK changes:

   • dotnet new sln now defaults to SLNX format — use --format sln if the old format is needed
   • Double quotes in file-level directives are disallowed
   • dnx.ps1 removed from .NET SDK
   • project.json no longer supported in dotnet restore

8. EF Core source changes (if applicable) — See references/efcore-dotnet9to10.md for:

   • ExecuteUpdateAsync now accepts a regular lambda (expression tree construction code must be rewritten)
   • IDiscriminatorPropertySetConvention signature changed
   • IRelationalCommandDiagnosticsLogger methods add logCommandText parameter

9. WinForms/WPF source changes (if applicable):

   • Applications referencing both WPF and WinForms must disambiguate MenuItem and ContextMenu types
   • Renamed parameter in HtmlElement.InsertAdjacentElement
   • Empty ColumnDefinitions and RowDefinitions are disallowed in WPF

10. Cryptography source changes (if applicable):

• MLDsa and SlhDsa members renamed from SecretKey to PrivateKey (e.g., ExportMLDsaSecretKey → ExportMLDsaPrivateKey, SecretKeySizeInBytes → PrivateKeySizeInBytes)
• Rfc2898DeriveBytes constructors are obsolete (SYSLIB0060) — replace with static Rfc2898DeriveBytes.Pbkdf2(password, salt, iterations, hashAlgorithm, outputLength)
• CoseSigner.Key can now be null — check for null before use
• X509Certificate.GetKeyAlgorithmParameters() and PublicKey.EncodedParameters can return null
• Environment variable renamed from CLR_OPENSSL_VERSION_OVERRIDE to DOTNET_OPENSSL_VERSION_OVERRIDE

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