Skills migrate-dotnet9-to-dotnet10

Name: migrate-dotnet9-to-dotnet10
Author: dotnet

install

source · Clone the upstream repo

git clone https://github.com/dotnet/skills

Claude Code · Install into ~/.claude/skills/

T=$(mktemp -d) && git clone --depth=1 https://github.com/dotnet/skills "$T" && mkdir -p ~/.claude/skills && cp -r "$T/plugins/dotnet-upgrade/skills/migrate-dotnet9-to-dotnet10" ~/.claude/skills/dotnet-skills-migrate-dotnet9-to-dotnet10 && rm -rf "$T"

manifest: plugins/dotnet-upgrade/skills/migrate-dotnet9-to-dotnet10/SKILL.md

source content

.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

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 10 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.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
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
```
net9.0
```
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>net9.0</TargetFramework>

to:

<TargetFramework>net10.0</TargetFramework>

For multi-targeted projects, add

net10.0

<TargetFrameworks>

or replace

net9.0

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.
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.
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 10 project	`references/csharp-compiler-dotnet9to10.md`
Any .NET 10 project	`references/core-libraries-dotnet9to10.md`
Any .NET 10 project	`references/sdk-msbuild-dotnet9to10.md`
ASP.NET Core	`references/aspnet-core-dotnet9to10.md`
Entity Framework Core	`references/efcore-dotnet9to10.md`
Cryptography APIs	`references/cryptography-dotnet9to10.md`
Microsoft.Extensions.Hosting, BackgroundService, configuration	`references/extensions-hosting-dotnet9to10.md`
System.Text.Json, XmlSerializer, HttpClient, MailAddress, Uri	`references/serialization-networking-dotnet9to10.md`
Windows Forms or WPF	`references/winforms-wpf-dotnet9to10.md`
Docker containers, single-file apps, native interop	`references/containers-interop-dotnet9to10.md`

Common source-incompatible changes to check for:

```
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.

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

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

ASP.NET Core obsoletions (if applicable):

WebHostBuilder

IWebHost

WebHost

are obsolete — migrate to

Host.CreateDefaultBuilder

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.

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
```
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
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
Cryptography source changes (if applicable):

MLDsa

and

SlhDsa

members renamed from

SecretKey

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

DOTNET_OPENSSL_VERSION_OVERRIDE

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):

SIGTERM signal handling removed — The .NET runtime no longer registers default SIGTERM handlers. If you rely on
```
AppDomain.ProcessExit
```
or
```
AssemblyLoadContext.Unloading
```
being raised on SIGTERM:
- ASP.NET Core and Generic Host apps are unaffected (they register their own handlers)
- Console apps and containerized apps without Generic Host must register
```
PosixSignalRegistration.Create(PosixSignal.SIGTERM, _ => Environment.Exit(0))
```
  explicitly
BackgroundService.ExecuteAsync runs entirely on a background thread — The synchronous portion before the first
```
await
```
no longer blocks startup. If startup ordering matters, move that code to
```
StartAsync
```
or the constructor, or implement
```
IHostedLifecycleService
```
.
Configuration null values are now preserved — JSON
```
null
```
values are no longer converted to empty strings. Properties initialized with non-default values will be overwritten with
```
null
```
. Review configuration binding code.
Microsoft.Data.Sqlite DateTimeOffset changes (all High impact):
- ```
GetDateTimeOffset
```
  without an offset now assumes UTC (previously assumed local)
- Writing
```
DateTimeOffset
```
  into REAL columns now converts to UTC first
- ```
GetDateTime
```
  with an offset now returns UTC with
```
DateTimeKind.Utc
```
- Mitigation:
```
AppContext.SetSwitch("Microsoft.Data.Sqlite.Pre10TimeZoneHandling", true)
```
  as a temporary workaround
EF Core parameterized collections —
```
.Contains()
```
on collections now uses multiple scalar parameters instead of JSON/OPENJSON. May affect query performance for large collections. Mitigation:
```
UseParameterizedCollectionMode(ParameterTranslationMode.Parameter)
```
to revert.
EF Core JSON data type on Azure SQL — Azure SQL and compatibility level ≥170 now use the
```
json
```
data type instead of
```
nvarchar(max)
```
. A migration will be generated to alter existing columns. Mitigation: set compatibility level to 160 or use
```
HasColumnType("nvarchar(max)")
```
explicitly.
System.Text.Json property name conflict validation — Polymorphic types with properties conflicting with metadata names (
```
$type
```
,
```
$id
```
,
```
$ref
```
) now throw
```
InvalidOperationException
```
. Add
```
[JsonIgnore]
```
to conflicting properties.

Other behavioral changes to review:

```
BufferedStream.WriteByte
```
no longer implicitly flushes — add explicit
```
Flush()
```
calls if needed
Default trace context propagator updated to W3C standard
```
DriveInfo.DriveFormat
```
returns actual Linux filesystem type names
LDAP
```
DirectoryControl
```
parsing is more stringent
Default .NET container images switched from Debian to Ubuntu (Debian images no longer shipped)
Single-file apps no longer look for native libraries in executable directory by default
```
DllImportSearchPath.AssemblyDirectory
```
only searches the assembly directory
```
MailAddress
```
enforces validation for consecutive dots
Streaming HTTP responses enabled by default in browser HTTP clients
```
Uri
```
length limits removed — add explicit length validation if
```
Uri
```
was used to reject oversized input from untrusted sources
Cookie login redirects disabled for known API endpoints (ASP.NET Core)
```
XmlSerializer
```
no longer ignores
```
[Obsolete]
```
properties — audit obsolete properties for sensitive data and add
```
[XmlIgnore]
```
to prevent unintended data exposure
```
dotnet restore
```
audits transitive packages
```
dotnet watch
```
logs to stderr instead of stdout
```
dotnet
```
CLI commands log non-command-relevant data to stderr
Various NuGet behavioral changes (see
```
references/sdk-msbuild-dotnet9to10.md
```
)
```
StatusStrip
```
uses System RenderMode by default (WinForms)
```
TreeView
```
checkbox image truncation fix (WinForms)
```
DynamicResource
```
incorrect usage causes crash (WPF)

Step 5: Update infrastructure

Dockerfiles: Update base images. Default tags now use Ubuntu instead of Debian. Debian images are no longer shipped for .NET 10.

# Before
FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
FROM mcr.microsoft.com/dotnet/aspnet:9.0
# After
FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
FROM mcr.microsoft.com/dotnet/aspnet:10.0

CI/CD pipelines: Update SDK version references. If using

global.json

, update:

{
  "sdk": {
    "version": "10.0.100"
  }
}

Environment variables renamed:

```
DOTNET_OPENSSL_VERSION_OVERRIDE
```
replaces the old name
```
DOTNET_ICU_VERSION_OVERRIDE
```
replaces the old name
```
NUGET_ENABLE_ENHANCED_HTTP_RETRY
```
has been removed

OpenSSL requirements: OpenSSL 1.1.1 or later is now required on Unix. OpenSSL cryptographic primitives are no longer supported on macOS.
Solution file format: If
```
dotnet new sln
```
is used in scripts, note it now generates SLNX format. Pass
```
--format sln
```
if the old format is needed.

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:
- Signal handling / graceful shutdown behavior
- Background services startup ordering
- Configuration binding with null values
- Date/time handling with Sqlite
- JSON serialization with polymorphic types
- EF Core queries using
```
.Contains()
```
  on collections
Security review — verify that the migration has not weakened security controls:
- TLS cipher validation logic is preserved after
```
SslStream
```
  API migration (SYSLIB0058)
- Obsolete properties containing sensitive data are excluded from serialization (
```
[XmlIgnore]
```
  ,
```
[JsonIgnore]
```
  )
- Input validation still rejects oversized URIs if
```
Uri
```
  was used as a length gate
- Exception handlers emit security-relevant telemetry (auth failures, access violations) before returning
```
true
```
- Connection strings set an explicit
```
Application Name
```
  that does not leak version info
- ```
dotnet restore
```
  vulnerability audit findings are addressed, not suppressed
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-dotnet9to10.md`	Always (C# 14 compiler breaking changes — field keyword, extension keyword, span overloads)
`references/core-libraries-dotnet9to10.md`	Always (applies to all .NET 10 projects)
`references/sdk-msbuild-dotnet9to10.md`	Always (SDK and build tooling changes)
`references/aspnet-core-dotnet9to10.md`	Project uses ASP.NET Core
`references/efcore-dotnet9to10.md`	Project uses Entity Framework Core or Microsoft.Data.Sqlite
`references/cryptography-dotnet9to10.md`	Project uses System.Security.Cryptography or X.509 certificates
`references/extensions-hosting-dotnet9to10.md`	Project uses Generic Host, BackgroundService, or Microsoft.Extensions.Configuration
`references/serialization-networking-dotnet9to10.md`	Project uses System.Text.Json, XmlSerializer, HttpClient, or networking APIs
`references/winforms-wpf-dotnet9to10.md`	Project uses Windows Forms or WPF
`references/containers-interop-dotnet9to10.md`	Project uses Docker containers, single-file publishing, or native interop (P/Invoke)