.NET 10 → .NET 11 Migration

Migrate a .NET 10 project or solution to .NET 11, systematically resolving all breaking changes. The outcome is a project targeting net11.0 that builds cleanly, passes tests, and accounts for every behavioral, source-incompatible, and binary-incompatible change introduced in .NET 11.

Note: .NET 11 is currently in preview. This skill covers breaking changes documented through Preview 1. It will be updated as additional previews ship.

When to Use

• Upgrading TargetFramework from net10.0 to net11.0
• Resolving build errors or new warnings after updating the .NET 11 SDK
• Adapting to behavioral changes in .NET 11 runtime, ASP.NET Core 11, or EF Core 11
• Updating CI/CD pipelines, Dockerfiles, or deployment scripts for .NET 11
• Fixing C# 15 compiler breaking changes after SDK upgrade

When Not to Use

• The project already targets net11.0 and builds cleanly — migration is done
• Upgrading from .NET 9 or earlier — address the .NET 9→10 breaking changes first
• Migrating from .NET Framework — that is a separate, larger effort
• Greenfield projects that start on .NET 11 (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, Cosmos DB, etc. Auto-detect from PackageReferences and SDK attributes if not provided

Workflow

Answer directly from the loaded reference documents for information about .NET 11 breaking changes. You may inspect the local repository (project/solution files, source code, configuration, build/test scripts) as needed to determine which changes apply. Do not fetch web pages or other external sources for breaking change information — the loaded references are the authoritative source. 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 11 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.EntityFrameworkCore.Cosmos → Cosmos DB provider
   • Dockerfile presence → Container changes relevant
   • Cryptography API usage → DSA on macOS affected
   • Compression API usage → DeflateStream/GZipStream/ZipArchive changes relevant
   • TAR API usage → Header checksum validation change relevant
   • NamedPipeClientStream usage with SafePipeHandle → SYSLIB0063 constructor obsoletion 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 net10.0 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>net10.0</TargetFramework>
   

   to:

   <TargetFramework>net11.0</TargetFramework>
   

   For multi-targeted projects, add net11.0 to <TargetFrameworks> or replace net10.0.

2. Update all Microsoft.Extensions.*, Microsoft.AspNetCore.*, Microsoft.EntityFrameworkCore.*, and other Microsoft package references to their 11.0.x versions. If using Central Package Management (Directory.Packages.props), update versions there.

3. Run dotnet restore. Fix any restore errors before continuing.

4. Run dotnet build. Capture all errors and warnings — these will be addressed in Step 3.

Step 3: Fix source-breaking and compilation changes

Load reference documents based on the project's technology areas:

Reference file	When to load
references/csharp-compiler-dotnet10to11.md	Always (C# 15 compiler breaking changes)
references/core-libraries-dotnet10to11.md	Always (applies to all .NET 11 projects)
references/sdk-msbuild-dotnet10to11.md	Always (SDK and build tooling changes)
references/efcore-dotnet10to11.md	Project uses Entity Framework Core (especially Cosmos DB provider)
references/cryptography-dotnet10to11.md	Project uses cryptography APIs or targets macOS
references/runtime-jit-dotnet10to11.md	Deploying to older hardware or embedded devices

Work through each build error systematically. Common patterns:

1. C# 15 Span collection expression safe-context — Collection expressions of Span<T>/ReadOnlySpan<T> type now have declaration-block safe-context. Code assigning span collection expressions to variables in outer scopes will error. Use array type or move the expression to the correct scope.

2. ref readonly delegates/local functions need InAttribute — If synthesizing delegates from ref readonly-returning methods or using ref readonly local functions, ensure System.Runtime.InteropServices.InAttribute is available.

3. nameof(this.) in attributes — Remove this. qualifier; use nameof(P) instead of nameof(this.P).

4. with() in collection expressions (C# 15) — with(...) is now treated as constructor arguments, not a method call. Use @with(...) to call a method named with.

5. Dynamic &&/|| with interface operand — Interface types as left operand of &&/|| with dynamic right operand now errors at compile time. Cast to concrete type or dynamic.

6. EF Core Cosmos sync I/O removal — ToList(), SaveChanges(), etc. on Cosmos provider always throw. Convert to async equivalents.

7. SYSLIB0063: NamedPipeClientStream isConnected parameter obsoleted — The constructor overload taking bool isConnected is obsoleted. Remove the isConnected argument and use the new 3-parameter constructor. Projects with TreatWarningsAsErrors will fail to build.

8. when switch-expression-arm parsing — (X.Y) when is now parsed as a constant pattern with a when clause instead of a cast expression, which can cause existing code to fail to compile or change meaning. Review switch expressions using when and adjust syntax as needed.

Step 4: Address behavioral changes

These changes compile successfully but alter runtime behavior. Review each one and determine impact:

1. DeflateStream/GZipStream empty payload — Now writes headers and footers even for empty payloads. If your code checks for zero-length output, update the check.

2. MemoryStream maximum capacity — Maximum capacity updated and exception behavior changed. Review code that creates large MemoryStreams or relies on specific exception types.

3. TAR header checksum validation — TAR-reading APIs now verify checksums. Corrupted or hand-crafted TAR files may now fail to read.

4. ZipArchive.CreateAsync eager loading — ZipArchive.CreateAsync eagerly loads entries. May affect memory usage for large archives.

5. Environment.TickCount consistency — Made consistent with Windows timeout behavior. Code relying on specific tick count behavior may need adjustment.

6. DSA removed from macOS — DSA cryptographic operations throw on macOS. Use a different algorithm (RSA, ECDSA).

7. Japanese Calendar minimum date — Minimum supported date corrected. Code using very early Japanese Calendar dates may be affected.

8. Minimum hardware requirements — x86/x64 baseline moved to x86-64-v2; Windows Arm64 requires LSE. Verify deployment targets meet requirements.

9. Mono launch target for .NET Framework — No longer set automatically. If using Mono for .NET Framework apps on Linux, specify explicitly.

Step 5: Update infrastructure

1. Dockerfiles: Update base images from 10.0 to 11.0:

   # Before
   FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
   FROM mcr.microsoft.com/dotnet/aspnet:10.0
   # After
   FROM mcr.microsoft.com/dotnet/sdk:11.0 AS build
   FROM mcr.microsoft.com/dotnet/aspnet:11.0
   

2. CI/CD pipelines: Update SDK version references. If using global.json, update the sdk.version in your existing file while preserving other keys (such as rollForward and test configuration):

    {
      "sdk": {
   -    "version": "10.0.100",
   -    "rollForward": "latestFeature"
   +    "version": "11.0.100-preview.1",
   +    "rollForward": "latestFeature"
      },
      "otherSettings": {
        "...": "..."
      }
    }
   

3. Hardware deployment targets: Verify all deployment targets meet the updated minimum hardware requirements (x86-64-v2 for x86/x64, LSE for Windows Arm64).

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:
   • Compression behavior with empty streams
   • TAR file reading
   • EF Core Cosmos DB operations (must be async)
   • DSA usage on macOS
   • Memory-intensive MemoryStream usage
   • Span collection expression assignments
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:

Reference file	When to load
references/csharp-compiler-dotnet10to11.md	Always (C# 15 compiler breaking changes)
references/core-libraries-dotnet10to11.md	Always (applies to all .NET 11 projects)
references/sdk-msbuild-dotnet10to11.md	Always (SDK and build tooling changes)
references/efcore-dotnet10to11.md	Project uses Entity Framework Core (especially Cosmos DB provider)
references/cryptography-dotnet10to11.md	Project uses cryptography APIs or targets macOS
references/runtime-jit-dotnet10to11.md	Deploying to older hardware or embedded devices