dotnet-8-to-9

name	Dotnet 8 to 9
description	Use when migrating a project from .NET 8 to .NET 9, or preparing .NET 8 code for that migration with only the .NET 8 SDK available (e.g. Claude for Web).

.NET 8 → 9 Migration

Two modes — pick by what SDK is available:

Mode A: Pre-migration prep (still on .NET 8)

Use this in environments without the .NET 9 SDK. All fixes below are compatible with both versions, so apply them now and the later migration is mostly a TargetFramework bump.

1. Backticks in verbatim strings (the big one — breaks compilation under .NET 9). Detect:

   grep -rn '@"[^"]*`' --include="*.cs"
   

   Typically JavaScript template literals embedded in C# @"..." strings. Fix by converting to JS string concatenation (see below).
2. ImplicitUsings + explicit Main: projects that define their own Main and have <ImplicitUsings>enable</ImplicitUsings> can hit CS0017 (multiple entry points) on .NET 9 — set <ImplicitUsings>disable</ImplicitUsings> in those projects.
3. TestData compilation: test projects with .cs files under TestData\ compile them by accident. Exclude:

   <ItemGroup>
     <Compile Remove="TestData\**\*.cs" />
     <None Include="TestData\**\*" />
   </ItemGroup>
   

4. Note (don't yet update) packages below the minimums listed in Mode B.

Mode B: Completing the migration (on .NET 9)

1. <TargetFramework>net9.0</TargetFramework> in all .csproj files.
2. Update packages — minimums that matter here:
   • Microsoft.CodeAnalysis.* (CSharp.Workspaces, Workspaces.MSBuild): 4.12.0+
   • Microsoft.Build.Locator: 1.7.8+
   • System.Text.Json: 9.0.0+
   • Microsoft.CodeAnalysis.Workspaces.MSBuild was implicitly available on .NET 8 but often needs an explicit PackageReference on .NET 9 (CS0234 on Microsoft.CodeAnalysis.MSBuild is the symptom).
3. Apply all Mode A fixes if not already done.
4. dotnet clean && dotnet restore && dotnet build && dotnet test.

The template-literal fix (condensed)

CS1002/CS1056/CS1010 in a file that embeds JavaScript = backtick template literals inside a verbatim string.

// Before (breaks on .NET 9):
return @"html += `<div class=""result"">${value}</div>`;";

// After (works on 8 and 9):
return @"html += '<div class=""result"">' + value + '</div>';";

Backticks → single quotes, ${expr} → ' + expr + ', keep "" for HTML attributes inside the verbatim string.