Add APICompat rule to detect static keyword changes on members and types

[Copilot]

• Add new diagnostic ID for static keyword changes (CP0022: CannotChangeStatic)
• Add localizable resource strings
• Create CannotAddOrRemoveStaticKeyword rule for members and types
• Create comprehensive tests for static keyword changes
• Add strict mode support for removing static from types
• Register the new rule in RuleFactory
• Test the implementation with the reproduction case
• Run linter and formatter
• Address all PR feedback

Latest Changes (this commit)

Split the resource string into two separate strings for proper localization:

• CannotAddStatic: "Cannot add static keyword to '{0}'."
• CannotRemoveStatic: "Cannot remove static keyword from '{0}'."

This allows translators to properly localize each message independently, instead of trying to substitute English words ("add"/"remove", "to"/"from") into a template which was not localizable.

The diagnostic ID remains CP0022 (CannotChangeStatic) with two different message formats depending on whether static is being added or removed.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>APICompat doesn't catch when a member changes static</issue_title>
<issue_description>### Describe the bug
APICompat does not catch the case when a member adds or removes static

To Reproduce

Create a class library that multi-targets and add/remove static on a member.

project.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>net8.0;net10.0</TargetFrameworks>
    <IsPackable>true</IsPackable>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <EnablePackageValidation>true</EnablePackageValidation>
  </PropertyGroup>

</Project>

Class1.cs

public class Class1
{
    #if NET10_0
    public string Foo => "net10.0";
    public static string Bar => "net10.0";
    #else
    public static string Foo => "net8.0";
    public string Bar => "net8.0";
    #endif
}

Expect validation errors for both Foo and Bar since both addition and removal is binary breaking. Removing static is source breaking as well.

We should make sure we include checking for all types of API that might change by static. That should be all members.

Adding static to a type that didn't have it before can be breaking, since static types cannot be used as return types, generic parameters, arguments. We have made such changes in the past - when we had non-constructable types. We should still flag this since we cannot be sure that someone didn't decide to use that in source (even if it wasn't callable).

Removing static from a type is compatible. We'll separately evaluate all members that existed before. </issue_description>

<agent_instructions>Be sure to first reproduce the problem by adding a test to https://github.com/dotnet/sdk/tree/main/test/Microsoft.DotNet.ApiCompatibility.Tests/Rules then fix the issue with a new rule in https://github.com/dotnet/sdk/tree/main/src/Compatibility/ApiCompat/Microsoft.DotNet.ApiCompatibility/Rules.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes APICompat doesn't catch when a member changes static #52111

---

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[krwq]

@copilot please review failures i.e. https://helixr1107v0xdeko0k025g8.blob.core.windows.net/dotnet-sdk-refs-pull-52112-merge-4c6167923cd5459290/Microsoft.NET.Build.Tests.dll.9/3/console.e907141d.log?helixlogtype=result&skoid=8eda00af-b5ec-4be9-b69b-0919a2338892&sktid=72f988bf-86f1-41af-91ab-2d7cd011db47&skt=2025-12-11T12%3A40%3A44Z&ske=2025-12-11T13%3A40%3A44Z&sks=b&skv=2024-11-04&sv=2024-11-04&se=2025-12-11T13%3A40%3A44Z&sr=b&sp=rl&sig=9aaiGTZdX27nnSN4VIm8k%2FN9AY9CaYsVSdSxVL3auLE%3D and see if this is related, if it is fix it

[krwq]

@ericstj I'd appreciate second person looking at this - especially the strict mode warning

[Copilot]

@copilot please review failures i.e. https://helixr1107v0xdeko0k025g8.blob.core.windows.net/dotnet-sdk-refs-pull-52112-merge-4c6167923cd5459290/Microsoft.NET.Build.Tests.dll.9/3/console.e907141d.log?h...

The log URL appears to be expired or inaccessible. I've verified:

• All ApiCompatibility tests pass (260 tests)
• The build succeeds without errors
• A simple multi-targeting package with EnablePackageValidation builds successfully with the changes

The changes I made:

1. Added CP0022/CP0023 for adding/removing static from members (always reported)
2. Added CP0024 for adding static to types (always reported)
3. Added CP0025 for removing static from types (only in strict mode)

Could you provide more details about the failure? Specifically:

• What error message appears?
• Is the test using strict mode?
• Is there a test asset that has static changing across target frameworks?

The failure might be unrelated to my changes, or there might be a test asset that legitimately changes static and now triggers the new rule.

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[Copilot]

Pull request overview

This PR adds a new APICompat rule to detect when the static keyword is added to or removed from members and types, which is a binary breaking change. The implementation follows the established patterns in the codebase for compatibility rules.

• Implements CannotAddOrRemoveStaticKeyword rule that checks for static modifier changes on methods, properties, fields, events, and types
• Adds 4 new diagnostic IDs (CP0022-CP0025) with appropriate localization strings
• Includes comprehensive test coverage for all member types and strict mode behavior
• Correctly handles the nuance that removing static from types is compatible (members checked separately) but reported in strict mode for developer awareness

Reviewed changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
CannotAddOrRemoveStaticKeyword.cs	New rule implementation with member and type-level checks for static keyword changes
CannotAddOrRemoveStaticKeywordTests.cs	Comprehensive test coverage including reproduction case, all member types, type-level changes, and strict mode
DiagnosticIds.cs	Adds four new diagnostic ID constants (CP0022-CP0025)
Resources.resx	Adds four new diagnostic message strings
Resources.*.xlf	Adds localization entries with state="new" for all supported languages
RuleFactory.cs	Registers the new rule to run by default
CannotChangeGenericConstraints.cs	Minor whitespace cleanup (trailing whitespace removal and spacing fix)

The implementation is clean, well-tested, and follows the established patterns in the codebase. No issues were found that require changes.

[ericstj]

@copilot please address feedback

[ericstj]

This type of substitution is not localizable since it has English words in code. Can you please split this into two resources. Keep a single diagnostic ID but allow it to have two different messages.

[ericstj]

@copilot please address this.

[Copilot]

Split into two separate resource strings for proper localization:

• CannotAddStatic: "Cannot add static keyword to '{0}'."
• CannotRemoveStatic: "Cannot remove static keyword from '{0}'."

The diagnostic ID remains CP0022 but uses the appropriate message based on the operation. Commit: 203993c