Migrate Static to Wrapper

Perform mechanical, codemod-style replacement of static dependency call sites with calls to injected wrapper interfaces or built-in abstractions. Operates on a bounded scope (single file, project, or namespace) so migrations can be done incrementally.

When to Use

• After wrappers have been generated (via generate-testability-wrappers) or built-in abstractions identified
• Migrating DateTime.UtcNow → TimeProvider.GetUtcNow() across a project
• Migrating File.* → IFileSystem.File.* across a namespace
• Adding constructor injection for the new abstraction to affected classes
• Incremental migration: one project or namespace at a time

When Not to Use

• No wrapper or abstraction exists yet (use generate-testability-wrappers first)
• The user wants to detect statics, not migrate them (use detect-static-dependencies)
• The code does not use dependency injection and the user hasn't chosen ambient context
• Migrating between test frameworks (use the appropriate migration skill)

Inputs

Input	Required	Description
Static pattern	Yes	What to replace (e.g., DateTime.UtcNow, File.ReadAllText)
Replacement abstraction	Yes	What to use instead (e.g., TimeProvider, IFileSystem)
Scope	Yes	File path, project (.csproj), namespace, or directory to migrate
Injection strategy	No	constructor (default), primary-constructor, or ambient

Workflow

Step 1: Verify prerequisites

Before modifying any code:

1. Confirm the wrapper/abstraction exists: Check that the interface or built-in abstraction is available in the project. For TimeProvider, verify the target framework is .NET 8+ or Microsoft.Bcl.TimeProvider is referenced. For System.IO.Abstractions, verify the NuGet package is referenced.

2. Confirm DI registration exists: Check Program.cs or Startup.cs for the service registration. If missing, add it before proceeding.

3. Identify all files in scope: List the .cs files that will be modified. Exclude test projects, obj/, bin/, and generated code.

Step 2: Plan the migration for each file

For each file containing the static pattern, determine:

1. Which class(es) contain the call sites — identify the class declarations
2. Whether the class already has the dependency injected — check constructors for existing TimeProvider, IFileSystem, etc. parameters
3. The replacement expression for each call site

Replacement mapping

Category	Original	DI replacement
Time	DateTime.Now	_timeProvider.GetLocalNow().DateTime
Time	DateTime.UtcNow	_timeProvider.GetUtcNow().DateTime
Time	DateTime.Today	_timeProvider.GetLocalNow().Date
Time	DateTimeOffset.UtcNow	_timeProvider.GetUtcNow()
File	File.ReadAllText(path)	_fileSystem.File.ReadAllText(path)
File	File.WriteAllText(path, text)	_fileSystem.File.WriteAllText(path, text)
File	File.Exists(path)	_fileSystem.File.Exists(path)
File	Directory.Exists(path)	_fileSystem.Directory.Exists(path)
Env	Environment.GetEnvironmentVariable(name)	_env.GetEnvironmentVariable(name)
Console	Console.WriteLine(msg)	_console.WriteLine(msg)
Process	Process.Start(info)	_processRunner.Start(info)

Apply the same pattern for other members in each category.

Step 3: Add constructor injection

Add the new dependency following the class's existing pattern:

• Primary constructor (C# 12+): Add parameter to primary constructor: public class OrderProcessor(ILogger<OrderProcessor> logger, TimeProvider timeProvider)
• Traditional constructor: Add private readonly field + constructor parameter, matching the existing field naming convention (_camelCase or m_camelCase)

Step 4: Replace call sites

Perform each replacement mechanically. For each call site:

1. Replace the static call with the wrapper call
2. Preserve the surrounding code structure (whitespace, comments, chaining)
3. Add required using directives if not already present

Adding using directives

Abstraction	Using directive
TimeProvider	None (in System namespace)
IFileSystem	using System.IO.Abstractions;
IHttpClientFactory	using System.Net.Http; (usually already present)
Custom wrappers	using <wrapper namespace>;

Step 5: Update affected test files

If test files exist for the migrated classes:

1. Update constructor calls — add the new parameter to test class instantiation
2. Use test doubles:
   • TimeProvider → new FakeTimeProvider() from Microsoft.Extensions.TimeProvider.Testing
   • IFileSystem → new MockFileSystem() from System.IO.Abstractions.TestingHelpers
   • Custom wrappers → new Mock<IWrapperName>() or hand-rolled fake

Step 6: Build verification

After all changes in the current scope:

dotnet build <project.csproj>

If the build fails:

• Missing using: Add the required using directive
• Missing NuGet package: Run dotnet add package <name>
• Constructor mismatch in tests: Update test instantiation (Step 5)
• Ambiguous call: Fully qualify the wrapper call

Step 7: Report changes

Summarize what was done:

## Migration Summary

**Pattern**: DateTime.UtcNow → TimeProvider.GetUtcNow()
**Scope**: MyProject/Services/

### Files Modified (production)
| File | Call Sites Replaced | Injection Added |
|------|--------------------:|:----------------|
| OrderProcessor.cs | 3 | Yes (constructor) |
| NotificationService.cs | 1 | Yes (primary ctor) |

### Files Modified (tests)
| File | Change |
|------|--------|
| OrderProcessorTests.cs | Added FakeTimeProvider parameter |

### Remaining (out of scope)
- MyProject/Legacy/ — 8 call sites not migrated (different namespace)

Validation

• All call sites in scope were replaced (none missed)
• Constructor injection added to all affected classes
• Field naming follows existing class conventions
• Required using directives added
• Required NuGet packages referenced
• Build succeeds after migration
• Test files updated with appropriate test doubles
• No behavioral changes introduced (wrapper delegates directly to the static)

Common Pitfalls

Pitfall	Solution
Replacing statics in test code	Only replace in production code; tests should use fakes/mocks
Breaking static classes	Static classes can't have constructors — use ambient context for these
Missing FakeTimeProvider NuGet	Add Microsoft.Extensions.TimeProvider.Testing to test project
Replacing in expression-bodied members without updating return type	DateTime → DateTimeOffset when using TimeProvider.GetUtcNow() — verify type compatibility
Migrating too much at once	Stick to the defined scope — one project or namespace per run
Forgetting DI registration	Always verify Program.cs/Startup.cs has the registration before replacing call sites