Migrate static to wrapper

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().LocalDateTime
Time	DateTime.UtcNow	_timeProvider.GetUtcNow().UtcDateTime
Time	DateTime.Today	_timeProvider.GetLocalNow().LocalDateTime.Date
Time	DateTimeOffset.Now	_timeProvider.GetLocalNow()
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.

Preserve DateTimeKind — this is the most common silent regression. TimeProvider.GetUtcNow() / GetLocalNow() return a DateTimeOffset. Converting back to DateTime must keep the original Kind, otherwise you introduce a behavioral change even though the code still compiles:

• DateTime.UtcNow has Kind == Utc → use .UtcDateTime (not .DateTime, which yields Kind == Unspecified).
• DateTime.Now has Kind == Local → use .LocalDateTime (not .DateTime).
• When a call site consumes a DateTimeOffset directly (a field/parameter/return already typed DateTimeOffset), drop the .UtcDateTime/.LocalDateTime suffix and assign the DateTimeOffset as-is — don't force it back through DateTime.

Match the target member's type: if the surrounding field/property is DateTime, keep it DateTime (via the Kind-correct property above); do not change it to DateTimeOffset as part of a "mechanical" migration — that is a design change, not a delegation.

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)

Static classes: use ambient context (no constructor injection)

A static class with only static members cannot receive constructor injection — adding an instance constructor or instance field would break it. Do not convert it to a non-static class just to inject the dependency; that changes its design and every call site. Instead, apply the ambient context pattern: expose a static, settable seam that defaults to the real implementation and is overridden once at composition/test setup.

When the user wants to keep the class static, the ambient seam below is the answer — present it as the solution and implement it directly. Do not hedge by offering "convert it to a non-static class" or "pass TimeProvider as a method parameter" as co-equal alternatives; those change the class's design or public API and are not what was asked. Lead with the seam, then note the parallelism trade-off.

public static class TimestampFormatter
{
    // Ambient seam — defaults to the real clock, swap in tests.
    public static TimeProvider Clock { get; set; } = TimeProvider.System;

    public static string Now() => Clock.GetUtcNow().ToString("O");
}

• Production: leave Clock at its TimeProvider.System default, or assign the DI-resolved TimeProvider once at startup (TimestampFormatter.Clock = app.Services.GetRequiredService<TimeProvider>();).

• Tests: override Clock with a FakeTimeProvider and always restore it in a finally so a failing assertion can't leak the fake into other tests:

  var original = TimestampFormatter.Clock;
  TimestampFormatter.Clock = new FakeTimeProvider(instant);
  try
  {
      // exercise code under test
  }
  finally
  {
      TimestampFormatter.Clock = original;
  }
  

• Parallelism caveat: a mutable static seam is process-global. Tests that mutate it must not run in parallel with each other (or with code that reads it) — put them in a non-parallel collection/class (e.g. xUnit [Collection] with parallelization disabled, or MSTest [DoNotParallelize]). Only if the class is not required to stay static and its tests must run fully parallel should you consider converting the caller to an instance with constructor injection instead — otherwise keep the ambient seam.

• The same seam works for other statics (IFileSystem, custom wrappers): a public static <Abstraction> X { get; set; } defaulting to the real implementation, with the same restore-in-finally and non-parallel discipline.

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)
• DateTimeKind preserved — former DateTime.UtcNow stays Utc (.UtcDateTime), former DateTime.Now stays Local (.LocalDateTime)

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 the ambient context seam (Step 3) instead of converting them to non-static
Missing FakeTimeProvider NuGet	Add Microsoft.Extensions.TimeProvider.Testing to test project
Replacing a DateTime value with .DateTime off a DateTimeOffset	DateTimeOffset.DateTime returns Kind == Unspecified — use .UtcDateTime (for former DateTime.UtcNow) or .LocalDateTime (for former DateTime.Now) to preserve the original DateTimeKind. Only change the field/return type to DateTimeOffset if the user asked for it.
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