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

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

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

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