migrate-static-to-wrapper

Original：🇺🇸 English

Translated

Mechanically replace static dependency call sites with wrapper or built-in abstraction calls across a bounded scope (file, project, or namespace). Performs codemod-style bulk replacement of DateTime.UtcNow to TimeProvider.GetUtcNow(), File.ReadAllText to IFileSystem, and similar transformations. Adds constructor injection parameters and updates DI registration. USE FOR: replace DateTime.UtcNow with TimeProvider, replace DateTime.Now with TimeProvider, migrate static calls to wrapper, bulk replace File.* with IFileSystem, codemod static to injectable, add constructor injection for time provider, mechanical migration of statics, refactor DateTime to TimeProvider, swap static for injected dependency, convert static calls to use abstraction, replace statics in a class, migrate one file to TimeProvider, scoped migration, update call sites. DO NOT USE FOR: detecting statics (use detect-static-dependencies), generating wrappers (use generate-testability-wrappers), migrating between test frameworks.

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Added on2026-04-14

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SKILL.md Content

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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:

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

Which class(es) contain the call sites — identify the class declarations
Whether the class already has the dependency injected — check constructors for existing
```
TimeProvider
```
,
```
IFileSystem
```
, etc. parameters
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:

Replace the static call with the wrapper call
Preserve the surrounding code structure (whitespace, comments, chaining)
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:

Update constructor calls — add the new parameter to test class instantiation

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:

bash

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