generate-testability-wrappers

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Generate wrapper interfaces and DI registration for hard-to-test static dependencies in C#. Produces IFileSystem, IEnvironmentProvider, IConsole, IProcessRunner wrappers, or guides adoption of TimeProvider and IHttpClientFactory. USE FOR: generate wrapper for static, create IFileSystem wrapper, wrap DateTime.Now, make static testable, make class testable, create abstraction for File.*, generate DI registration, TimeProvider adoption, IHttpClientFactory setup, testability wrapper, mock-friendly interface, mock time in tests, create the right abstraction to mock, how to mock DateTime, test code using File.ReadAllText, what abstraction for Environment, how to make statics injectable, adopt System.IO.Abstractions, make file calls testable. DO NOT USE FOR: detecting statics (use detect-static-dependencies), migrating call sites (use migrate-static-to-wrapper), general interface design not about testability.

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Sourcedotnet/skills

Added on2026-04-14

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SKILL.md Content

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Generate Testability Wrappers

Generate wrapper interfaces, default implementations, and DI service registration code for untestable static dependencies. For statics that already have .NET built-in abstractions (

TimeProvider

IHttpClientFactory

), guide adoption of the built-in. For statics without built-in alternatives, generate custom minimal wrappers.

When to Use

After running
```
detect-static-dependencies
```
and identifying which statics to wrap
When the user asks to make a class testable by replacing statics with injected abstractions
When adopting
```
TimeProvider
```
(.NET 8+) or
```
System.IO.Abstractions
```
When creating a custom wrapper for
```
Environment.*
```
,
```
Console.*
```
, or
```
Process.*
```

When Not to Use

The user wants to find statics first (use
```
detect-static-dependencies
```
)
The user wants to bulk-replace call sites (use
```
migrate-static-to-wrapper
```
)
The static is already behind an interface
The project does not use dependency injection and the user does not want to add it

Inputs

Input	Required	Description
Static category	Yes	Which category: `time` , `filesystem` , `environment` , `network` , `console` , `process`
Target framework	Yes	The `TargetFramework` from `.csproj` (affects which built-in abstractions exist)
DI container	No	Which DI framework: `microsoft` (default), `autofac` , `none` (ambient context)
Namespace	No	Target namespace for generated wrapper code

Workflow

Step 1: Determine the abstraction strategy

Based on the category and target framework:

Category	.NET 8+	.NET 6-7	.NET Framework
Time	`TimeProvider` (built-in)	`TimeProvider` via `Microsoft.Bcl.TimeProvider` NuGet	Custom `ISystemClock`
File system	`System.IO.Abstractions` (NuGet)	Same	Same
HTTP	`IHttpClientFactory` (built-in)	Same	Same
Environment	Custom `IEnvironmentProvider`	Same	Same
Console	Custom `IConsole`	Same	Same
Process	Custom `IProcessRunner`	Same	Same

Step 2: Generate built-in abstraction adoption (Time, HTTP)

TimeProvider (.NET 8+)

No wrapper code needed — guide the user:

csharp

builder.Services.AddSingleton(TimeProvider.System);

Inject into classes:

csharp

public class OrderProcessor(TimeProvider timeProvider)
{
    public bool IsExpired(Order order)
        => timeProvider.GetUtcNow() > order.ExpiresAt;
}

Test with
```
FakeTimeProvider
```
:

csharp

// Requires Microsoft.Extensions.TimeProvider.Testing NuGet
var fakeTime = new FakeTimeProvider(new DateTimeOffset(2026, 1, 15, 0, 0, 0, TimeSpan.Zero));
var processor = new OrderProcessor(fakeTime);
fakeTime.Advance(TimeSpan.FromDays(1));
Assert.True(processor.IsExpired(order));

TimeProvider (pre-.NET 8)

Guide: install

Microsoft.Bcl.TimeProvider

NuGet. Same API as above.

IHttpClientFactory

No wrapper code needed — register typed clients via

builder.Services.AddHttpClient<MyService>()

and inject

HttpClient

directly into the class constructor.

Step 3: Generate custom wrappers (Environment, Console, Process)

For categories without built-in abstractions, follow this template:

Interface — define the minimal surface

Only include methods that were actually detected in the codebase. Do NOT generate a wrapper for every possible member — wrap only what is used.

csharp

namespace <Namespace>;

/// <summary>
/// Abstraction over <static class> for testability. 
/// </summary>
public interface I<WrapperName>
{
    // One method per detected static call
    <return type> <MethodName>(<parameters>);
}

Default implementation — delegate to the real static

csharp

namespace <Namespace>;

/// <summary>
/// Default implementation that delegates to <static class>.
/// </summary>
public sealed class <WrapperName> : I<WrapperName>
{
    public <return type> <MethodName>(<parameters>)
        => <StaticClass>.<Method>(<arguments>);
}

DI registration

csharp

// In Program.cs or Startup.cs:
builder.Services.AddSingleton<I<WrapperName>, <WrapperName>>();

Step 4: Generate file system wrapper adoption

Prefer the established

System.IO.Abstractions

NuGet package over custom wrappers:

Install the package:

dotnet add package System.IO.Abstractions

csharp

builder.Services.AddSingleton<IFileSystem, FileSystem>();

Inject
```
IFileSystem
```
into classes:

csharp

public class ConfigLoader(IFileSystem fileSystem)
{
    public string LoadConfig(string path)
        => fileSystem.File.ReadAllText(path);
}

Test with
```
MockFileSystem
```
:

dotnet add <TestProject> package System.IO.Abstractions.TestingHelpers

csharp

var mockFs = new MockFileSystem(new Dictionary<string, MockFileData>
{
    { "/config.json", new MockFileData("{\"key\": \"value\"}") }
});
var loader = new ConfigLoader(mockFs);
Assert.Equal("{\"key\": \"value\"}", loader.LoadConfig("/config.json"));

Step 5: Generate ambient context alternative (when DI is not available)

If the codebase does not use DI (e.g., old console app, library code), offer the ambient context pattern:

csharp

public static class Clock
{
    private static readonly AsyncLocal<Func<DateTimeOffset>?> s_override = new();
    public static DateTimeOffset UtcNow
        => s_override.Value?.Invoke() ?? TimeProvider.System.GetUtcNow();

    public static IDisposable Override(DateTimeOffset fixedTime)
    {
        s_override.Value = () => fixedTime;
        return new Scope();
    }
    private sealed class Scope : IDisposable
    {
        public void Dispose() => s_override.Value = null;
    }
}

Key trade-offs:

AsyncLocal<T>

ensures parallel tests don't interfere; production cost is one null check per call; the

static readonly

field is essentially free.

Step 6: Place generated files

Generate files following the project's existing conventions:

If there is an
```
Abstractions/
```
or
```
Interfaces/
```
folder, place the interface there
If there is an
```
Infrastructure/
```
or
```
Services/
```
folder, place the implementation there
Otherwise, create files next to the code that uses the static

Always generate:

The interface file (or adoption instructions for built-in abstractions)
The default implementation file
The DI registration snippet (as a code comment at the bottom of the implementation, or as separate instructions)

Validation

Generated interface only wraps statics that were actually detected (not speculative)
Default implementation delegates to the real static with no behavior changes
DI registration uses
```
AddSingleton
```
for stateless wrappers,
```
AddTransient
```
for stateful ones
NuGet packages are recommended where established libraries exist (System.IO.Abstractions, etc.)
For .NET 8+,
```
TimeProvider
```
is recommended over custom
```
ISystemClock
```
Ambient context pattern includes
```
AsyncLocal<T>
```
, scoped disposal, and trade-off explanation

Common Pitfalls

Pitfall	Solution
Wrapping ALL members of a static class	Only wrap methods actually called in the codebase
Custom time wrapper on .NET 8+	Use built-in `TimeProvider` instead
Custom file system wrapper	Prefer `System.IO.Abstractions` NuGet — battle-tested, complete
Registering scoped when singleton suffices	Stateless wrappers should be `AddSingleton`
Forgetting test helper packages	`Microsoft.Extensions.TimeProvider.Testing` for time, `System.IO.Abstractions.TestingHelpers` for filesystem
Ambient context without `AsyncLocal`	Non-async `[ThreadStatic]` breaks with `async` / `await` — always use `AsyncLocal<T>`

generate-testability-wrappers

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Tags

SKILL.md Content

Generate Testability Wrappers

When to Use

When Not to Use

Inputs

Workflow

Step 1: Determine the abstraction strategy

Step 2: Generate built-in abstraction adoption (Time, HTTP)

TimeProvider (.NET 8+)

TimeProvider (pre-.NET 8)

IHttpClientFactory

Step 3: Generate custom wrappers (Environment, Console, Process)

Interface — define the minimal surface

Default implementation — delegate to the real static

DI registration

Step 4: Generate file system wrapper adoption

Step 5: Generate ambient context alternative (when DI is not available)

Step 6: Place generated files

Validation

Common Pitfalls