generate-testability-wrappers

name	generate-testability-wrappers
description	Generate wrapper interfaces and DI registration for hard-to-test static dependencies in C#, when the abstraction does NOT exist yet. Produces IFileSystem, IEnvironmentProvider, IConsole, IProcessRunner wrappers, or guides first-time adoption of TimeProvider and IHttpClientFactory and registering them in DI. 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, set up/adopt TimeProvider when it is not registered yet, IHttpClientFactory setup, testability wrapper, create the right abstraction to mock, what abstraction for Environment, how to make statics injectable, adopt System.IO.Abstractions. DO NOT USE FOR: detecting statics (use detect-static-dependencies), migrating call sites or replacing existing DateTime.*/File.* usages once the wrapper is created or already registered in DI (use migrate-static-to-wrapper), general interface design.
license	MIT

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:

1. Register in DI:

builder.Services.AddSingleton(TimeProvider.System);

2. Inject into classes:

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

3. Test with FakeTimeProvider:

// 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.

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

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

// 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:

1. Install the package:

dotnet add package System.IO.Abstractions

2. Register in DI:

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

3. Inject IFileSystem into classes:

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

4. Test with MockFileSystem:

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

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:

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:

1. The interface file (or adoption instructions for built-in abstractions)
2. The default implementation file
3. 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>