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 there is no DI container and the seam has to be ambient rather than injected

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

A project with no DI container, or a user who does not want to add one, is not a reason to skip this skill — that is exactly what the ambient context seam in Step 5 is for. Choose the seam over constructor injection in that case; do not decline the request and do not propose registering anything in a service collection.

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

The table picks which abstraction. How it reaches the code under test is a separate axis: constructor injection when a DI container exists, and the ambient context seam of Step 5 when one does not. Decide that axis first — check for a host builder, IServiceCollection, or an existing container registration — because a static class cannot take a constructor and a project without a container has nowhere to register anything. In that case skip Steps 2–4 and go to Step 5; the abstraction chosen above still applies, it is just reached through the ambient seam.

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);

1. Inject into classes:

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

1. 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

1. Register in DI:

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

1. Inject IFileSystem into classes:

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

1. 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.

Three properties this pattern must keep, because each has broken a real migration:

• Scope the override and make it reversible. Return an IDisposable that restores the previous value, so a test cannot leak a pinned time into the next one. A bare setter, or a manual try/finally at each call site, puts that burden on every test author.
• Use AsyncLocal<T>, never [ThreadStatic]. [ThreadStatic] does not flow across await, so the override silently disappears mid-test.
• Preserve the semantics of the member you are replacing. Substituting DateTime.UtcNow with a local-time source changes the DateTimeKind every existing caller and stored value depends on — pair UtcNow with GetUtcNow(), and Now with GetLocalNow().

The same shape works for non-time statics: swap TimeProvider.System.GetUtcNow() for the real static call and keep the override slot, the disposable scope, and the original semantics.

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) — skip this one entirely on the ambient-seam path: there is no container to register into, and offering one anyway is the failure mode that made a user ask for the seam in the first place

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>, a scoped IDisposable that restores the previous value, and trade-off explanation
• On the ambient-seam path, no IServiceCollection registration is proposed and the replaced member's semantics (UtcNow vs Now, and its DateTimeKind) are preserved

Common Pitfalls

Pitfall	Solution
Declining because the project has no DI container	The ambient seam in Step 5 is the answer for that case — offer it instead of asking the user to adopt a container
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>