Generate Testability Wrappers

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);

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.

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)

Related skills