umbrella-dotnet-scaffold-file-handler

name	umbrella-dotnet-scaffold-file-handler
description	Scaffold a file handler (interface, implementation, DirectoryNames constant, DI registration) in the Core.Logic project, following the Umbrella UmbrellaFileHandler pattern. Authorization is separate — see umbrella-dotnet-scaffold-file-authorization-handler.

Scaffold File Handler

Purpose

Add a new file handler to the Core.<AppName>.Core.Logic project. File handlers plug into the Umbrella file storage infrastructure and are responsible for storage operations only: saving, retrieving, deleting files, generating web-relative URLs, caching file lookups, and optional post-save processing (e.g. image resizing).

Authorization is decoupled and lives in a separate UmbrellaFileAuthorizationHandler — see the umbrella-dotnet-scaffold-file-authorization-handler skill. A file handler can exist without a matching authorization handler if access control is handled elsewhere, but be aware that the default storage provider behaviour is to deny access for directories with no registered auth handler.

A file handler can represent any logical grouping of files — files attached to a database record, files in a SharePoint-style folder, user uploads, generated reports, or anything else. The group ID (int) is whatever identifier separates one group of files from another for this particular handler.

How the provider uses the handler

The provider uses the handler's DirectoryName to construct the file path (/<directoryName>/<groupId>/<fileName>). The DirectoryName property on the handler must exactly match the constant registered in DirectoryNames, as authorization lookup (in the separate auth handler) uses the same value.

Discovery (read these before writing anything)

1. Read existing file handler implementations in Core\<AppName>.Core.Logic\FileSystem\ to understand the pattern.
2. Read the interfaces in Core\<AppName>.Core.Logic\FileSystem\Abstractions\.
3. Read Core\<AppName>.Core.Common\FileSystem\Constants\DirectoryNames.cs to see existing constants and the All collection.
4. Read Core\<AppName>.Core.Logic\IServiceCollectionExtensions.cs to see where handlers are registered.

---

Step 1 -- Add the DirectoryNames constant

File: Core\<AppName>.Core.Common\FileSystem\Constants\DirectoryNames.cs

Add a new public const string entry using lowercase, hyphenated naming (kebab-case):

public const string <Name> = "<name-in-kebab-case>";

Also add the new constant to the All collection:

public static readonly IReadOnlyCollection<string> All = [
    // existing entries ...
    <Name>
];

---

Step 2 -- Create the interface

File location: Core\<AppName>.Core.Logic\FileSystem\Abstractions\I<Name>FileHandler.cs

using Umbrella.FileSystem.Abstractions;

namespace <AppName>.Core.Logic.FileSystem.Abstractions;

public interface I<Name>FileHandler : IUmbrellaFileHandler<int>;

Rules:

• Empty marker interface — all behaviour comes from the base interface and implementation
• Always extends IUmbrellaFileHandler<int>
• Needs the explicit using Umbrella.FileSystem.Abstractions; — not in global usings for this project

---

Step 3 -- Create the implementation

File location: Core\<AppName>.Core.Logic\FileSystem\<Name>FileHandler.cs

Minimal pattern (no post-save processing):

using <AppName>.Core.Common.FileSystem.Constants;
using <AppName>.Core.Logic.FileSystem.Abstractions;
using Umbrella.FileSystem.Abstractions;
using Umbrella.Utilities.Caching.Abstractions;

namespace <AppName>.Core.Logic.FileSystem;

internal sealed class <Name>FileHandler : UmbrellaFileHandler<int>, I<Name>FileHandler
{
    public <Name>FileHandler(
        ILogger<<Name>FileHandler> logger,
        IHybridCache cache,
        ICacheKeyUtility cacheKeyUtility,
        IUmbrellaFileStorageProvider fileProvider,
        IUmbrellaFileStorageProviderOptions options)
        : base(logger, cache, cacheKeyUtility, fileProvider, options)
    {
    }

    public override string DirectoryName => DirectoryNames.<Name>;
}

With post-save image resizing:

using <AppName>.Core.Common.FileSystem.Constants;
using <AppName>.Core.Logic.FileSystem.Abstractions;
using Umbrella.DynamicImage.Abstractions;
using Umbrella.FileSystem.Abstractions;
using Umbrella.Utilities.Caching.Abstractions;

namespace <AppName>.Core.Logic.FileSystem;

internal sealed class <Name>FileHandler : UmbrellaFileHandler<int>, I<Name>FileHandler
{
    private readonly IDynamicImageResizer _dynamicImageResizer;
    private readonly IDynamicImageUtility _dynamicImageUtility;

    public <Name>FileHandler(
        ILogger<<Name>FileHandler> logger,
        IHybridCache cache,
        ICacheKeyUtility cacheKeyUtility,
        IUmbrellaFileStorageProvider fileProvider,
        IUmbrellaFileStorageProviderOptions options,
        IDynamicImageResizer dynamicImageResizer,
        IDynamicImageUtility dynamicImageUtility)
        : base(logger, cache, cacheKeyUtility, fileProvider, options)
    {
        _dynamicImageResizer = dynamicImageResizer;
        _dynamicImageUtility = dynamicImageUtility;
    }

    public override string DirectoryName => DirectoryNames.<Name>;

    protected override async Task AfterSavingAsync(IUmbrellaFileInfo fileInfo, int groupId, CancellationToken cancellationToken = default)
    {
        cancellationToken.ThrowIfCancellationRequested();

        byte[] imageBytes = await fileInfo.ReadAsByteArrayAsync(cancellationToken: cancellationToken);
        var format = _dynamicImageUtility.ParseImageFormat(Path.GetExtension(fileInfo.Name));
        var (resizedBytes, _, _) = _dynamicImageResizer.ResizeImage(imageBytes, 1600, 1600, DynamicResizeMode.UseWidth, format, DynamicImageFilterQuality.High, 100);
        await fileInfo.WriteFromByteArrayAsync(resizedBytes, cancellationToken: cancellationToken);
    }
}

Rules:

• Always internal sealed class inheriting UmbrellaFileHandler<int> and the marker interface
• Base 5 constructor params in this exact order: ILogger<T>, IHybridCache, ICacheKeyUtility, IUmbrellaFileStorageProvider, IUmbrellaFileStorageProviderOptions — all passed to : base(...)
• Extra dependencies go AFTER the 5 base params and are stored as private readonly fields
• DirectoryName must return DirectoryNames.<Name> — the same constant added in Step 1
• Do NOT add AuthorizeAsync here — authorization belongs in a separate UmbrellaFileAuthorizationHandler (see umbrella-dotnet-scaffold-file-authorization-handler)
• Override AfterSavingAsync only when post-save processing is needed; always call cancellationToken.ThrowIfCancellationRequested() first
• using Umbrella.DynamicImage.Abstractions; is only needed when overriding AfterSavingAsync for image work — confirm the package is referenced in the .csproj

---

Step 4 -- Register in DI

File: Core\<AppName>.Core.Logic\IServiceCollectionExtensions.cs

Add one line in the // File Handlers section, in alphabetical order:

_ = services.AddSingleton<I<Name>FileHandler, <Name>FileHandler>();

File handlers are always AddSingleton — they are stateless and safe to share across requests.

---

Verification

1. DirectoryNames.<Name> constant exists and is added to the All collection.
2. The interface is an empty marker extending IUmbrellaFileHandler<int> with the using Umbrella.FileSystem.Abstractions; directive.
3. The implementation is internal sealed, the constructor passes exactly the 5 base params to : base(...), and DirectoryName returns the correct constant.
4. There is no AuthorizeAsync on the file handler — authorization is in a separate handler.
5. AddSingleton<I<Name>FileHandler, <Name>FileHandler>() is present in IServiceCollectionExtensions.cs.
6. If authorization is needed, the umbrella-dotnet-scaffold-file-authorization-handler skill has been used to create a matching <Name>FileAuthorizationHandler with the same DirectoryName.