File Provider

Microsoft.Extensions.FileProviders

Overview

Microsoft.Extensions.FileProviders provides a unified abstraction for reading files from different sources including the physical file system, embedded resources in assemblies, and composite providers that merge multiple sources. The core interface IFileProvider exposes methods for getting file info, listing directory contents, and watching for changes.

File providers integrate deeply with ASP.NET Core for static file serving, configuration reloading, and Razor view discovery. They are also useful in non-web applications for abstracting file access behind an interface that can be swapped for testing or to support embedded resources.

Install via NuGet:

dotnet add package Microsoft.Extensions.FileProviders.Abstractions
dotnet add package Microsoft.Extensions.FileProviders.Physical
dotnet add package Microsoft.Extensions.FileProviders.Embedded
dotnet add package Microsoft.Extensions.FileProviders.Composite

PhysicalFileProvider

PhysicalFileProvider reads files from a directory on disk. It supports glob-based file watching via IChangeToken.

using System.IO;
using Microsoft.Extensions.FileProviders;

// Create a provider rooted at a directory
var contentRoot = Path.Combine(Directory.GetCurrentDirectory(), "content");
using var provider = new PhysicalFileProvider(contentRoot);

// Get a single file
IFileInfo fileInfo = provider.GetFileInfo("templates/welcome.html");
if (fileInfo.Exists)
{
    using var stream = fileInfo.CreateReadStream();
    using var reader = new StreamReader(stream);
    string content = await reader.ReadToEndAsync();
    Console.WriteLine($"File: {fileInfo.Name}, Length: {fileInfo.Length}");
}

// List directory contents
IDirectoryContents directory = provider.GetDirectoryContents("templates");
foreach (var file in directory)
{
    Console.WriteLine($"  {file.Name} ({file.Length} bytes, Dir={file.IsDirectory})");
}

EmbeddedFileProvider (ManifestEmbeddedFileProvider)

ManifestEmbeddedFileProvider reads files embedded in an assembly. Files must be included as embedded resources in the .csproj.

<!-- In your .csproj file -->
<ItemGroup>
  <EmbeddedResource Include="Resources/**/*" />
</ItemGroup>

<PropertyGroup>
  <GenerateEmbeddedFilesManifest>true</GenerateEmbeddedFilesManifest>
</PropertyGroup>

<ItemGroup>
  <PackageReference Include="Microsoft.Extensions.FileProviders.Embedded" Version="8.0.0" />
</ItemGroup>

using System.Reflection;
using Microsoft.Extensions.FileProviders;

// Access embedded resources with directory structure preserved
var assembly = Assembly.GetExecutingAssembly();
var embedded = new ManifestEmbeddedFileProvider(assembly, "Resources");

IFileInfo template = embedded.GetFileInfo("email/welcome.html");
if (template.Exists)
{
    using var stream = template.CreateReadStream();
    using var reader = new StreamReader(stream);
    var html = await reader.ReadToEndAsync();
}

// List embedded files
var files = embedded.GetDirectoryContents("email");
foreach (var file in files)
{
    Console.WriteLine($"Embedded: {file.Name}");
}

CompositeFileProvider

CompositeFileProvider merges multiple providers so files can be resolved from any source in priority order. The first provider to contain a matching file wins.

using Microsoft.Extensions.FileProviders;

// Overlay: user overrides first, then embedded defaults
var userOverrides = new PhysicalFileProvider("/app/custom-templates");
var defaults = new ManifestEmbeddedFileProvider(typeof(Program).Assembly, "Resources");
var composite = new CompositeFileProvider(userOverrides, defaults);

// Resolves from userOverrides if present, otherwise from defaults
IFileInfo file = composite.GetFileInfo("email/welcome.html");

Watching for File Changes

IFileProvider.Watch returns an IChangeToken that triggers when files matching a glob pattern are created, modified, or deleted. Use ChangeToken.OnChange for convenient callback registration.

using Microsoft.Extensions.FileProviders;
using Microsoft.Extensions.Primitives;

using var provider = new PhysicalFileProvider("/app/config");

// Watch for changes to any JSON file in the config directory
ChangeToken.OnChange(
    () => provider.Watch("**/*.json"),
    () =>
    {
        Console.WriteLine("Configuration files changed, reloading...");
        ReloadConfiguration(provider);
    });

// Watch a specific file
ChangeToken.OnChange(
    () => provider.Watch("appsettings.json"),
    () => Console.WriteLine("appsettings.json was modified"));

void ReloadConfiguration(IFileProvider fileProvider)
{
    var settingsFile = fileProvider.GetFileInfo("appsettings.json");
    if (settingsFile.Exists)
    {
        using var stream = settingsFile.CreateReadStream();
        // Parse and apply new settings
    }
}

Integrating with Dependency Injection

Register file providers in the DI container and inject IFileProvider where needed.

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.FileProviders;
using Microsoft.Extensions.Hosting;

var builder = Host.CreateApplicationBuilder(args);

// Register a named file provider
builder.Services.AddSingleton<IFileProvider>(sp =>
{
    var env = sp.GetRequiredService<IHostEnvironment>();
    var physical = new PhysicalFileProvider(
        Path.Combine(env.ContentRootPath, "templates"));
    var embedded = new ManifestEmbeddedFileProvider(
        typeof(Program).Assembly, "Resources");
    return new CompositeFileProvider(physical, embedded);
});

builder.Services.AddTransient<TemplateService>();

using var host = builder.Build();

using System.IO;
using System.Threading.Tasks;
using Microsoft.Extensions.FileProviders;

public class TemplateService
{
    private readonly IFileProvider _fileProvider;

    public TemplateService(IFileProvider fileProvider)
    {
        _fileProvider = fileProvider;
    }

    public async Task<string> LoadTemplateAsync(string templateName)
    {
        var fileInfo = _fileProvider.GetFileInfo($"email/{templateName}.html");
        if (!fileInfo.Exists)
            throw new FileNotFoundException($"Template not found: {templateName}");

        using var stream = fileInfo.CreateReadStream();
        using var reader = new StreamReader(stream);
        return await reader.ReadToEndAsync();
    }
}

Provider Comparison

Best Practices

1. Use CompositeFileProvider for override patterns where user-customizable files take priority over embedded defaults, eliminating conditional file-exists logic.
2. Inject IFileProvider through DI rather than creating providers inline so the file source can be swapped for testing with NullFileProvider or an in-memory implementation.
3. Dispose PhysicalFileProvider when the application shuts down because it holds a FileSystemWatcher internally that leaks if not disposed.
4. Use ManifestEmbeddedFileProvider with GenerateEmbeddedFilesManifest in the .csproj to preserve directory structure; without the manifest, directory listings are not supported.
5. Prefer ChangeToken.OnChange over manual IChangeToken polling because it handles re-registration automatically when a token is consumed.
6. Scope PhysicalFileProvider to a specific directory rather than the filesystem root to limit access and prevent path traversal.
7. Check IFileInfo.Exists before calling CreateReadStream() because non-existent files return a NotFoundFileInfo that throws on stream creation.
8. Use glob patterns in Watch() like **/*.json for recursive watching or config/*.yaml for directory-scoped watching instead of watching individual files one at a time.
9. Cache file contents in memory for frequently accessed templates rather than reading from the provider on every request, using IChangeToken to invalidate the cache.
10. Avoid using PhysicalFileProvider in unit tests -- create a test implementation of IFileProvider or use NullFileProvider to keep tests fast and deterministic.