public void ConfigureServices(IServiceCollection services)
{
 services.AddSingleton<IFileProvider>(
  new PhysicalFileProvider(Directory.GetCurrentDirectory()));
}

public class HelloFileProviderMiddleware
{
 private readonly RequestDelegate _next;
 private readonly IFileProvider _fileProvider;
 public HelloFileProviderMiddleware(RequestDelegate next, IFileProvider fileProvider)
 {
  _next = next;
  _fileProvider = fileProvider;
 }
 public async Task Invoke(HttpContext context)
 {
  var output = new StringBuilder("");
  IDirectoryContents dir = _fileProvider.GetDirectoryContents("");
  foreach (IFileInfo item in dir)
  {
   output.AppendLine(item.Name);
  }
  await context.Response.WriteAsync(output.ToString());
 }
}
public static class UseHelloFileProviderExtensions
{
 public static IApplicationBuilder UseHelloFileProvider(this IApplicationBuilder app)
 {
  return app.UseMiddleware<HelloFileProviderMiddleware>();
 }
}

public class HelloFileProviderMiddleware
{
 private readonly RequestDelegate _next;
 private readonly IFileProvider _fileProvider;
 public HelloFileProviderMiddleware(RequestDelegate next, IFileProvider fileProvider)
 {
  _next = next;
  _fileProvider = fileProvider;
 }
 public async Task Invoke(HttpContext context)
 {
  IFileInfo file = _fileProvider.GetFileInfo("Startup.cs");
  using (var stream = file.CreateReadStream())
  {
   using (var reader = new StreamReader(stream))
   {
    var output = await reader.ReadToEndAsync();
    await context.Response.WriteAsync(output.ToString());
   }
  }
 }
}

public class HelloFileProviderMiddleware
{
 private readonly RequestDelegate _next;
 private readonly IFileProvider _fileProvider;
 public HelloFileProviderMiddleware(RequestDelegate next, IFileProvider fileProvider)
 {
  _next = next;
  _fileProvider = fileProvider;
 }
 public async Task Invoke(HttpContext context)
 {
  var output = new StringBuilder("");
  ResolveDirectory(output, "", "");
  await context.Response.WriteAsync(output.ToString());
 }
 private void ResolveDirectory(StringBuilder output, string path, string prefix)
 {
  IDirectoryContents dir = _fileProvider.GetDirectoryContents(path);
  foreach (IFileInfo item in dir)
  {
   if (item.IsDirectory)
   {
    output.AppendLine(prefix + "[" + item.Name + "]");
    ResolveDirectory(output,
     item.PhysicalPath.Substring(Directory.GetCurrentDirectory().Length),
     prefix + " ");
   } else
   {
    output.AppendLine(prefix + item.Name);
   }
  }
 }
}