ASP.NET Core ETAg middleware

ETagMiddleware.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.WebUtilities;
using Microsoft.Net.Http.Headers;
using System.IO;
using System.Security.Cryptography;
using System.Threading.Tasks;

public class ETagMiddleware
{
    private readonly RequestDelegate _next;

    public ETagMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        var response = context.Response;
        var originalStream = response.Body;

        using (var ms = new MemoryStream())
        {
            response.Body = ms;

            await _next(context);

            if (IsEtagSupported(response))
            {
                string checksum = CalculateChecksum(ms);

                response.Headers[HeaderNames.ETag] = checksum;

                if (context.Request.Headers.TryGetValue(HeaderNames.IfNoneMatch, out var etag) && checksum == etag)
                {
                    response.StatusCode = StatusCodes.Status304NotModified;
                    return;
                }
            }

            ms.Position = 0;
            await ms.CopyToAsync(originalStream);
        }
    }

    private static bool IsEtagSupported(HttpResponse response)
    {
        if (response.StatusCode != StatusCodes.Status200OK)
            return false;

        // The 20kb length limit is not based in science. Feel free to change
        if (response.Body.Length > 20 * 1024)
            return false;

        if (response.Headers.ContainsKey(HeaderNames.ETag))
            return false;

        return true;
    }

    private static string CalculateChecksum(MemoryStream ms)
    {
        string checksum = "";

        using (var algo = SHA1.Create())
        {
            ms.Position = 0;
            byte[] bytes = algo.ComputeHash(ms);
            checksum = $"\"{WebEncoders.Base64UrlEncode(bytes)}\"";
        }

        return checksum;
    }
}

public static class ApplicationBuilderExtensions
{
    public static void UseETagger(this IApplicationBuilder app)
    {
        app.UseMiddleware<ETagMiddleware>();
    }
}

SettingsController.cs

        /// <returns>
        /// Enough settings to start the app.
        /// </returns>
        [HttpGet]
        [AllowAnonymous]
        [Route("bootstrap")]
        [ResponseCache(CacheProfileName = "OneHourWithOverride")]
        public IActionResult Bootstrap()
        {
            if (Request?.Headers != null && Request.Headers.ContainsKey("If-Modified-Since"))
            {
                var result = DateTime.TryParse(Request.Headers["If-Modified-Since"], out var when);
                if (result)
                {
                    // It's very rare for app settings to change; they're environment-specific.
                    // But, if you're testing you may want to override the default 3600 (in appSettings.Development.json).
                    var seconds = this.configuration.GetValue<int?>("OneHourWithOverrideCacheDuration") ?? 3600;
                    if (when.AddSeconds(seconds).CompareTo(DateTime.Now) > 0)
                    {
                        Response.StatusCode = 304;
                        Response.ContentLength = 0;
                        return Content(string.Empty);
                    }
                }
            }

			...
              
#pragma warning disable CA1305 // Specify IFormatProvider: "O" is invariant
            this.HttpContext.Response.Headers.Add("Last-Modified", DateTime.UtcNow.ToString("O"));
#pragma warning restore CA1305 // Specify IFormatProvider
            return this.Ok(/* app settings */);
        }

Startup.cs

// Add "app.UseETagger();" to "Configure" method in Startup.cs
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseStaticFiles();
    
    // Add this after static files but before MVC in order to provide ETags to MVC Views and Razor Pages.
    app.UseETagger();

    app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });
}