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>();
    }
}

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?}");
    });
}

@Memnarch But how can the Server know if the result he wants to return is already the one the client has cached? The ETag saves you the bandwith to actually transfer the data from the server to the client. The backend therefore saves resources because he can answer more efficiently, even though he had to process the content of the answer beforehand. If you configured the cache to store and it is not yet expired, there will be no connection made to the server. If it is expired, but the ETag stays the same, the client can refresh the cache with the specified max age of the server and server does not have to actually send the data again.

Also, if you cache the result on server side, using the ETAG as key, then you also save resources on the server side. If the request is cachable.