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ASP.NET Core ETAg middleware
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>();
}
}
// 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?}");
});
}
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