Add ASP.NET Core middleware

Prerender.AspNetCore.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <PackageId>Prerender.AspNetCore</PackageId>
+    <Version>1.0.0</Version>
+    <Authors>Prerender.io</Authors>
+    <Description>ASP.NET Core middleware for prerendering JavaScript-rendered pages for SEO via Prerender.io</Description>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
+    <RepositoryUrl>https://github.com/prerender/integrations</RepositoryUrl>
+    <RepositoryType>git</RepositoryType>
+  </PropertyGroup>
+  <ItemGroup>
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
+  </ItemGroup>
+  <ItemGroup>
+    <None Include="README.md" Pack="true" PackagePath="\" />
+  </ItemGroup>
+</Project>

PrerenderMiddleware.cs

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+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace Prerender.AspNetCore;
+
+public class PrerenderMiddleware : IMiddleware
+{
+    private static readonly string[] CrawlerUserAgents =
+    [
+        "googlebot", "yahoo", "bingbot", "baiduspider",
+        "facebookexternalhit", "twitterbot", "rogerbot", "linkedinbot",
+        "embedly", "quora link preview", "showyoubot", "outbrain",
+        "pinterest", "slackbot", "w3c_validator", "perplexity",
+        "oai-searchbot", "chatgpt-user", "gptbot", "claudebot", "amazonbot",
+    ];
+
+    private static readonly string[] ExtensionsToIgnore =
+    [
+        ".js", ".css", ".xml", ".less", ".png", ".jpg", ".jpeg", ".gif",
+        ".pdf", ".doc", ".txt", ".ico", ".rss", ".zip", ".mp3", ".rar",
+        ".exe", ".wmv", ".avi", ".ppt", ".mpg", ".mpeg", ".tif", ".wav",
+        ".mov", ".psd", ".ai", ".xls", ".mp4", ".m4a", ".swf", ".dat",
+        ".dmg", ".iso", ".flv", ".m4v", ".torrent", ".ttf", ".woff", ".svg",
+    ];
+
+    private readonly IHttpClientFactory _httpClientFactory;
+    private readonly PrerenderOptions _options;
+    private readonly ILogger<PrerenderMiddleware> _logger;
+
+    public PrerenderMiddleware(
+        IHttpClientFactory httpClientFactory,
+        IOptions<PrerenderOptions> options,
+        ILogger<PrerenderMiddleware> logger)
+    {
+        _httpClientFactory = httpClientFactory;
+        _options = options.Value;
+        _logger = logger;
+    }
+
+    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
+    {
+        if (!ShouldPrerender(context))
+        {
+            await next(context);
+            return;
+        }
+
+        try
+        {
+            var client = _httpClientFactory.CreateClient("prerender");
+            var apiUrl = BuildApiUrl(context);
+
+            using var request = new HttpRequestMessage(HttpMethod.Get, apiUrl);
+            request.Headers.TryAddWithoutValidation(
+                "User-Agent", context.Request.Headers["User-Agent"].ToString());
+            if (!string.IsNullOrWhiteSpace(_options.Token))
+                request.Headers.TryAddWithoutValidation("X-Prerender-Token", _options.Token);
+            request.Headers.TryAddWithoutValidation("X-Prerender-Int-Type", "AspNetCore");
+
+            using var response = await client.SendAsync(request, context.RequestAborted);
+            context.Response.StatusCode = (int)response.StatusCode;
+            var body = await response.Content.ReadAsStringAsync();
+            await context.Response.WriteAsync(body);
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogWarning(ex, "Prerender service unreachable, falling back");
+            await next(context);
+        }
+    }
+
+    private static bool ShouldPrerender(HttpContext context)
+    {
+        if (context.Request.Method != HttpMethods.Get) return false;
+
+        var path = context.Request.Path.Value ?? string.Empty;
+        if (IsStaticAsset(path)) return false;
+
+        if (context.Request.Query.ContainsKey("_escaped_fragment_")) return true;
+        if (context.Request.Headers.ContainsKey("X-Bufferbot")) return true;
+
+        var ua = context.Request.Headers["User-Agent"].ToString();
+        return !string.IsNullOrEmpty(ua) && IsBot(ua);
+    }
+
+    private string BuildApiUrl(HttpContext context)
+    {
+        var serviceUrl = _options.ServiceUrl.TrimEnd('/') + "/";
+        var scheme = context.Request.Scheme;
+        var host = context.Request.Host.Value;
+        var pathAndQuery = context.Request.Path + context.Request.QueryString;
+        return $"{serviceUrl}{scheme}://{host}{pathAndQuery}";
+    }
+
+    private static bool IsBot(string userAgent)
+    {
+        var ua = userAgent.ToLowerInvariant();
+        return CrawlerUserAgents.Any(bot => ua.Contains(bot));
+    }
+
+    private static bool IsStaticAsset(string path)
+    {
+        var lower = path.ToLowerInvariant();
+        return ExtensionsToIgnore.Any(ext => lower.EndsWith(ext));
+    }
+}

PrerenderOptions.cs

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+namespace Prerender.AspNetCore;
+
+public class PrerenderOptions
+{
+    public string? Token { get; set; }
+    public string ServiceUrl { get; set; } = "https://service.prerender.io/";
+}

PrerenderServiceExtensions.cs

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+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Prerender.AspNetCore;
+
+public static class PrerenderServiceExtensions
+{
+    public static IServiceCollection AddPrerender(this IServiceCollection services)
+    {
+        services.AddOptions<PrerenderOptions>().BindConfiguration("Prerender");
+        services.AddHttpClient("prerender");
+        services.AddTransient<PrerenderMiddleware>();
+        return services;
+    }
+
+    public static IApplicationBuilder UsePrerender(this IApplicationBuilder app)
+        => app.UseMiddleware<PrerenderMiddleware>();
+}

README.md

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+# prerender-aspnetcore
+
+ASP.NET Core middleware for [Prerender.io](https://prerender.io). Intercepts requests from bots and crawlers and serves prerendered HTML, so your JavaScript-rendered app is fully indexable by search engines and social media scrapers.
+
+Compatible with **ASP.NET Core 8+** and **.NET 8+**.
+
+## Installation
+
+```bash
+dotnet add package Prerender.AspNetCore
+```
+
+## Setup
+
+Register the middleware in `Program.cs`:
+
+```csharp
+builder.Services.AddPrerender();
+
+var app = builder.Build();
+app.UsePrerender(); // place before routing middleware
+```
+
+Add your token to `appsettings.json`:
+
+```json
+{
+  "Prerender": {
+    "Token": "YOUR_PRERENDER_TOKEN"
+  }
+}
+```
+
+The middleware must be placed **before** routing to intercept bot requests early.
+
+## Settings
+
+| Setting | Default | Description |
+|---------|---------|-------------|
+| `Prerender:Token` | `null` | Your Prerender.io token |
+| `Prerender:ServiceUrl` | `https://service.prerender.io/` | Prerender service URL (override for self-hosted Prerender) |
+
+## Self-hosted Prerender
+
+```json
+{
+  "Prerender": {
+    "ServiceUrl": "http://your-prerender-server:3000"
+  }
+}
+```
+
+## How it works
+
+Requests are prerendered when **all** of the following are true:
+
+- The HTTP method is `GET`
+- The `User-Agent` matches a known bot/crawler (Googlebot, Bingbot, Twitterbot, GPTBot, ClaudeBot, etc.)  
+  — OR the URL contains `_escaped_fragment_`  
+  — OR the `X-Bufferbot` header is present
+- The URL does not end with a static asset extension (`.js`, `.css`, `.png`, etc.)
+
+Everything else passes through to your normal ASP.NET Core pipeline.
+
+If the Prerender service is unreachable, the middleware falls back gracefully and serves the normal response.
+
+## License
+
+MIT

tests/Prerender.AspNetCore.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="8.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
+    <PackageReference Include="xunit" Version="2.7.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.7" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\Prerender.AspNetCore.csproj" />
+  </ItemGroup>
+</Project>