--- name: openui-forge-csharp description: OpenUI generative UI with C# ASP.NET Core Minimal API backend. Direct OpenAI API SSE streaming via HttpClient on .NET 10. version: 1.2.0 author: OthmanAdi --- # OpenUI Forge — C# Build generative UI apps with a React frontend + C# backend. Streams OpenAI API responses directly via an ASP.NET Core Minimal API (.NET 10 LTS) using `HttpClient`. ## Activation Triggers - "openui csharp", "openui c#", "openui dotnet", "openui aspnet" - "generative ui csharp", "c# streaming ui backend", "asp.net core openui" ## Prerequisites - Node.js >= 22 (24 LTS recommended) + React >= 18.3.1 (19+ recommended) (frontend) - .NET SDK 10.0 (backend; .NET 10 is the current LTS, supported until Nov 2028. .NET 8 LTS also works but reaches end of support Nov 2026.) - `OPENAI_API_KEY` environment variable set ## Quick Start 1. Create the React frontend and install OpenUI deps: ```bash npm install @openuidev/react-ui @openuidev/react-headless @openuidev/react-lang lucide-react zod ``` 2. Generate the system prompt: ```bash npx @openuidev/cli generate ./src/lib/library.ts --out backend/system-prompt.txt ``` 3. Create the C# backend (see Full Code below) 4. Run: `dotnet run` on `:5000`, frontend on `:3000` ## Full Code ### Backend: `backend/openui-backend.csproj` ```xml net10.0 enable enable ``` > No NuGet packages required. ASP.NET Core, `HttpClient`/`IHttpClientFactory`, and `System.Text.Json` all ship in the .NET 10 shared framework referenced by `Microsoft.NET.Sdk.Web`. ### Backend: `backend/Program.cs` ```csharp using System.Text; using System.Text.Json; using System.Text.Json.Serialization; var builder = WebApplication.CreateBuilder(args); // Pooled, correctly-disposed HttpClient instances. Infinite timeout because // this is a long-lived streaming proxy; client disconnects cancel the request. builder.Services.AddHttpClient("openai", client => { client.Timeout = Timeout.InfiniteTimeSpan; }); // CORS: lock to the configured frontend origin. Do NOT use AllowAnyOrigin — // a wildcard would let any site call this backend and burn your API key. var frontendOrigin = Environment.GetEnvironmentVariable("FRONTEND_ORIGIN") ?? "http://localhost:3000"; builder.Services.AddCors(options => { options.AddDefaultPolicy(policy => policy.WithOrigins(frontendOrigin) .WithMethods("POST", "OPTIONS") .AllowAnyHeader()); }); var app = builder.Build(); app.UseCors(); // Load the generated system prompt ONCE at startup; fail fast if missing. var promptPath = Path.Combine(Directory.GetCurrentDirectory(), "system-prompt.txt"); if (!File.Exists(promptPath)) { throw new FileNotFoundException( "system-prompt.txt not found. Generate it with: " + "npx @openuidev/cli generate ./src/lib/library.ts --out system-prompt.txt", promptPath); } var systemPrompt = await File.ReadAllTextAsync(promptPath); var apiKey = Environment.GetEnvironmentVariable("OPENAI_API_KEY"); var baseUrl = (Environment.GetEnvironmentVariable("OPENAI_BASE_URL") ?? "https://api.openai.com/v1") .TrimEnd('/'); var model = Environment.GetEnvironmentVariable("OPENAI_MODEL") ?? "gpt-5.5"; app.MapPost("/api/chat", async (ChatRequest req, IHttpClientFactory httpClientFactory, HttpContext ctx) => { if (string.IsNullOrEmpty(apiKey)) return Results.Json(new { error = "OPENAI_API_KEY not set" }, statusCode: 500); if (req.Messages is null || req.Messages.Length == 0) return Results.Json(new { error = "messages must be a non-empty array" }, statusCode: 400); // Prepend the server-side system prompt; never trust the client to supply it. var messages = new List { new("system", systemPrompt) }; messages.AddRange(req.Messages); var payload = JsonSerializer.Serialize(new OpenAiRequest(model, true, messages)); using var upstreamRequest = new HttpRequestMessage( HttpMethod.Post, $"{baseUrl}/chat/completions") { Content = new StringContent(payload, Encoding.UTF8, "application/json"), }; upstreamRequest.Headers.Add("Authorization", $"Bearer {apiKey}"); var client = httpClientFactory.CreateClient("openai"); // ResponseHeadersRead returns as soon as headers arrive, so we read the // body incrementally off the socket instead of buffering it into memory. var upstream = await client.SendAsync( upstreamRequest, HttpCompletionOption.ResponseHeadersRead, ctx.RequestAborted); if (!upstream.IsSuccessStatusCode) { var errorBody = await upstream.Content.ReadAsStringAsync(ctx.RequestAborted); upstream.Dispose(); return Results.Json( new { error = $"OpenAI returned {(int)upstream.StatusCode}: {errorBody}" }, statusCode: (int)upstream.StatusCode); } return Results.Extensions.SseProxy(upstream); }); app.Run(); // SSE passthrough: forward OpenAI's native `data: {chunk}\n\n` lines verbatim, // flushing after each so tokens appear as they arrive. Pair with openAIAdapter(). // // Idiomatic .NET 10 alternative: if you parse each chunk into a typed payload, // return TypedResults.ServerSentEvents(IAsyncEnumerable>) from // System.Net.ServerSentEvents — the framework writes the SSE framing for you. // We keep the raw passthrough because the upstream is ALREADY valid SSE. static class ResultExtensions { public static IResult SseProxy(this IResultExtensions _, HttpResponseMessage upstream) => new SseProxyResult(upstream); } sealed class SseProxyResult(HttpResponseMessage upstream) : IResult { public async Task ExecuteAsync(HttpContext httpContext) { var response = httpContext.Response; response.StatusCode = 200; response.ContentType = "text/event-stream"; response.Headers.CacheControl = "no-cache"; response.Headers.Connection = "keep-alive"; response.Headers["X-Accel-Buffering"] = "no"; // defeat proxy buffering try { await using var upstreamStream = await upstream.Content.ReadAsStreamAsync(httpContext.RequestAborted); using var reader = new StreamReader(upstreamStream, Encoding.UTF8); while (await reader.ReadLineAsync(httpContext.RequestAborted) is { } line) { if (line.Length == 0) continue; // re-emit our own framing below await response.WriteAsync(line + "\n", httpContext.RequestAborted); if (line.StartsWith("data:", StringComparison.Ordinal)) await response.WriteAsync("\n", httpContext.RequestAborted); await response.Body.FlushAsync(httpContext.RequestAborted); if (line == "data: [DONE]") break; } } catch (OperationCanceledException) { // Client aborted — nothing to do. } finally { upstream.Dispose(); } } } // JsonPropertyName pins wire names to lowercase: incoming binding is // case-insensitive, but Serialize defaults to PascalCase and OpenAI needs // lowercase role/content/model. record ChatMessage( [property: JsonPropertyName("role")] string Role, [property: JsonPropertyName("content")] string Content); record ChatRequest( [property: JsonPropertyName("messages")] ChatMessage[] Messages); record OpenAiRequest( [property: JsonPropertyName("model")] string Model, [property: JsonPropertyName("stream")] bool Stream, [property: JsonPropertyName("messages")] List Messages); ``` ### Frontend: `app/chat/page.tsx` ```tsx "use client"; import { FullScreen } from "@openuidev/react-ui"; import { openuiChatLibrary } from "@openuidev/react-ui/genui-lib"; import { openAIAdapter, openAIMessageFormat, } from "@openuidev/react-headless"; export default function ChatPage() { return ( ); } ``` > The C# backend forwards OpenAI's SSE stream line-by-line with a `StreamReader` loop, calling `Response.Body.FlushAsync()` after each line (no `CopyToAsync` buffering), so the client sees tokens as they arrive. Pair it with `openAIAdapter()` on the frontend. `openAIReadableStreamAdapter()` is for NDJSON (no `data:` prefix) and will silently produce no output here. > > `HttpCompletionOption.ResponseHeadersRead` is the key to incremental streaming: it returns control once the upstream headers arrive instead of buffering the whole response body into memory first. > > An official OpenAI .NET SDK exists (`OpenAI` on NuGet) as an alternative to hand-rolling the HTTP call. This skill keeps raw `HttpClient` as the dependency-free default (everything used here ships in the .NET 10 shared framework). .NET 10 also added a native typed SSE result, `TypedResults.ServerSentEvents` (from `System.Net.ServerSentEvents`), which writes the `data:` framing for you when you yield strongly-typed `SseItem` values — use it if you want to parse and reshape chunks rather than pass them through. ## System Prompt Generation ```bash npx @openuidev/cli generate ./src/lib/library.ts --out backend/system-prompt.txt ``` ## Validation Checklist - [ ] `system-prompt.txt` exists in the C# backend directory (next to the `.csproj`) - [ ] `OPENAI_API_KEY` is set in environment (`.env` is not auto-loaded; export it or use `dotnet user-secrets`) - [ ] `OPENAI_BASE_URL` honored for OpenAI-compatible providers (defaults to `https://api.openai.com/v1`) - [ ] CORS policy allows the frontend origin via `WithOrigins(FRONTEND_ORIGIN)` (not `AllowAnyOrigin`) - [ ] Response streams SSE directly from OpenAI API (passthrough), `Content-Type: text/event-stream` - [ ] `HttpCompletionOption.ResponseHeadersRead` used so the body is not buffered - [ ] `Response.Body.FlushAsync()` called after each line - [ ] Frontend `apiUrl` points to `http://localhost:5000/api/chat` - [ ] Frontend uses `streamProtocol={openAIAdapter()}` and `openAIMessageFormat` - [ ] `componentLibrary={openuiChatLibrary}` prop passed to `FullScreen` - [ ] CSS import in root layout (`@openuidev/react-ui/components.css`) ## Error Patterns | Error | Cause | Fix | |-------|-------|-----| | CORS blocked | Origin mismatch | Set `FRONTEND_ORIGIN` to match, ensure `app.UseCors()` runs before the endpoint | | `FileNotFoundException: system-prompt.txt` | File missing from backend dir | Run the CLI generate command into the project directory | | 500 `OPENAI_API_KEY not set` | Env var missing | Export `OPENAI_API_KEY` (or set it in `launchSettings.json` for local dev) | | Upstream status forwarded (e.g. 401/429) | Key invalid or rate limited | Check `OPENAI_API_KEY`, model name, and provider quota | | Empty response / components show as raw text | Frontend adapter mismatch | Backend emits SSE — use `openAIAdapter()`, not `openAIReadableStreamAdapter()` | | Stream arrives all at once at the end | Buffering somewhere | Confirm `ResponseHeadersRead` + per-line `FlushAsync()`; keep `X-Accel-Buffering: no` | | `role`/`content` rejected by API | PascalCase JSON | Keep the `[JsonPropertyName(...)]` attributes on the DTO records |