Add C# billing service

Author: cghyzel

services/billing-csharp/Billing.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.3.2" />
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="Billing.csproj" />
+  </ItemGroup>
+
+</Project>

services/billing-csharp/Billing.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- existing packages… -->
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="6.4.0" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Remove="Tests\**\*.cs" />
+  </ItemGroup>
+</Project>

services/billing-csharp/Controllers/BillingController.cs

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+using Microsoft.AspNetCore.Mvc;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Threading.Tasks;
+using System;
+
+public class ChargeRequest
+{
+    [JsonPropertyName("username")]
+    public string Username { get; set; }
+
+    [JsonPropertyName("productId")]
+    public string ProductId { get; set; }
+
+    [JsonPropertyName("quantity")]
+    public int Quantity { get; set; }
+}
+
+[ApiController]
+[Route("billing")]
+public class BillingController : ControllerBase
+{
+    private readonly string EXPECTED_SECRET = Environment.GetEnvironmentVariable("BILLING_SECRET");
+
+    [HttpPost("charge")]
+    public async Task<IActionResult> Charge([FromBody] ChargeRequest request)
+    {
+        var receivedSecret = Request.Headers["X-Service-Secret"].ToString();
+        if (string.IsNullOrEmpty(receivedSecret) || receivedSecret != EXPECTED_SECRET)
+            return Unauthorized("Missing or invalid service secret");
+
+        if (string.IsNullOrEmpty(request.Username) || string.IsNullOrEmpty(request.ProductId) || request.Quantity <= 0)
+            return BadRequest("Invalid request payload");
+
+        var responsePayload = new
+        {
+            status = "charged",
+            user = request.Username,
+            product = request.ProductId,
+            quantity = request.Quantity
+        };
+
+        return Ok(JsonSerializer.Serialize(responsePayload));
+    }
+}

services/billing-csharp/Dockerfile

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+FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
+WORKDIR /src
+COPY . .
+RUN dotnet restore Billing.csproj && dotnet restore Billing.Tests.csproj
+RUN dotnet test Billing.Tests.csproj --no-restore --logger "trx;LogFileName=test_results.trx"
+RUN dotnet publish Billing.csproj -c Release -o /app/publish
+
+FROM mcr.microsoft.com/dotnet/aspnet:6.0 AS runtime
+WORKDIR /app
+COPY --from=build /app/publish .
+ENTRYPOINT ["dotnet", "Billing.dll"]

services/billing-csharp/Program.cs

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+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.OpenApi.Models;
+
+
+var builder = WebApplication.CreateBuilder(args);
+
+builder.Services.AddControllers();
+builder.Services.AddSwaggerGen(c =>
+{
+    c.SwaggerDoc("v1", new OpenApiInfo { Title = "Billing API", Version = "v1" });
+});
+
+var app = builder.Build();
+
+app.UseSwagger();
+app.UseSwaggerUI(c =>
+{
+    app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "Billing API v1"));
+});
+
+app.MapControllers();
+app.Run();

services/billing-csharp/Tests/Controllers/BillingControllerTests.cs

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+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Http;
+using Xunit;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+public class BillingControllerTests
+{
+    [Fact]
+    public async Task ReturnsOkWithValidPayload()
+    {
+        System.Environment.SetEnvironmentVariable("BILLING_SECRET", "topsecretvalue");
+
+        var controller = new BillingController();
+        controller.ControllerContext = new ControllerContext
+        {
+            HttpContext = new DefaultHttpContext()
+        };
+        controller.Request.Headers["X-Service-Secret"] = "topsecretvalue";
+
+        var result = await controller.Charge(new ChargeRequest
+        {
+            Username = "alice",
+            ProductId = "widget-123",
+            Quantity = 1
+        });
+
+        var okResult = Assert.IsType<OkObjectResult>(result);
+        var json = okResult.Value as string;
+        Assert.NotNull(json);
+
+        var doc = JsonSerializer.Deserialize<JsonElement>(json);
+        Assert.Equal("charged", doc.GetProperty("status").GetString());
+        Assert.Equal("alice", doc.GetProperty("user").GetString());
+        Assert.Equal("widget-123", doc.GetProperty("product").GetString());
+        Assert.Equal(1, doc.GetProperty("quantity").GetInt32());
+    }
+
+    [Fact]
+    public async Task ReturnsUnauthorizedWithoutSecret()
+    {
+        var controller = new BillingController();
+        controller.ControllerContext = new ControllerContext
+        {
+            HttpContext = new DefaultHttpContext()
+        };
+        controller.Request.Headers["X-Service-Secret"] = "wrongsecret";
+
+        var result = await controller.Charge(new ChargeRequest
+        {
+            Username = "bob",
+            ProductId = "widget-123",
+            Quantity = 1
+        });
+
+        Assert.IsType<UnauthorizedObjectResult>(result);
+    }
+
+    [Fact]
+    public async Task ReturnsBadRequestWithMissingFields()
+    {
+        System.Environment.SetEnvironmentVariable("BILLING_SECRET", "topsecretvalue");
+
+        var controller = new BillingController();
+        controller.ControllerContext = new ControllerContext
+        {
+            HttpContext = new DefaultHttpContext()
+        };
+        controller.Request.Headers["X-Service-Secret"] = "topsecretvalue";
+
+        var result = await controller.Charge(new ChargeRequest
+        {
+            Username = "",
+            ProductId = "",
+            Quantity = 0
+        });
+
+        Assert.IsType<BadRequestObjectResult>(result);
+    }
+}