1// .NET 8 — App Insights with distributed tracing + custom telemetry
2// Program.cs
3builder.Services.AddApplicationInsightsTelemetry(options =>
4{
5    // Use connection string, NOT instrumentation key (deprecated)
6    options.ConnectionString = builder.Configuration["ApplicationInsights:ConnectionString"];
7});
8
9// Custom middleware — enriches every request with user context
10builder.Services.AddSingleton<ITelemetryInitializer, UserTelemetryInitializer>();
11
12public class UserTelemetryInitializer : ITelemetryInitializer
13{
14    public void Initialize(ITelemetry telemetry)
15    {
16        // Adds custom dimension to EVERY piece of telemetry
17        if (telemetry is ISupportProperties item)
18            item.Properties["UserId"] = httpContextAccessor.HttpContext?.User.GetUserId();
19    }
20}
21
22// Service — track custom event + dependency manually
23public class OrderService
24{
25    private readonly TelemetryClient _telemetry;
26
27    public async Task PlaceOrder(Order order)
28    {
29        var sw = Stopwatch.StartNew();
30        try
31        {
32            await _db.SaveAsync(order);
33            sw.Stop();
34
35            // Custom event with properties (avoid high cardinality keys!)
36            _telemetry.TrackEvent("OrderPlaced", new Dictionary<string, string>
37            {
38                ["ProductCategory"] = order.Category, // LOW cardinality — good
39                // ["OrderId"] = order.Id,  // HIGH cardinality — BAD, hits data cap fast
40            }, new Dictionary<string, double>
41            {
42                ["OrderValueGBP"] = order.TotalGbp,
43            });
44        }
45        catch (Exception ex)
46        {
47            _telemetry.TrackException(ex, new Dictionary<string, string>
48            {
49                ["OrderId"] = order.Id, // OK in exceptions — low volume
50            });
51            throw;
52        }
53    }
54}
55
56// Azure Functions — MUST flush before function exits!
57public class ProcessOrderFunction
58{
59    private readonly TelemetryClient _telemetry;
60
61    [FunctionName("ProcessOrder")]
62    public async Task Run([ServiceBusTrigger("orders")] string message)
63    {
64        try { /* process */ }
65        finally
66        {
67            // Without this, telemetry is lost when function host recycles
68            await _telemetry.FlushAsync(CancellationToken.None);
69        }
70    }
71}