HostingEventSourceListener.cs

using System.Diagnostics.Tracing;

namespace WebApplication1;

/// <summary>
/// Subscribes to the current request counter as it is updated, once per second.
/// Follows the simple event listener pattern here:
/// https://learn.microsoft.com/en-us/dotnet/core/diagnostics/event-counters
/// </summary>
public class HostingEventSourceListener : EventListener, IHostingEventSourceListener
{
    private int? _currentRequests;
    public int? CurrentRequests => _currentRequests;

    protected override void OnEventSourceCreated(EventSource source)
    {
        // https://github.com/dotnet/aspnetcore/blob/be51b1aa73343e45a1d00afd436abad794f471fb/src/Hosting/Hosting/src/Internal/HostingEventSource.cs
        if (!source.Name.Equals("Microsoft.AspNetCore.Hosting"))
        {
            return;
        }

        EnableEvents(source, EventLevel.Verbose, EventKeywords.All, new Dictionary<string, string>()
        {
            ["EventCounterIntervalSec"] = "1"
        }!);
    }

    protected override void OnEventWritten(EventWrittenEventArgs eventData)
    {
        if (eventData.EventName != "EventCounters")
        {
            return;
        }

        for (int i = 0; i < eventData.Payload?.Count; ++ i)
        {
            if (eventData.Payload[i] is IDictionary<string, object> eventPayload)
            {
                // https://learn.microsoft.com/en-us/dotnet/core/diagnostics/available-counters
                // https://github.com/dotnet/aspnetcore/blob/be51b1aa73343e45a1d00afd436abad794f471fb/src/Hosting/Hosting/src/Internal/HostingEventSource.cs
                if (eventPayload.TryGetValue("Name", out var parsedName) && (string) parsedName == "current-requests")
                {
                    // Min, Max, Mean will all have the same values for current-requests.
                    eventPayload.TryGetValue("Min", out var currentRequests);
                    _currentRequests = ((int?) ((double?) currentRequests));
                }
            }
        }
    }
}