dotnet add package Rollbar.Net.AspNet.Mvc

//...
using Rollbar;
//...

public class Global : HttpApplication
{
  void Application_Start(object sender, EventArgs e)
  {
    ConfigureRollbar();

    // Code that runs on application startup
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
  }

  /// <summary>
  /// Handles the Error event on the Application level.
  /// </summary>
  /// <param name="sender">The source of the event.</param>
  /// <param name="e">The <see cref="EventArgs"/> instance containing the event data.</param>
  void Application_Error(object sender, EventArgs e)
  {
    Exception exception = Server.GetLastError();

    // Let's report to Rollbar on the Application/Global Level: 
    var metaData = new Dictionary<string, object>();
    metaData.Add("reportLevel", "GlobalLevel");
    RollbarLocator.RollbarInstance.Error(exception, metaData);

    if (exception is HttpUnhandledException)
    {
      // Pass the error on to the error page. 
      Server.Transfer("ErrorPage.aspx?handler=Application_Error%20-%20Global.asax", true);
    }
  }

  /// <summary>
  /// Handles the Error event on the Page level.
  /// </summary>
  /// <param name="sender">The source of the event.</param>
  /// <param name="e">The <see cref="EventArgs"/> instance containing the event data.</param>
  private void Page_Error(object sender, EventArgs e)
  {
    // Get last error from the server.
    Exception exception = Server.GetLastError();

    // Let's report to Rollbar on the Page Level:
    var metaData = new Dictionary<string, object>();
    metaData.Add("reportLevel", "PageLevel");
    RollbarLocator.RollbarInstance.Error(exception, metaData);

    // Handle specific exception.
    if(exception is InvalidOperationException)
    {
      // Pass the error on to the error page.
      Server.Transfer(
        "ErrorPage.aspx?handler=Page_Error%20-%20Default.aspx",
        true
      );
    }
  }
  
  /// <summary>
  /// Configures the Rollbar singleton-like notifier.
  /// </summary>
  private void ConfigureRollbar()
  {
    RollbarInfrastructureConfig rollbarInfrastructureConfig = new RollbarInfrastructureConfig(
      RollbarSamplesSettings.AccessToken,
      RollbarSamplesSettings.Environment
    );
    RollbarInfrastructure.Instance.Init(rollbarInfrastructureConfig);

    // optionally, if you would like to monitor Rollbar internal events within your application:
    RollbarInfrastructure.Instance.QueueController.InternalEvent += OnRollbarInternalEvent;
  }

  private static void OnRollbarInternalEvent(object sender, RollbarEventArgs e)
  {
    Console.WriteLine(e.TraceAsString());

    RollbarApiErrorEventArgs apiErrorEvent = e as RollbarApiErrorEventArgs;
    if (apiErrorEvent != null)
    {
      //TODO: handle/report Rollbar API communication error event...
      return;
    }
    CommunicationEventArgs commEvent = e as CommunicationEventArgs;
    if (commEvent != null)
    {
      //TODO: handle/report Rollbar API communication event...
      return;
    }
    CommunicationErrorEventArgs commErrorEvent = e as CommunicationErrorEventArgs;
    if (commErrorEvent != null)
    {
      //TODO: handle/report basic communication error while attempting to reach Rollbar API service... 
      return;
    }
    InternalErrorEventArgs internalErrorEvent = e as InternalErrorEventArgs;
    if (internalErrorEvent != null)
    {
      //TODO: handle/report basic internal error while using the Rollbar Notifier... 
      return;
    }
  }

}