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
    );
  }
}

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");
    metaData.Add("failedPage", this.AppRelativeVirtualPath);
    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
      );
    }
  }

try
{
  // Let's simulate an error:
  throw new NullReferenceException("WebForms.Site sample: simulated exception");
}
catch (Exception exception)
{
  // Let's report to Rollbar on the Code Level:
  var metaData = new Dictionary<string, object>();
  metaData.Add("reportLevel", "CodeLevel");
  metaData.Add("failedPage", this.AppRelativeVirtualPath);
  RollbarLocator.RollbarInstance.Error(exception, metaData);
  throw exception;
}