.Net core log Startup errors to app insights - asp.net-core

Question

I'm trying to log any startup errors but the logs are not flowing into application insights. I tried configuring app insights in program.cs but still I don't see any logs.
public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureLogging(logging =>
{
logging.ClearProviders();
logging.AddApplicationInsights();
logging.SetMinimumLevel(LogLevel.Information);
});

Answer 1

To Capture logs from program.cs or startup.cs, you need to configure like this.
.ConfigureLogging((context, builder) =>
{
// Providing a connection string is required if need to capture logs during application startup, such as
// in Program.cs or Startup.cs itself.
builder.AddApplicationInsights(
configureTelemetryConfiguration: (config) => config.ConnectionString = context.Configuration["APPLICATIONINSIGHTS_CONNECTION_STRING"],
configureApplicationInsightsLoggerOptions: (options) => { }
);
// Capture all log-level entries from Program
builder.AddFilter<ApplicationInsightsLoggerProvider>(
typeof(Program).FullName, LogLevel.Trace);
// Capture all log-level entries from Startup
builder.AddFilter<ApplicationInsightsLoggerProvider>(
typeof(Startup).FullName, LogLevel.Trace);
});

Answer 2

For those using .NET 6 projects with minimal hosting model, this worked for me to capture the startup/shutdown messages into Application Insights:
// Program.cs
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddApplicationInsightsTelemetry();
builder.Logging.AddFilter<ApplicationInsightsLoggerProvider>("Microsoft.Hosting.Lifetime", LogLevel.Information);
...

Answer 3

By default it is not supported to handle exceptions from application startup, see this link.
What I did to get the logs into the application insights is to add a try/catch block in the Main of Program.cs and initialize the TelemetryClient manually.
See the following code:
public static void Main(string[] args)
{
try
{
CreateHostBuilder(args)
.Build().Run();
}
catch(Exception ex)
{
TelemetryConfiguration telemetryConfiguration = TelemetryConfiguration.CreateDefault();
telemetryConfiguration.ConnectionString = "application-insights-connection-string";
var telemetryClient = new TelemetryClient(telemetryConfiguration);
telemetryClient.TrackException(ex);
telemetryClient.Flush();
throw;
}
}