Make UseNCronJobAsync() mandatory when startup jobs have been defined

docs/features/startup-jobs.md

@@ -33,6 +33,8 @@ app.Run();
 
 The `RunAtStartup` in combination with `UseNCronJobAsync` method ensures that the job is executed as soon as the application starts. This method is useful for scenarios where certain tasks need to be performed immediately upon application launch.
 
+Failure to call `UseNCronJobAsync` when startup jobs are defined will lead to a fatal exception during the application start.
+
 ## Example Use Case
 
 Consider an application that needs to load initial data from a database or perform some cleanup tasks whenever it starts. You can define and configure a startup job to handle this:

src/NCronJob/NCronJobExtensions.cs

@@ -116,12 +116,18 @@ public static IServiceCollection AddNCronJob(this IServiceCollection services, D
     /// <summary>
     /// Configures the host to use NCronJob. This will also start any given startup jobs and their dependencies.
     /// </summary>
+    /// <remarks>
+    /// Failure to call this method (or <see cref="UseNCronJobAsync(IHost)"/>) when startup jobs are defined will lead to a fatal exception during the application start.
+    /// </remarks>
     /// <param name="host">The host.</param>
     public static IHost UseNCronJob(this IHost host) => UseNCronJobAsync(host).ConfigureAwait(false).GetAwaiter().GetResult();
 
     /// <summary>
     /// Configures the host to use NCronJob. This will also start any given startup jobs and their dependencies.
     /// </summary>
+    /// <remarks>
+    /// Failure to call this method (or <see cref="UseNCronJob(IHost)"/>) when startup jobs are defined will lead to a fatal exception during the application start.
+    /// </remarks>
     /// <param name="host">The host.</param>
     public static async Task<IHost> UseNCronJobAsync(this IHost host)
     {

src/NCronJob/Scheduler/QueueWorker.LogMessage.cs

@@ -39,7 +39,4 @@ internal sealed partial class QueueWorker
 
     [LoggerMessage(LogLevel.Information, "Job removed from queue: {JobType} at {RunAt}")]
     private partial void LogJobRemovedFromQueue(string jobType, DateTime? runAt);
-
-    [LoggerMessage(LogLevel.Warning, "The UseNCronJob(Async) method was not called. Startup jobs might not have been executed.")]
-    private partial void LogUseNCronJobNotCalled();
 }

src/NCronJob/Scheduler/QueueWorker.cs

@@ -122,10 +122,20 @@ protected override async Task ExecuteAsync(CancellationToken stoppingToken)
 
     private void AssertUseNCronJobWasCalled()
     {
-        if (!missingMethodCalledHandler.UseWasCalled)
+        if (missingMethodCalledHandler.UseWasCalled)
         {
-            LogUseNCronJobNotCalled();
+            return;
+        }
+
+        if (jobRegistry.GetAllOneTimeJobs().Count == 0)
+        {
+            return;
         }
+
+        throw new InvalidOperationException(
+            $"""
+            Startup jobs have been registered. However, neither IHost.UseNCronJobAsync(), nor IHost.UseNCronJob() have been been called.
+            """);
     }
 
     private void CreateWorkerQueues(CancellationToken stopToken)

tests/NCronJob.Tests/RunAtStartupJobTests.cs

@@ -7,6 +7,26 @@ namespace NCronJob.Tests;
 
 public class RunAtStartupJobTests : JobIntegrationBase
 {
+    [Fact]
+    public async Task UseNCronJobIsMandatoryWhenStartupJobsAreDefined()
+    {
+        var builder = Host.CreateDefaultBuilder();
+        var storage = new Storage();
+        builder.ConfigureServices(services =>
+        {
+            services.AddNCronJob(s => s.AddJob<SimpleJob>().RunAtStartup());
+            services.AddSingleton(_ => storage);
+        });
+
+        using var app = builder.Build();
+
+#pragma warning disable IDISP013 // Await in using
+        Func<Task> act = () => RunApp(app);
+#pragma warning restore IDISP013 // Await in using
+
+        await act.ShouldThrowAsync<InvalidOperationException>();
+    }
+
     [Fact]
     public async Task UseNCronJobShouldTriggerStartupJobs()
     {