using Database in IHostedService

Question

Hi guys I try to use my Database in an IHostedService. Asp .net, SQLite I get the context with the IServiceScopeFactory. After that I Start a Timer and then I want to access the database every time the timer gets called around all 2 seconds.

If I start the application this exception will be thrown immediately after starting

System.InvalidOperationException: 'An attempt was made to use the model while it was being created. A DbContext instance cannot be used inside 'OnModelCreating' in any way that makes use of the model that is being created.'

Here the code where I am calling the Database. In the line : if (CurrentIndex > db.Municipalities.Count()) CurrentIndex = 1; the exception occours.

public WeatherRefreshService(IServiceScopeFactory serviceProvider)
{
    this.db = serviceProvider.CreateScope().ServiceProvider.GetRequiredService<SunfinderDbContext>();
}

public Task StartAsync(CancellationToken cancellationToken)
{
    var task = new Task(async () =>
    {
        timer = new Timer(
            RefreshWeatherData,
            null,
            TimeSpan.Zero,
            TimeSpan.FromMilliseconds(RefreshTime)
            );
    });
    task.Start();
    return task;
}

public async void RefreshWeatherData(object? state)
{
    if (CurrentIndex > db.Municipalities.Count()) CurrentIndex = 1;
    var currentMunicipality = db.Municipalities.Where(x => x.Id == CurrentIndex).First();
    ...
}

Answer 1

You should resolve the database dependency inside your task (the processing loop), not in the constructor.

See here for more information and examples. The constructor captures the IServiceProvider into a field, which is used to create a scope and then the required service as part of the work loop.