C#•10mo ago

Primary constructor parameter is null after being run in a Task

The following snippet of code is responsible for starting a background service

public class PeriodicCallingIdentificationService(CallService callService)
{
    private Task? _executeTask;
    private CancellationTokenSource? _stoppingCts;
    private Modem? _modem;

    public void StartExecuting(CancellationToken cancellationToken)
    {
        if (_modem is null) throw new InvalidOperationException("Can't start executing without modem");
        _stoppingCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        _executeTask = Task.Run(async () =>
        {
            await DoWork(_stoppingCts.Token); // <-- here
            _modem.Close();
            _modem = null;
        }, cancellationToken);
    }

public class PeriodicCallingIdentificationService(CallService callService)
{
    private Task? _executeTask;
    private CancellationTokenSource? _stoppingCts;
    private Modem? _modem;

    public void StartExecuting(CancellationToken cancellationToken)
    {
        if (_modem is null) throw new InvalidOperationException("Can't start executing without modem");
        _stoppingCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        _executeTask = Task.Run(async () =>
        {
            await DoWork(_stoppingCts.Token); // <-- here
            _modem.Close();
            _modem = null;
        }, cancellationToken);
    }

This service is being registered with Microsoft DI as a singleton. When the StartExecuting function is called, the callService is correctly being passed. However if I set a breakpoint inside the lambda of Task.Run, callService is null there... Any ideas?

6 Replies

UltraWelfareOP•10mo ago

Small Update: it isn't null, the debugger is messing something up :/ Also in before you tell me about BackgroundServices: I'm in blazor hybrid where these type of stuff dont work.

i like chatgpt•10mo ago

There will be no a backing field for callService if it is not used in your class. Where is it used?

public class Student(string name) // name will have no a backing field because this parameter is not used in the class.
{
    public int Id { get; set; }
}

public class Student(string name) // name will have no a backing field because this parameter is not used in the class.
{
    public int Id { get; set; }
}

is translated to

public class Student
{
    [CompilerGenerated]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private int <Id>k__BackingField;

    public int Id
    {
        [CompilerGenerated]
        get
        {
            return <Id>k__BackingField;
        }
        [CompilerGenerated]
        set
        {
            <Id>k__BackingField = value;
        }
    }

    [NullableContext(1)]
    public Student(string name)
    {
    }
}

public class Student
{
    [CompilerGenerated]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private int <Id>k__BackingField;

    public int Id
    {
        [CompilerGenerated]
        get
        {
            return <Id>k__BackingField;
        }
        [CompilerGenerated]
        set
        {
            <Id>k__BackingField = value;
        }
    }

    [NullableContext(1)]
    public Student(string name)
    {
    }
}

Complete case:

public class Student(string name)
{
    public int Id { get; set; }
    public string Name => name; // generate a shared backing field for name
    public void Print() => Console.WriteLine(name); // generate a shared backing field for name
    
    
    public string Hahaha { get; set; } = name; // does NOT generate a shared backing field for name
    public string hihihi = name; // does NOT generate a shared backing field for name
}

public class Student(string name)
{
    public int Id { get; set; }
    public string Name => name; // generate a shared backing field for name
    public void Print() => Console.WriteLine(name); // generate a shared backing field for name
    
    
    public string Hahaha { get; set; } = name; // does NOT generate a shared backing field for name
    public string hihihi = name; // does NOT generate a shared backing field for name
}

UltraWelfareOP•10mo ago

It's used inside DoWork It works as intended, it's just the debugger is showing weird information about the variables You can take a look at this small snippet:

internal class Program
{
    static void Main(string[] args)
    {
        var printService = new PrintService();
        var testService = new TestService(printService);
        
        testService.StartExecuting();
        Thread.Sleep(5000);
        testService.StopExecuting();
        Console.WriteLine("Done");
    }
}

class PrintService
{
    public void PrintSomething()
    {
        Console.WriteLine("Something");
    }
}

class TestService(PrintService printService)
{

    private CancellationTokenSource _cts;
    private Task _task;
    
    public void StartExecuting(CancellationToken cancellationToken = default)
    {
        _cts = new();
        _task = Task.Run(async () =>
        {
            await DoWork(_cts.Token);
        }, cancellationToken);
    }
    
    public void StopExecuting(CancellationToken cancellationToken = default)
    {
        _cts.Cancel();
        _task.Wait(cancellationToken);
    }
    
    public async Task DoWork(CancellationToken cancellationToken)
    {
        while (true)
        {
            try
            {
                Console.WriteLine("Doing work");
                printService.PrintSomething();
                
                await Task.Delay(1000, cancellationToken);
            } catch (OperationCanceledException)
            {
                Console.WriteLine("Operation cancelled");
                break;
            }
        }
    }
}

internal class Program
{
    static void Main(string[] args)
    {
        var printService = new PrintService();
        var testService = new TestService(printService);
        
        testService.StartExecuting();
        Thread.Sleep(5000);
        testService.StopExecuting();
        Console.WriteLine("Done");
    }
}

class PrintService
{
    public void PrintSomething()
    {
        Console.WriteLine("Something");
    }
}

class TestService(PrintService printService)
{

    private CancellationTokenSource _cts;
    private Task _task;
    
    public void StartExecuting(CancellationToken cancellationToken = default)
    {
        _cts = new();
        _task = Task.Run(async () =>
        {
            await DoWork(_cts.Token);
        }, cancellationToken);
    }
    
    public void StopExecuting(CancellationToken cancellationToken = default)
    {
        _cts.Cancel();
        _task.Wait(cancellationToken);
    }
    
    public async Task DoWork(CancellationToken cancellationToken)
    {
        while (true)
        {
            try
            {
                Console.WriteLine("Doing work");
                printService.PrintSomething();
                
                await Task.Delay(1000, cancellationToken);
            } catch (OperationCanceledException)
            {
                Console.WriteLine("Operation cancelled");
                break;
            }
        }
    }
}

It's a recreation of the debugger bug If you debug inside the lambda or DoWork the printService would be shown as null (although it isn't)

i like chatgpt•10mo ago

The generated state machines are confusing. 🙂

UltraWelfareOP•10mo ago

What do you mean

i like chatgpt•10mo ago

I attempted to understand the flow but the generated state machines by sharplab.io are confusing.

Gaming

Programming

Primary constructor parameter is null after being run in a Task