C# List - list can be filled or taken from two threads

Question

C# List - list can be filled or taken from two threads

Markus Freitag 3,101

Hello,

static object LockPackageList = new object();

void Test1()
{    
    lock (LockPackageList)
    {
     switch (type)
     {
     case L.FiFo:
     ListPackageLabel.Add(currentPackage);
     // BlockingCollectionPackage.Add(currentPackage);
     break;
     case L.LiFo:
     ListPackageLabel.Insert(0, currentPackage);
     break;
     }
    }
}

public void DeletePackage()
        {
            lock (LockPackageList)
            {
                ListPackageLabel.RemoveAt(0);
            }

Is that sufficient with the lock?

Need I one static object LockPackageList = new object();
or two?
For Add and Remove?

Answer 1

Karen Payne MVP 30,441

Hello @Markus Freitag

Have you considered using a thread-safe Singleton? The following is based off this web page sixth version.

public class Address  
{  
    public string Type { get; set; }  
    public string Company { get; set; }  
    public string Number { get; set; }  
    public string Street { get; set; }  
    public string City { get; set; }  
}  

public partial class Person  
{  
    public int Id { get; set; }  
    public string FirstName { get; set; }  
    public string LastName { get; set; }  
    public DateTime? DateOfBirth { get; set; }  
    public IList<Address> Addresses { get; set; }     
}

Singleton

public sealed class Sample  
{  
    private static readonly Lazy<Sample> Lazy =  
        new Lazy<Sample>(() =>  
            new Sample());  
  
    /// <summary>  
    /// Entry point to access information in this class  
    /// </summary>  
    public static Sample Instance => Lazy.Value;  
    public List<Person> People { get; set; }  
  
    private Sample()  
    {  
        People = new List<Person>();  
    }  
}

Add in one thread

Sample.Instance.People.Add(new Person() {FirstName = "Karen", LastName = "Payne"});

Read in another thread (or add etc if you want)

foreach (var person in Sample.Instance.People)  
{  
    Console.WriteLine($"{person.FirstName} {person.LastName}");  
}

Markus Freitag 3,101 Reputation points

2020-12-10T13:42:55.633+00:00
private static readonly Lazy<Sample> Lazy =

new Lazy<Sample>(() => new Sample());

Have you considered using a thread-safe Singleton?

No. Is difficult for understanding your example. Can you show an object add, delete? Lazy is new for me. When do I use this?

Best with my example.

[XmlRoot("PACKAGELABEL")] public class PackageLabel { [XmlAttribute("quantityPackage")] public int QuantityPackage { get; set; } [XmlAttribute("batchNumber")] public string BatchNumber { get; set; }

If the class thread safe, so is the list of this class also thread safe?
Karen Payne MVP 30,441 Reputation points

2020-12-10T15:10:40.79+00:00

@Markus Freitag I provided about as simple as you can get, Lazy is described here. If you have not read this you need to as me writing up what is written there serves nobody.

Since I exposed a list you have methods and properties which are always available without a singleton.
Markus Freitag 3,101 Reputation points

2020-12-10T15:48:16.317+00:00
static object LockPackageListAdd = new object(); static object LockPackageListDelete = new object(); //or ist one object enough?

You're right, I haven't read it in detail yet. It just looks complicated. If I would only go via Lock, do I need one Lock or two Lock objects? Can you answer that please?
Thanks!

For what?

Add

Delete items
Karen Payne MVP 30,441 Reputation points

2020-12-10T16:13:37.04+00:00

Never used locks like this so I can't say, only have used singleton and classes under System.Collections.Concurrent. Best guess is you need a lock on both and test for the possibilities for deadlocks. In any case I'd refactor as I've shown or similar.
Markus Freitag 3,101 Reputation points

2020-12-11T16:55:28.26+00:00

Yes you are locking a single object, not the list I think.
Maybe you have another good example.
Sorry for asking again. I'm just looking for a good and workable solution.

I hope you understand what I want. Do you?

C# List - list can be filled or taken from two threads

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