Use a referenced object across threads

Question

What I am doing is creating an object(A) that holds a reference to another object(B). The UI portion of my code holds those objects(A) in a BindingList that is used as the data source for a DevExpress grid view. The controller sends the newly created objects(A) to the UI via an event. The controller also has a thread that updates the referenced object(B). The exception thrown comes from the DevExpress GridView and reads "Cross thread operation detected. To suppress this exception, set DevExpress.Data.CurrencyDataController.DisableThreadingProblemsDetection = true".

Now I don't want to suppress this exception because the code will eventually end up in a critical application.

So how can I update a reference object across threads without causing problems? Here is the code from my Test application. It will be essentially the same in the actual program.

UPDATE The error in the UI was fixed by the answer from Nicholas Butler but now the exception has moved into the Employee class. I have updated the code to reflect the changes.

Here is my code

*UI *

    public partial class Form1 : Form
{
    private BindingList<IEmployee> empList;
    EmployeeController controller;
    private delegate void AddEmployeInvoke(IEmployee employee);
    public Form1()
    {
        controller = new EmployeeController();
        controller.onNewEmployee += new EmployeeController.NewEmployee(controller_onNewEmployee);
        empList = new BindingList<IEmployee>();
        InitializeComponent();
    }

    void controller_onNewEmployee(IEmployee emp)
    {
        AddEmployee(emp);
    }

    private void AddEmployee(IEmployee empl)
    {
        if (InvokeRequired)
        {
            this.Invoke(new AddEmployeInvoke(AddEmployee), new Object[] {empl});
        }
        else
        {
             empList.Add(empl);
        }
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        this.gridControl1.DataSource = empList;
        this.gridControl1.RefreshDataSource();
        controller.Start();
    }
 }

Controller:

    class EmployeeController
{
    List<IEmployee> emps;
    Task empUpdater;
    CancellationToken cancelToken;
    CancellationTokenSource tokenSource;
    Pay payScale1;
    Pay payScale2;

    public EmployeeController()
    {
        payScale1 = new Pay(12.00, 10.00);
        payScale2 = new Pay(14.00, 11.00);
        emps = new List<IEmployee>();
    }

    public void Start()
    {
        empUpdater = new Task(AddEmployee, cancelToken);
        tokenSource = new CancellationTokenSource();
        cancelToken = tokenSource.Token;
        empUpdater.Start();
    }

    public bool Stop()
    {
        tokenSource.Cancel();
        while (!empUpdater.IsCompleted)
        { }
        return true;
    }

    private void AddEmployee()
    {
        IEmployee emp = new Employee("steve", ref payScale1);
        ThrowEmployeeEvent(emp);
        emps.Add(emp);
        emp = new Employee("bob", ref payScale2);
        ThrowEmployeeEvent(emp);
        emps.Add(emp);
        int x = 0;

        while (!cancelToken.IsCancellationRequested)
        {
            emp = new Employee("Emp" + x, ref payScale1);
            ThrowEmployeeEvent(emp);
            x++;
            emp = new Employee("Emp" + x, ref payScale2);
            ThrowEmployeeEvent(emp);

            Thread.Sleep(1000);

            payScale2.UpdatePay(10.0);
            payScale1.UpdatePay(11.0);

            Thread.Sleep(5000);
        }
    }

    private void ThrowEmployeeEvent(IEmployee emp)
    {
        if (onNewEmployee != null)
            onNewEmployee(emp);
    }

    public delegate void NewEmployee(IEmployee emp);
    public event NewEmployee onNewEmployee;
}

Employee Class: (Exception thrown in this class)

    class Employee : IEmployee
{
    private string _name;
    private double _salary;
    private Pay _myPay;
    public string Name 
    { 
        get { return _name; } 
        set { _name = value; 
            //if (PropertyChanged != null) this.PropertyChanged(this, new PropertyChangedEventArgs("Name")); 
            } 
    }        
    public double Salary
    {
        get { return _salary; }
    }
    int x = 1;

    public Employee(string name, ref Pay pay)
    {
        _myPay = pay;
       _myPay.PropertyChanged += new PropertyChangedEventHandler(_myPay_PropertyChanged);
       _salary = _myPay.Salary;
        Name = name;
    }

    void _myPay_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "Salary")
        {
            _salary = _myPay.Salary;
            if (this.PropertyChanged != null)
                // exception thrown on the line below
                this.PropertyChanged(this, new PropertyChangedEventArgs("Salary"));
        }
    }

    public void ChangeName()
    {
        Name = "Me " + x;
        x++;
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

Employee Interface:

    interface IEmployee : INotifyPropertyChanged
{
    string Name { get; set; }
    double Salary { get;}
}

Pay Class:

    class Pay : INotifyPropertyChanged
{
    private double _salary;
    private double _bonus;
    public double Salary { get { return _salary; } set { _salary = value; if(PropertyChanged != null) this.PropertyChanged(this, new PropertyChangedEventArgs("Salary"));} }
    public double Bonus { get { return _bonus; } set { _bonus = value; if (PropertyChanged != null) this.PropertyChanged(this, new PropertyChangedEventArgs("Bonus")); } }

    public Pay(double salary, double bonus)
    {
        Salary = salary;
        Bonus = bonus;
    }

    public void UpdatePay(double salary)
    {
        Salary += salary;
        if (onChange != null)
            this.onChange();
    }

    public void UpdatePay(double salary, double bonus)
    {
        Salary += salary;
        Bonus += bonus;

        if (onChange != null)
            this.onChange();
    }

    public delegate void Change();
    public event Change onChange;

    public event PropertyChangedEventHandler PropertyChanged;
}

I greatly appreciate any help.

Answer 1

The problem is that EmployeeController.onNewEmployee is being fired on a non UI thread. Use the event based async pattern to raise the events on a specific (in this case the UI) thread: http://msdn.microsoft.com/en-us/library/hkasytyf.aspx.

Alternatively you can check IsInvokeRequired in each event handler and if so use .Invoke to get back onto the UI thread.. This is more cumbersome but may in your case be easier/quicker to implement.

Answer 2

You are calling empList.Add(empl); even when InvokeRequired == true. Try:

private void AddEmployee(IEmployee empl)
{
    if (InvokeRequired)
    {
        this.Invoke(new AddEmployeInvoke(AddEmployee), new Object[] {empl});
    }
    else
    {
        empList.Add(empl); //exception thrown here
    }
}

---

You also need to raise your INotifyPropertyChanged events on the UI thread, but you do not have a UI Control to call Invoke on. The easy way to do this is to store a reference to your main form and make it public static:

public partial class Form1 : Form
{
    public static Control UI { get; private set; }

    public Form1()
    {
        UI = this;
    }
}

You can then use Form1.UI.InvokeRequired and Form1.UI.Invoke from anywhere in your app.

---

I was trying to take one step at a time, but if you want a more correct solution, you can pass the UI SynchronizationContext to the controller and use its Post or Send methods:

public Form1()
{
    controller = new EmployeeController( SynchronizationContext.Current );
    ...

class EmployeeController
{
    private SynchronizationContext _SynchronizationContext = null;

    public EmployeeController( SynchronizationContext sc )
    {
        _SynchronizationContext = sc;
        ...

You then have to get it to your objects. To raise an event you would then do this:

var evt = this.PropertyChanged;
if ( evt != null ) sc.Send(
    new SendOrPostCallback( state => evt( this, ...EventArgs... ) ),
    null );