BackgroundWorker with anonymous methods?

Question

I'm gonna create a BackgroundWorker with an anonymous method.
I've written the following code :

BackgroundWorker bgw = new BackgroundWorker();
bgw.DoWork += new DoWorkEventHandler(
    () =>
    {
        int i = 0;
        foreach (var item in query2)
        {
            ....
            ....
        }
    }
);

But Delegate 'System.ComponentModel.DoWorkEventHandler' does not take '0' arguments and I have to pass two objects to the anonymous method : object sender, DoWorkEventArgs e

Could you please guide me, how I can do it ? Thanks.

Answer 1

You just need to add parameters to the anonymous function:

bgw.DoWork += (sender, e) => { ... }

Or if you don't care about the parameters you can just:

bgw.DoWork += delegate { ... }

Answer 2

If you specify a lambda, you must ensure it takes the same number of arguments:

bgw.DoWork += (s, e) => ...;

But if you're not using the arguments, you could just use an anonymous delegate without parameters:

bgw.DoWork += delegate
{
    ...
};

Answer 3

If you have written the above without lambdas how it would be?

backgroundWorker1.DoWork += 
                new DoWorkEventHandler(backgroundWorker1_DoWork);

and the named method:

private void backgroundWorker1_DoWork(object sender, 
        DoWorkEventArgs e)
    {   
        // Get the BackgroundWorker that raised this event.
        BackgroundWorker worker = sender as BackgroundWorker;

        // Assign the result of the computation
        // to the Result property of the DoWorkEventArgs
        // object. This is will be available to the 
        // RunWorkerCompleted eventhandler.
        e.Result = ComputeFibonacci((int)e.Argument, worker, e);
    }

But now you are using lambdas with no bound variables ()=> You should provide two objects sender and e (which they will get type inferred later).

backgroundWorker1.DoWork += (sender, e) => ...

Answer 4

Lets make it simple

Lambda expressions are really handy to make the code shorter and more readable. However entry level programmers might find it a bit difficult to deal with. There are three separate concepts one should go through: anonymous methods, delegates and lambda expressions. A detailed walk-through of each of them is beyond the scope of this answer. I hope that the code example given below will serve the purpose of giving a quick view of the different approaches available.

class TestBed
{
    BackgroundWorker bgw = new BackgroundWorker();
    void sample()
    {            
        //approach #1
        bgw.DoWork += new DoWorkEventHandler(bgw_DoWork);
        //DoWorkEventHandler is nothing but a readily available delegate written by smart Microsoft guys

        //approach #2, to make it a little shorter
        bgw.DoWork += (s,e) => 
        {
            //...
        };
        //this is called lambda expression (see the => symbol)

        //approach #3, if lambda scares you
        bgw.DoWork += delegate 
        { 
            //... (but you can't have parameters in this approach
        };

        //approach #4, have a helper method to prepare the background worker
        prepareBgw((s,e)=>
        {
            //...
        }
        );

        //approach #5, helper along with a simple delegate, but no params possible
        prepareBgw(delegate 
        {
            //...
        }
        );

        //approach #6, helper along with passing the methodname as a delegate
        prepareBgw(bgw_DoWork);

        //approach #7, helper method applied on approach #1
        prepareBgw(new DoWorkEventHandler(bgw_DoWork));

    }

    void bgw_DoWork(object sender, DoWorkEventArgs e)
    {
        //...
    }
    void prepareBgw(DoWorkEventHandler doWork)
    {
        bgw.DoWork+= doWork;
    }
}

Note that we used "delegate" and not "Delegate" in this example (there is a difference between the two)