asked on

Invoke() and BeginInvoke() questions

I have the following codes.
private void UpdateUI(MethodInvoker del)
{
if(this.InvokeRequired)
this.Invoke(del);
else
del.Invoke();
}
public void AddGreeting(string str)
{
MethodInvoker del = new MethodInvoker(delegate
{
this.richtextbx.AppendText(str);
}
);
UpdateUI(del);
}

1. If I understand correctly, this.Invoke(del); is like
this.Invoke((MethodInvoker)(delegate()
{
this.richtextbx.AppendText(str);
}
correct?

2. Then, what does del.Invoke() do? Why do I need this line of code?
3. I thought that MethodInvoker can be used only for the method that has void type and has no parameter. But AddGreeting method has a string type parameter. How come this is working?
4. I heard that BeginInvoke() is better than Invoke(), so I was going to replace Invoke() wit BeginInvoke(), but I am getting errors on both 1) and 2) when I tried to use BeginInvoke(). Why is that?
private void UpdateUI(MethodInvoker del)
{
if(this.InvokeRequired)
this.BeginInvoke(del);----1)
else
del.BeginInvoke();-----2)
}

ASKER CERTIFIED SOLUTION

Aruspex

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IzzyTwinkly

ASKER

Hi Aruspex,

Thank u so much for your help.
i) so for the second question, I don't need the following part in UpdateUI method, right?
else
del.Invoke();

ii) U said that MethodInvoker is executing the delegate. I don't really understand what msdn tells about MethodInvoker: 'MethodInvoker provides a simple delegate that is used to invoke a method with a void parameter list. ' I am using anonymous method here. what does it mean by 'a void parameter list' here?

iii) Lastly if I use BeginInvoke() in 2), I am getting an error saying that no overload method 'BeginInvoke()' takes 0 parameter.

When a button is clicked, the following codes are executed for the checked items in myArrayList.
.....
for (int i = 0; i < myArrayList.Count; i++)
{
// If the checked item is not existing
if (CheckMyItemExists(myArrayList[i].ToString()) == false)
{
MessageBox.Show(myArrayList[i].ToString() + " is not in the List.");
del = new MethodInvoker(delegate
{
this.ProgressBar1.Value = (i + 1) * 100 / myArrayList.Count;---------------3)
}
);
UpdateUI(del);
continue;
}
....//Code for existing checked items
And if I use BeginInvoke() in 1), I am getting "System.DivideByZeroException" on 3)
Could u plz tell me y?

Aruspex

Hello,

i) You should leave it in so the delegate will execute if .InvokeRequired() returns false

ii) By no parameters it means a method like Save() instead of Save(true), with true being a parameter.

iii) I would just stick to invoke, as I mentioned before BeginInvoke is async and can be more complex to use correctly. You would need to create a callback method to pass to del.BeginInvoke() to fix that error

iv)
maybe try something like;

       
if (CheckMyItemExists(myArrayList[i].ToString()) == false)
{
   MessageBox.Show(myArrayList[i].ToString() + " is not in the List.");
    int itemCount = myArrayList.Count;
 
    del = new MethodInvoker(delegate
    {
        if (itemCount != 0)
        {
            this.ProgressBar1.Value = (i + 1) * 100 / itemCount;
        }
        else
        {
            //code to handle 0 item count
        }
     }
 
    UpdateUI(del);
    continue;

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IzzyTwinkly

ASKER

Thank u very much Aruspex