Accessing Control.Focused property from Thread

Sukhjinder_K

Hi I need to get the Focused Property of My ListBox list from within a thread. The thread makes calls to following function bool listFocused() { if (this.list.InvokeRequired) { ListFocusCallBack d = new ListFocusCallBack(listFocused); Object obj = this.list.Invoke(d, new object[] {}); // ProblemLine // Should Return something here } else { return this.list.Focused; } } If I compile the above then I get a "not all code paths return a value" error, because I'm not returning anything in "ProblemLine" I tried replacing "ProblemLine" with if( obj.Equals(Boolean.TrueString )) return true; return false; but I'm not sure if it is the right approach. Please Advice Thanks...

Martin 0

Hello, You don't need the else statement in your method. If invoke is called the code after that will not been worked threw. Set a breakpoint at the first line of the method and Debug it to see it better. Your method could look like that: bool listFocused() { if(this.list.InvokeRequired) { ListFocusCallBack d = new ListFocusCallBack(listFocused); this.list.Invoke(d, new object[] {}); } return this.list.Focused; } -- modified at 8:07 Wednesday 21st November, 2007 Wrong Information Sorry

All the best, Martin

Stevo Z

To my knowledge, it's not thread safe this way, let me explain :

bool listFocused()
{
// lets say this method is called from thread [T1], different from main application thread [T0] (thread that that listbox was created on)

if(this.list.InvokeRequired) // if this condition is true, we are on \[T1\]
{
    ListFocusCallBack d = new ListFocusCallBack(listFocused); // still \[T1\]
    this.list.Invoke(d, new object\[\] {}); // 'd' executes on \[T0\] !! \[T1\] is blocked until Invoke finishes

   // now we're back at \[T1\] !!
}

return this.list.Focused; // !!! again \[T1\] - if u invoke a delegate using control invoke, it doesn't mean
                          // that the rest of this method will be executed on that invoked thread. it
                          // remains on thread \[T1\], where it's not safe to get the Focused value.

}

what you need to do is to return value that Control.Invoke method returns

bool listFocused()
{
// [T1]

if(this.list.InvokeRequired) // if on other than \[T0\]
{
     ListFocusCallBack d = new ListFocusCallBack(listFocused);
     return (bool)this.list.Invoke(d, new object\[\] {}); // wait for Invoked method to finish and return result (again on \[T1\])       
}

else // \[T0\] now we are sure this is thread safe 
{
    return this.list.Focused; 
}

}

zilo

Martin 0

Hello Zilo, I do not agree with you.

Zilo(svk) wrote:

return this.list.Focused; // !!! again [T1] - if u invoke a delegate using control invoke, it doesn't mean // that the rest of this method will be executed on that invoked thread. it // remains on thread [T1], where it's not safe to get the Focused value.

This code will never be executed by [T1] AFAIK. After the Invoke call, the method will no longer go threw the rest of the method code. I often debugged such a situation, and hope what I just told is not complete rubbish! -- modified at 8:09 Wednesday 21st November, 2007 I wrote: hope what I just told is not complete rubbish It was rubbish!

All the best, Martin

Stevo Z

Test it. It really is the way I wrote. I mean I don't see any reason why Control.Invoke(delegate Method); should for any reason make calling method to return whatever the delegate returns. It does just doesn't make any sense. Execution continues. It just blocks until delegate completed.

zilo

Martin 0

Hello Zilo, Just testet it, and have to say THANK YOU SO MUCH for pointing that out. First of all, I was not consistend in my Projects. And second, I was completely wrong. Thats what I like at CP. By answering (or trying to do so at least), you also get support! Got my '5'. Thanks again!

All the best, Martin

Stevo Z

np, It's good to know that you know something :)) . I wasn't sure either about this one.

zilo

Sukhjinder_K

I was also thinking on similar lines what I actually did was if (this.list.InvokeRequired) { ListFocusCallBack d = new ListFocusCallBack(listFocused); Object obj = this.list.Invoke(d, new object[] {}); if( obj.Equals(Boolean.TrueString )) return true; return false; } else { return this.list.Focused; } Your solution about converting Object to bool as (bool)obj never came to my mind. I never knew it was possible. Thanks... The Show Ain't Over Yet...