Question

I am trying to read value from a listbox item that is on another thread.

I tried to make a new method to run the invoke command, I can manage to send a command to the listbox like add via the invoke method but i cant seem to get a response, i cant seem to get the value of the item, i have tried a few ways, once i change it from a void to a string things start to get hairy…

  thread t1 = new thread(thethread)
    t1.start()

    public void thethread()
    {
    string text = readListBoxSelected(listBox1) + " lala" ;
    }



    public static string readListBoxSelected(ListBox listbox)
    {
        if (listbox.InvokeRequired)
        {
            return (string)listbox.Invoke(
              new Func<String>(() => readListBoxSelected(listbox))
            );
        }
        else
        {

            string varText = listbox.SelectedValue.ToString();
            return varText;
        }
        }

Above is a example of what i am trying to do.

Here is the error:

System.NullReferenceException was
unhandled by user code
Message=Object reference not set to an
instance of an object.
Source=** StackTrace:
at **.Form1.readListBoxSelected(ListBox listbox) in e:\documents and
settings\scott\my documents\visual
studio
2010\Projects*****\Form1.cs:line
133
at ***.Form1.<>c_DisplayClass5.b_3()
in e:\documents and settings\scott\my
documents\visual studio
2010\Projects******\Form1.cs:line
127 InnerException:

I imagine what is wrong is exactly what it says “Object reference not set to an instance of an object”……. All my variables seem to be declared as fair as i am aware, how can i correct this??

I get the feeling i am going about the entire thing wrongly…. 0_o
Thanks in Advance,
Scott

Answer 1

Try This

public static string readListBoxSelected(ListBox listbox)
    {
        if (listbox.InvokeRequired)
        {
            return (string)listbox.Invoke(
              new Func<String>(() => readListBoxSelected(listbox))
            );
        }
        else
        {
if(istbox.SelectedValue != null)

            return  listbox.SelectedValue.ToString();
else
return String.Empty
        }
        }