Hi Ghanshyam,
In your 'Select All' pushbutton callback, you can add the following code to select all elements currently displayed in your listbox:
sizeOfListBox = numel(get(handles.listbox1, 'String'));
set(handles.listbox1,'value',1:sizeOfListBox);
You can replace 'listbox1' with the Tag of your custom listbox.
Note: You will also need to adjust the 'Max' and 'Min' properties of the Listbox so that multiple item selection is allowed. 'Max' must be at least 2 more than 'Min'.
Best Answer