Hi Ei Shwe,
Note: Since the version information is not mentioned, we have provided response for 11.2 version. If this is not your version, please revert back to us with the Version informations.
We have done a simple to meet your requirements. Kindly download the sample from the below link.
In the sample, we have checked all the items of the ComboBoxAdv when the ShowAll is checked in the Selected event of the ShowAll item.
And likewise, in the Unselected event, we have unchecked all the items.
Query #2:
You could find the Dashboard sample in the below location in your system,
C:\Users\UserName\AppData\Local\Syncfusion\EssentialStudio\11.2.0.25\WPF\Tools.WPF\Samples\ComboBox\ComboBox Demo
In the above mentioned Demo, you can find the SelectionChangedCommand, and if you set SelectedIndex, it will be selected at application startup.
Please let us know if you have any queries.
Regards,
Karuppasamy P
ComboBox_873e789b.zip