Extremely sorry the below code also works fine
sid = ""
For Each st As String In ListBox1.SelectedItems
If sid = "" Then
sid = st
Else
sid += st
End If
Next
MessageBox.Show(sid)
problem is i have set the datasource and displaymember property of ListBox in that case for and for-each loop don't work, i don't know why, if some one have any idea please share with us.