WinForms FAQ - CheckedListBox

To check all items, you can use code such as:

C#
for( int i=0 ; i < myCheckedListBox.Items.Count; i++ ) 
{ 
   myCheckedListBox.SetItemChecked(myCheckedListBox.Items[i], true); 
}

VB.NET
For i As Integer = 0 To myCheckedListBox.Items.Count - 1 
    myCheckedListBox.SetItemChecked(myCheckedListBox.Items(i), True) 
Next