Hello Joe,

This is not a built-in functionality in the XamDataGrid. If you want to have Checked,Unchecked and Undeterminate states, you would have to handle the checked/unchecked events of the CheckBox in the header and set the values manually in the cells below it.

Another way would be to create a one-way binding between the checkboxes in the cells and the one in the header. You would not be able to do that directly, so you could use an attached property or the Tag property.