public class CustomCheckBoxHeaderCellElement : GridCheckBoxHeaderCellElement
{
    public CustomCheckBoxHeaderCellElement(GridViewColumn column, GridRowElement row) : base(column, row)
    {
    }

    protected override void SetTwoStateCheckBoxState()
    {
        List<GridViewRowInfo> rows = new List<GridViewRowInfo>();
        MethodInfo mi = typeof(GridCheckBoxHeaderCellElement).GetTypeInfo().GetMethod("GetRowsToIterateOver", BindingFlags.Instance | BindingFlags.NonPublic);
        rows = mi.Invoke(this, null) as List<GridViewRowInfo>;

        bool? foundChecked = null;
        bool? foundUnchecked = null;
        bool? foundIndeterminate = null;

        bool isStateEvaluated = this.EvaluateCheckBoxState(rows, ref foundChecked, ref foundUnchecked, ref foundIndeterminate);
        if (!isStateEvaluated)
        {
            if (foundChecked.HasValue && !foundUnchecked.HasValue && !foundIndeterminate.HasValue)
            {
                this.SetCheckBoxState(ToggleState.On);
            }
            else
            {
                this.SetCheckBoxState(ToggleState.Off);
            }
        }
    }
}