如何在xamarin上为ios创建自定义警报视图(How to create custom alert view for ios on xamarin)

 你的循环错了 - $ columns ['EGGS']不存在： 
 
$columns = array('EGGS', 'SALAD', 'TREES', 'REVISED');
foreach($columns as $column) {
    $$column = $columns[$column] ? 'YES' : 'NO';
}
 
 它应该是： 
 
$columns = array('EGGS', 'SALAD', 'TREES', 'REVISED');
foreach($columns as $column) {
    $$column = $$column ? 'YES' : 'NO';
}
 
 或更好： 
 
$tmp = array();
$columns = array('EGGS', 'SALAD', 'TREES', 'REVISED');
foreach($columns as $column) {
    $tmp[$column] = $$column ? 'YES' : 'NO';
}
 
 这样你就不会覆盖你的绑定变量。 
 
 还要注意，出于性能方面的原因，您应该在while()循环之外移动该常量数组声明。 

Your loop is wrong - $columns['EGGS'] doesn't exist:
 
$columns = array('EGGS', 'SALAD', 'TREES', 'REVISED');
foreach($columns as $column) {
    $$column = $columns[$column] ? 'YES' : 'NO';
}
 
it should be:
 
$columns = array('EGGS', 'SALAD', 'TREES', 'REVISED');
foreach($columns as $column) {
    $$column = $$column ? 'YES' : 'NO';
}
 
or better still:
 
$tmp = array();
$columns = array('EGGS', 'SALAD', 'TREES', 'REVISED');
foreach($columns as $column) {
    $tmp[$column] = $$column ? 'YES' : 'NO';
}
 
so that you're not over-writing your bound variables.
 
also note that you should move that constant array declaration outside of your while() loop for performance reasons.