DefaultIfEmpty

int[] arr1 = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };//構造帶元素的數組
int[] arr2 = { }; //構造一個空數組

var query1 = arr1.DefaultIfEmpty(); //DefaultIfEmpty方法判斷元素是否為空
var query2 = arr2.DefaultIfEmpty(-1); //DefaultIfEmpty方法判斷如果元素為空返回-1
//顯示查詢結果
foreach (var item in query1)
{
Response.Write(item + " , ");//結果query1={1,2,3,4,5,6,7,8,9}
}
Response.Write("<br/>");
foreach (var item in query2)
{
Response.Write(item + " , ");// 結果query2={-1}
}