DefaultIfEmpty

int[] arr1 = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };//构造带元素的数组
int[] arr2 = { }; //构造一个空数组

var query1 = arr1.DefaultIfEmpty(); //DefaultIfEmpty方法判断元素是否为空
var query2 = arr2.DefaultIfEmpty(-1); //DefaultIfEmpty方法判断如果元素为空返回-1
//显示查询结果
foreach (var item in query1)
{
Response.Write(item + " , ");//结果query1={1,2,3,4,5,6,7,8,9}
}
Response.Write("<br/>");
foreach (var item in query2)
{
Response.Write(item + " , ");// 结果query2={-1}
}