c# - 使用 LINQ 在 C# 中的基于服务的数据库中搜索值是否存在

Question

我制作了一个文本框供用户输入id，我想看看它是否存在于我的表中

HotelDatabaseDataContext db = new HotelDatabaseDataContext();//database 

customer_id = int.Parse(textBox1.Text);//value from textbox
var selected = from x in db.Customers
               where x.Id == customer_id//selecting value from database
               select x; 

/* I want to see if "selected" is null or not and want to store it's value in another variable */
if(selected.id==null)
{
    int _id = selected.id;
}

Answer 1

linq 查询的结果是 IEnumerable/IQueryable。如果您想检查您是否拥有该记录，请使用 FirstOrDefault:

var item = selected.FirstOrDefault();
if(item != null)  
{
    int id = item.id;
}

//or:
int id = selected.FirstOrDefault()?.id;