Question

我在跟踪LINQ语句时遇到错误

错误

＆＃34;无法将隐式转换为bool类型转换为字符串

string UserId = Context.Users.Where(x => x.UserName == username).FirstOrDefault(x => x.Id);

这里有什么问题？

Answer 1

如果您只想这样的身份

string UserId = Context.Users.Where(x => x.UserName == username).Select(g=> g.Id.ToString()).FirstOrDefault();

如果您希望用户像这样对象

   var Usersobj=Context.Users.Where(x => x.UserName == username).FirstOrDefault();

Answer 2

您需要检查用户是否存在提供的用户名，否则将引发空引用异常;

string id = string.Empty;
User user = Context.Users.FirstOrDefault(x => x.UserName == username);
if(user != null)
{
    id=user.Id.ToString(); //if id is already a string no need for ToString()
}

或c＃6的单行;

string userId = Context.Users.FirstOrDefault(x => x.UserName == username)?.Id.ToString();

Answer 3

我会用：

string UserId = Context.Users.Where(x => x.UserName == username).Select(s => s.Id).DefaultIfEmpty(String.Empty).FirstOrDefault();

Answer 4

x =＆gt; x.Id用于枚举集合并应用条件来检查哪一个匹配，然后返回第一个匹配。如果FirstOrDefault<Tsource>为空，则<Tsource>为源中的第一个元素。这里FirstOrDefault仅通过传递id，所以使用x =＆gt; x.Id == id

var user =  Context.Users.Where(x => x.UserName == Username ).FirstOrDefault(x=>x.Id==id)

或

string UserId = Context.Users.Where(x => x.UserName == Username ).FirstOrDefault(x=>x.Id==id).ToString();

linq语句出错

4 个答案: