LINQ to SQL实体到对象的转换

Question

如您所知,LINQ to SQL生成自己的实体.我尝试从数据库中获取数据并将其放在我自己的实体中,我创建的类.

当我这样做时:

        AppData.MyDBDataContext context = new AppData.MyDBDataContext();
        List<User> users = (from user in context.Users.Select(x => new User
        {
            Id = x.Id,
            Name = x.Name,
            Password = x.Password
        }) where user.Password == "123456" select user).ToList();

它有效(User是我创建的一个类).但是当我尝试概括和构建一个强制转换函数时:

User castToUser (DataAccess.AppData.User linqEntity)
{
    return new User
    {
        Id = linqEntity.Id,
        Name = linqEntity.Name,
        Password = linqEntity.Password
    };
}

(DataAccess.AppData.User是LINQ to SQL生成的实体).

    AppData.MyDBDataContext context = new AppData.MyDBDataContext();
    List<User> users = (from user in context.Users.Select(x => castToUser(x)) where user.Password == "123456" select user).ToList();

它抛出一个错误: Method 'DataAccess.Entities.User castToUser(DataAccess.AppData.User)' has no supported translation to SQL.

错误仅在更改时出现

new User
            {
                Id = x.Id,
                Name = x.Name,
                Password = x.Password
            }

to castToUser(x),返回相同的东西!

Answer 1

这是因为l2sql试图提出您的投影的翻译.相反,只需在检索数据后进行投影即可:

AppData.MyDBDataContext context = new AppData.MyDBDataContext();
List<User> users = context.Users
   .Where( u => u.Password == "123456" )
   .ToList() // fetch
   .Select( u => castToUser( u ));

建议 - 不要混合使用linq样式和扩展方法样式这两种样式.这降低了可读性.