使用LINQ中的select new返回列表

Question

这是我的方法,它给了我错误.

public List<Project> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
        var query = from pro in db.Projects
                    select new { pro.ProjectName, pro.ProjectId };

        return query.ToList();
    }
}

如果我改变它:

public List<Project> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
        var query = from pro in db.Projects
                    select pro;

        return query.ToList();
    }
}

然后它工作正常没有错误.

能告诉我,我怎样才能返回ProjectId和ProjectName.

谢谢.

Answer 1

方法不能返回匿名类型.它必须与方法返回类型中定义的类型相同.检查GetProjectForCombo的签名并查看您指定的返回类型.

创建一个具有必需属性的ProjectInfo类,然后在新表达式中创建ProjectInfo类型的对象.

class ProjectInfo
{
   public string Name {get; set; }
   public long Id {get; set; }
}

public List<ProjectInfo> GetProjectForCombo()
{
    using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
    {
        var query = from pro in db.Projects
                    select new ProjectInfo(){ Name = pro.ProjectName, Id = pro.ProjectId };

        return query.ToList();
    }
}

Answer 2

public List<Object> GetProjectForCombo()
{
   using (MyDataContext db = new MyDataContext (DBHelper.GetConnectionString()))
   {
     var query = db.Project
     .Select<IEnumerable<something>,ProjectInfo>(p=>
                 return new ProjectInfo{Name=p.ProjectName, Id=p.ProjectId);       

     return query.ToList<Object>();
   }

}

Answer 3

你不能从类中返回匿名类型...(嗯,你可以,但你必须先将它们转换为对象,然后再使用另一侧的反射来重新获取数据)所以你必须创建一个小类要包含的数据.

class ProjectNameAndId
{
    public string Name { get; set; }
    public string Id { get; set; }
}

然后在你的LINQ语句中:

select new ProjectNameAndId { Name = pro.ProjectName, Id = pro.ProjectId };