无法转换类型&#39;System.Linq.IQueryable <int> &#39;到&#39;int&#39;

Question

I have a problem sending data from the database to a list in the Controller. I am new to C# and MVC so any help will be useful! Code follows

public static List<FilterTree> GetAllUsers()
    {
        FilterTreeDBContext db = new FilterTreeDBContext();
        var userList = new List<FilterTree>();
        var device =  new FilterTree();
        //This is the line where I get the error
        device.ID = from a in db.FilterTree select a.ID;

        userList.Add(device);
        return userList;
    }

Thanks & Happy Holidays!! :)

Answer 1

device.ID = (from a in db.FilterTree select a.ID).First();

Linq query is lazy, and executes only after you request the value

BTW don't forgot to close the context, otherwise you will leak connections

using (var db = new FilterTreeDBContext())
{
    ...
    return userList;
}

Answer 2

至于框架知道查询可能有多个对象，所以像这样使用它：

device.ID = (from a in db.FilterTree select a.ID).FirstOrDefault();