Linq 通过将 Id 与具有 id 的对象列表进行比较来获取列表

Question

I have the following code:

var translations = _context.Translations
       Where(t => t.LineId == lines.Id)

I got a variable named lines which is of type List<Line> every Line object has Line.Id

Now I have another table named Translations I would like to get all Translations which have Line.Id that is equal to every Line.Id from the lines list.

How can I accomplish this in a single LINQ expression?

I need something like this:

 var translations = new List<Translation>();

 foreach (var line in lines)
 {
    var translation =
    _context.Translations.FirstOrDefault(t => t.LineId == line.Id);
    translations.Add(translation);
 }

Answer 1

Project an IEnumerable<T> of your Line.Id property, then use the result in your query with the Contains method:

var lineIDs = lines.Select(l => l.Id).ToArray();

var translations = _context.Translations
    .Where(t => lineIDs.Contains(t.LineId))
    .ToList();

Answer 2

Since you want only those translations that have IDs that exist in lines , this will get you what you want:

var translations = from translation in _context.Translations
                   join line in lines on translation.LineID equals line.LineID
                   select translation;

A join will be faster than invoking a LINQ query for each item in the Translations "table."