C＃中的LINQ中的Foreach循环

Question

I would like to replace the foreach loop in the following code with LINQ ForEach() Expression:

    List<int> idList = new List<int>() { 1, 2, 3 };

    IEnumerable<string> nameList = new List<string>();

    foreach (int id in idList)
    {
       var Name = db.Books.Where(x => x.BookId == id).Select(x => x.BookName);
        nameList.Add(Name);
    }

Any Help Please!!

Answer 1

Your code doesn't quite work (you're adding an IEnumerable<string> to a List<string> ). You also won't need ForEach , since you're constructing the list:

You can do this:

var nameList = idList.SelectMany(id => db.Books.Where(x => x.BookId == id)
                     .Select(x => x.BookName)).ToList();

But then you're hitting the database for each ID. You can grab all the books at once with :

var nameList = db.Books.Where(b => idList.Contains(b.BookId))
                       .Select(b => b.BookName).ToList();

Which will only hit the database once.

Answer 2

Why not a select?

List<int> idList = new List<int>() { 1, 2, 3 };

List<string> nameList = idList
    .Select(id => db.Books.Where(x => x.BookId == id).Select(x => x.BookName))
    .ToList();

Or better yet: refactorise and select...

int[] idList = new int[] { 1, 2, 3 };

List<string> nameList = db.Books
    .Where(x => idList.Contains(x.BookId))
    .Select(x => x.BookName))
    .ToList();

Answer 3

    nameList.AddRange(
             db.Books.Where(x => idList.Contains(x.BookId))
                     .Select(x => x.BookName)
                     .ToList());

This will generate an IN statement in the SQL, thereby only doing a single select.

One thing to be aware of is the performance of IN degrades as the set (idList in this case) gets bigger. In the case of a large set, you can batch the set and do multiple queries:

int start = 0;
int batch = 1000;
while (start < idList.Count())
{
  var batchSet = idList.Skip(start).Take(batch);
  nameList.AddRange(
             db.Books.Where(x => batchSet.Contains(x.BookId))
                     .Select(x => x.BookName)
                     .ToList());
  start += batch;
}

Answer 4

To answer your specific question, you can do this:

List<int> idList = new List<int>() { 1, 2, 3 };

List<string> nameList = new List<string>();

idList.ForEach(id => {
    var Name = db.Books.Where(x => x.BookId == id).Select(x => x.BookName);
    nameList.Add(Name);
});