Question

I have a query like below. anyone have idea why the ?: parts are always return false values although there is item contains in LIST. Or anyone have better idea to write this query, feel free to reply here. thanks.

List EList = new List();
EList.Add(new EmployeeInfo(1, "a1", "b1"));
EList.Add(new EmployeeInfo(2, "a2", "b2"));
List OList = new List();
OList.Add(new EmployeeInfo(1, "a1", "b1"));
OList.Add(new EmployeeInfo(2, "a2", "b2"));
OList.Add(new EmployeeInfo(3, "aa2", "bb2"));

var results = (
    from e in b
    select new
    {
        Id = e.ID,
        Name = e.Name,
        Email = e.Email,
        IS_Elist = (EList.Contains(e))?true:false,
        IS_Olist = (OList.Contains(e)) ? true : false,
    }
    ).ToList();

Answer 1

First you can simplify your query – expression ? : true : false is equivalent to just expression and we get the following.

var results = b.Select(e => new
                            {
                                Id = e.ID,
                                Name = e.Name,
                                Email = e.Email,
                                IS_Elist = EList.Contains(e),
                                IS_Olist = OList.Contains(e)
                             })
               .ToList();

This should and will definitely yield true if an item from b is contained in EList or OList. If you always get false this means that EList and OList do not contain any items from b. Remember that you are probably doing a comparison of references and therefore

new EmployeeInfo(1, "a", "b") == new EmployeeInfo(1, "a", "b")

will usually yield false.

I guess you actually intended a comparison by ID.

var results = b.Select(e => new
                            {
                                Id = e.ID,
                                Name = e.Name,
                                Email = e.Email,
                                IS_Elist = EList.Any(x => x.ID == e.ID),
                                IS_Olist = OList.Any(x => x.ID == e.ID)
                             })
               .ToList();

The alternative solution is to implement IEquatable<T> on EmployeeInfo or overriding Equals(), GetHashCode() and the equality operator. You should take care that all your implementations yield consistent results or you will end with a lot of confusion why for example Object.Equals() and the equality operator yield different results.