Linq and Contains

Question

First example which works:

public class Test
{
    public int ID;
    public string Name;
}

List<int> list1 = Load1();
List<Test> list2 = Load2();

var query = list2.Where(c => list1.Contains(c.ID));

Now I would like to use two lists of objects as a source and get list of objects that have this same values for member ID.

List<Test> list1 = Load2();
List<Test> list2 = Load2();

Below doesn't compile:

var query = list2.Where(c => **list1.ID.Contains**(c.ID));

I know it is wrong but placed it here for better understanding. I would appreciate someone to show me the right path :-)

Regards Mariusz

Answer 1

You can perform an inner join as follows:

var query = from x in list1
            join y in list2 on x.ID equals y.ID
            select new { x, y };

or

var query = list1.Join(list2,
                       x => x.ID,
                       y => y.ID,
                       (x, y) => new { x, y });

Answer 2

You could make your Test class equatable:

public class Test : IEquatable<Test>
{
  public int Id {get;set;}
  public bool Equals(Test other)
  {
    return this.Id == other.Id;
  }
}

Then this would work:

list1.Where(item => list2.Contains(item));

Answer 3

Try

list2.Where(c => list1.Any(d => d.ID == c.ID)); 

You need to enumerate the other collection with your own code to get what you want, because .Contains uses the .Equals function, which for Objects will just match against the reference.

Answer 4

enter code heresimple version would be

var query = list2.Where(c=> list1.Select(l=>l.ID).Contains(c.ID))

or you could use the linq version of an inner join if you have no repetition

var query = list2.Join(list1,a=>a.ID,b=>b.ID,(a,b)=>a);

or you could use a System.Collections.Generic.IEQualityComparer

public class TestComparerer : IEqualityComparer<Test> {
    bool IEqualityComparer<Test>.Equals(Test a, Test b) {
        return a!=null && b!=null && a.ID.Equals(b.ID);
    }

    int IEqualityComparer<Test>.GetHashCode(Test a){
     return a.ID.GetHashCode();
    }
}

var query = list2.intersect(list1,new TestComparer());

finally if you override Equals and GetHashCode() on test you can make them comparable

public override bool Equals(object o) {
var other=o as Test;
return other!=null && this.ID==other.ID;
}

public override int GetHashCode() {
return ID.GetHashCode();
}

again this will allow you to do list1.Intersect(list2) or list2.Intersect(list1)