Hello,

I have two Lists:

A = {ID, Name} = { (Null, John), (Null, Mary), (Null, Andrew), (Null,
Peter) }

B = {ID, Name} = { (1, John), (2, Robert), (3, Angela), (4, Andrew) }

I want to find which items in A do not exist in B then:

1. Add the items to B and using an ID from the function GetID(). B
would become:

    B = {ID, Name} = { (1, John), (2, Robert), (3, Angela), (4,
Andrew), (231, Mary), (45, Peter)  }

2. Then update the A list, or create a new one (C), to get the created
items:

    C = {ID, Name} = { (231, Mary), (45, Peter) }

How can I do this with LINQ?

I have been trying a join but with the something as "not equals" but
it does not work.

I think I might need to do this in 3 steps:

1. Get items in B that do not exist in C;
2. Insert those items in C;
3. Get those items again from C to get the created ID's with the
correspondent names.

Could someone help me out with this?

 Well See the example below. Key is that you  have to have an IEqualityComparer implemented

1     static void Main(string[] args)
2            {
3    
4                List A = new List { new Test { ID = 1, Name = "John" }, new Test { ID = 2, Name = "Mary" }, 
5                    new Test { ID = 3, Name = "Andrew" },new Test {ID=4,Name="Peter"} };
6                List B = new List { new Test { ID = 1, Name = "John" }, new Test { ID = 21, Name = "Robert" }, 
7                    new Test { ID = 9, Name = "Angela" },new Test {ID=3,Name="Andrew"} };
8          
9                var ItemsinAThatDoNotExistinB = A.Except(B,new TestComparer()).ToList();
10           }
11   
12   public class TestComparer : IEqualityComparer
13       {
14           #region IEqualityComparer Members
15   
16           public bool Equals(Test x, Test y)
17           {
18               if (x.Equals(y))
19               {
20                   return true;
21               }
22               else
23               {
24                   return false;
25               }
26           }
27   
28           public int GetHashCode(Test obj)
29           {
30               return obj.ToString().ToLower().GetHashCode();
31   
32           }
33   
34           #endregion
35       }
36       public class Test:IEquatable
37       {
38           public int? ID {get;set;}
39           public string Name { get; set; }
40   
41           #region IEquatable Members
42   
43          
44   
45           public bool Equals(Test other)
46           {
47               if (this.ID == other.ID  && this.Name.Equals(other.Name))
48               {
49                   return true;
50               }
51               else
52               {
53                   return false;
54               }
55           }
56   
57           #endregion
58   
59           
60       }

Assuming you have a suitable equality comparer, or have implemented IEquatable<T>, you can use the Except and Concat methods:

1. C = A.Except(B) [or A.Except(B, comparer) to pass an equality comparer];
2. Update the ID of all elements in C;
3. B = B.Concat(C)
   

Try this

// You must have some class like this already
public class Info {
    public int? ID {get; set;}
    public string Name {get; set;}
}
// Load some data so I can test
List<Info> A = new List<Info> { new Info { Name = "John" }, new Info { Name = "Mary" }, new Info { Name = "Peter" } };
List<Info> B = new List<Info> { new Info { ID = 1, Name = "John" }, new Info { ID = 2, Name = "Robert" }, new Info { ID = 3, Name = "Angela" }, new Info { ID = 4, Name = "Andrew" } };

// Make List C
List<Info> C = A.Select(a => new { Name = a.Name }).Except(B.Select(b => new { Name = b.Name })).Select(c => new Info { ID = GetID(c.Name), Name = c.Name }).ToList();
// Append C to B
B = B.Union(C).ToList();

To create list C you need to make an IEnumerable with just the Name field, this is what the Select(a=> new {Name = a.Name}) does.  The Except query operator is one way to do the 'not equals' join.  Then create a new IEnumerable<Info> using GetID and the Names.

Appending C to B is just the Union query operator.

Hope this helps

If anyone wanders in here...

the public class TestComparer : IEqualityComparer  needs to have the type specified:

public class TestComparer : IEqualityComparer<Test>