Compare and merge two List into new List

Question

I'm trying to figure out how best to compare and merge two List<T> with a new List<T> being generated that compares multiple properties within each object.

class Account
{
    public Account() { }
    public string ID { get; set; }
    public string Name { get; set; }
    public string Value { get; set; }
}

List<Account> Mine = new List<Account>();
List<Account> Yours = new List<Account>();
List<Account> Ours = new List<Account>();

Account m1 = new Account(){ ID = null, Name = "C_First", Value = "joe" };
Account m2 = new Account(){ ID = null, Name = "C_Last", Value = "bloggs" };
Account m3 = new Account(){ ID = null, Name = "C_Car", Value = "ford" };

Mine.Add(m1);
Mine.Add(m2);
Mine.Add(m3);

Account y1 = new Account(){ ID = "1", Name = "C_First", Value = "john" };
Account y2 = new Account(){ ID = "2", Name = "C_Last", Value = "public" };

Yours.Add(y1);
Yours.Add(y2);

The resulting List<Account> Ours would have the following List<Account> objects:

{ ID = "1", Name = "C_First", Value = "joe" };
{ ID = "2", Name = "C_Last", Value = "bloggs" };
{ ID = null, Name = "C_Car", Value = "ford" };

I need to figure out how best to compare the ID and Value properties between both List<Account> objects where the List<Account> Yours ID takes precedence over the List<Account> Mine and the List<Account> Mine Value takes precedence over List<Account> Yours along with any object that's not in List<Account> Yours being added as well.

I've tried the following:

Ours = Mine.Except(Yours).ToList();

which results in List<Ours> being empty.

I've read this post Difference between two lists in which Jon Skeet mentions using a custom IEqualityComparer<T> to do what I need but I'm stuck on how to create an IEqualityComparer<T> that compares more than 1 property value.

How do you define that two items represent the same but with different data? is just by name and value? — Ivo, Oct 31 '13 at 05:40
By the Name property, so if the objects name properties are the same but the Value is different from one object to the other the List Mine would take precedence but the ID of List Yours would override the null value for ID in the List Mine. Hope that makes sense. — devin M. arnold, Oct 31 '13 at 05:42

score 3 · Accepted Answer · answered Oct 31 '13 at 05:51

3

Not sure if it can be done in "pue" LINQ, but a bit of procedural code would do the trick:

var dict = Yours.ToDictionary(y => y.Name);
foreach (var m in Mine) {
    Account y;
    if (dict.TryGetValue(m.Name, out y))
        Ours.Add(new Account { ID = y.ID, Name = m.Name, Value = m.Value });
    else
        Ours.Add(m);
}

After that, printing Ours...

foreach (var o in Ours)
    Console.WriteLine("{0}\t{1}\t{2}", o.ID, o.Name, o.Value);

...gives the following result:

1       C_First joe
2       C_Last  bloggs
        C_Car   ford

answered Oct 31 '13 at 05:51

Branko Dimitrijevic

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Is there a way to keep it from adding objects with the same Name and Value?Account m4 = new Account(){ ID = null, Name = "C_Hair", Value = "long" }; Account y3 = new Account(){ ID = "1", Name = "C_Hair", Value = "long" }; – devin M. arnold Oct 31 '13 at 06:35
@devinM.arnold If I understood correctly what you wish to accomplish, you'll need to create a "key" class that combines `Name` and `Value`, and then use _it_ as the dictionary key (instead of the string). Just be careful to properly implement [IEquatable](http://msdn.microsoft.com/en-us/library/ms131187(v=vs.110).aspx) - override `GetHashCode`. – Branko Dimitrijevic Oct 31 '13 at 06:48
adding brackets around the if statement then adding this conditional did the trick (y.Value != m.Value) – devin M. arnold Oct 31 '13 at 06:55
that makes perfect sense about creating a separate "key" class, thank you for pointing me in the right direction. cheers! – devin M. arnold Oct 31 '13 at 07:02

Ivo · Answer 2 · 2013-10-31T05:55:30.987

0

Try this:

var index = Mine.ToDictionary(x => x.Name);

foreach(var account in Yours)
{
    if(index.ContainsKey(account.Name))
    {
        var item = index[account.Name]; 
        if(item.ID == null)
            item.ID = account.ID;
    }
    index.Add(account.Name, account);
}

Ours = index.Values.ToList();

edited Oct 31 '13 at 05:55

answered Oct 31 '13 at 05:47

Ivo

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Intellisense is saying that the last x in (x => x.Name, x) doesn't exist in the current context. – devin M. arnold Oct 31 '13 at 05:53
Fixed. Also, the code is just for giving an idea of a possible solution, you have to put a little bit of effort – Ivo Oct 31 '13 at 05:56
1

enough `Mine.ToDictionary(x=>x.Name)` – Grundy Oct 31 '13 at 05:56
Kept getting an exceptionerror regarding duplicate keys ivowiblo – devin M. arnold Oct 31 '13 at 06:03
Grundy, that sort of worked but the ID values were all null in the resulting Dictionary – devin M. arnold Oct 31 '13 at 06:04
That's why you need to execute the foreach on the Yours list for getting the IDs for those values who have null ID. – Ivo Nov 12 '13 at 14:11

Grundy · Answer 3 · 2013-10-31T06:29:42.983

0

try this code for IEqualityComparer:

public class D : IEqualityComparer<Account>
{
    public bool Equals(Account x, Account y)
    {
        return x.ID == y.ID && x.Value==y.Value;
    }

    public int GetHashCode(Account obj)
    {
        return obj.ID.GetHashCode() ^ obj.Value.GetHashCode();
    }
}

used like this: Ours = Mine.Except(Yours, new D()).ToList();

edited Oct 31 '13 at 06:29

answered Oct 31 '13 at 06:02

Grundy

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Is it used like this: Mine.ToDictionary(x => x.Name, new D()); – devin M. arnold Oct 31 '13 at 06:12

Compare and merge two List into new List

3 Answers3