LINQ groupby with condition dependent on other sequence elements - linq

Question

LINQ. I have an IEnumerable of type Transaction:
private class Transaction{
public string Debitor { get; set; }
public double Debit { get; set; }
public string Creditor { get; set; } }
Sample IEnumerable<Transaction>:
[Debitor] | [Spend] | [Creditor]
luca 10 alessio
giulia 12 alessio
alessio 7 luca
alessio 6 giulia
marco 5 giulia
alessio 3 marco
luca 1 alessio
I would like to group the Transactions where Debitor == Creditor; else in a separate group. For the previous example, I should get:
Group 1:
luca 10 alessio
alessio 7 luca
luca 1 alessio
Group 2:
giulia 12 alessio
alessio 6 giulia
Group 3:
marco 5 giulia
Group 4:
alessio 3 marco
I have solved it using 2 for loops (one nested) over the same IEnumerable, performing the check and using separate lists for the output, but I wonder if there is a less clunky way of doing this using LINQ.
A similar question might be: LINQ Conditional Group, however in this case the grouping condition is variable dependent on the other elements of the IEnumerable.

Answer 1

The simplest way would indeed be to create a combined key, such as in Anu's answer.
Another way to do that (not necessarily better, but avoids sub collections and string joining), is:
var groups = transactions.GroupBy(t=> t.Debitor.CompareTo(t.Creditor) > 0 ? (t.Debitor,t.Creditor) : (t.Creditor,t.Debitor));
NB The above assumes you can use implicit Tuple creation. If you have a lower C# version and/or don't have the ValueTuple NuGet package installed, you can use: var groups = transactions.GroupBy(t=> t.Debitor.CompareTo(t.Creditor) > 0 ? Tuple.Create(t.Debitor,t.Creditor) : Tuple.Create(t.Creditor,t.Debitor));
Purely for the sake of mentioning it, another way, is to use a custom equality comparer for the group by. This might be overkill depending on your needs, collection size, need for reusabillity, etc, but to show the possibility all the same: first create a class (or implement it in Transaction directly)
class TransactionDebCredComparer : EqualityComparer<Transaction>
{
public override bool Equals(Transaction t1, Transaction t2) => (t1.Debitor == t2.Creditor && t2.Debitor == t1.Creditor) || (t1.Debitor == t2.Debitor && t2.Creditor == t1.Creditor);
public override int GetHashCode(Transaction t) => t.Debitor.GetHashCode() ^ t.Creditor.GetHashCode();
}
Then you can group your enumerable by using
var groups = transactions.GroupBy(t=>t, new TransactionDebCredComparer() );

Answer 2

For a simple solution, you could group using a key created with Creditor and Debitor. For example.
string CreateKey(params string[] names)=>string.Join(",",names.OrderBy(x => x));
var result = transactionCollection.GroupBy(x=> CreateKey(x.Debitor,x.Creditor));
Please note I have used a collection in CreateKey, in case you have more similar grouping factors, but you can write a simpler version for CreateKey if the condition is always involving Creditor and Debitor alone.