How can I list classes with similar IDs in a list with Linq?

March 6, 2023

I have NodeViewModel class. There is ID variable in NodeViewModel class. It can be two classes but with the same ID variable. IDs are not sequential numbers. Example ID; 010105.

I have a list containing my NodeViewModels as follows;

List<NodeViewModel> nodes = new List<NodeViewModel>((IEnumerable<NodeViewModel>)diagram.Nodes);

It is necessary to export the information in the content of two matching classes by combining them. Therefore I created a class called IdNodeInteractionModel. I want to collect matching NodeViewModels in a List.

public class IdNodeInteractionModel
{
    public int ID;
    public NodeViewModel firstNode;
    public NodeViewModel secondNode;
}

I can match similar ids with multiple foreach loops. But I am using similar loops. There are if else queries and all of them reduce the readability of the code. How can I make practical readability with Linq?

I would be glad if you help.

>Solution :

It can be two classes but with the same ID variable

Sounds like you are looking for GroupBy/ToLookup. For example:

List<NodeViewModel> nodes = new List<NodeViewModel>();
var result = nodes
    .GroupBy(n => n.ID)
    .Select(g =>
    {
        var pair = g.Take(2).ToArray(); // ignore the rest if we have more then 2
        return new IdNodeInteractionModel
        {
            ID = g.Key,
            FirstNode = pair[0],
            SecondNode = pair.Length > 1 ? pair[1] : null
        };
    })
    .ToList();