How to map a list of objects on another one with condition

May 10, 2022

I have two lists. One of them has an Id

var a = new List<MyModel>
            {
                new MyModel()
                {
                    id = 1,
                    prop1 = 1,
                    prop2 = 1
                },
                new MyModel()
                {
                    id = 2,
                    prop1 = 2,
                    prop2 = 3
                },
            };

and another one hasn’t, but they are identical on combination of prop1 and prop2

var b = new List<MyModel>
            {
                new MyModel()
                {
                    prop1 = 1,
                    prop2 = 1
                },
                new MyModel()
                {
                    prop1 = 2,
                    prop2 = 3
                },
            };

I need to populate list b with ids from list a when a.prop1 = b.prop1 && a.prop2 == b.prop2 but I don’t know how to write this linq properly

The two list are always identical, but unordered. I think I can sort them and just assign them one by one, but I wonder if there another way

>Solution :

This works for me:

var query =
    from bb in b
    join aa in a on new { bb.prop1, bb.prop2 } equals new { aa.prop1, aa.prop2 }
    select new { bb, aa.id };
    
foreach (var q in query)
    q.bb.id = q.id;