Question

How do I replace objects in Collection1 with a matching object(by name and description) from Collection2?

Also the objects for the matching operation in Collection2 have to be within a certain range, say from 0-20. Objects with index > 20 in Collection2 need to be appended to Collection1.

Answer 1

This is one way:

foreach (var item in collection2.Take(20)) // grab replacement range
{
    int index;
    if ((index = collection1.FindIndex(x => 
                                  x.Name == item.Name && 
                                  x.Description == item.Description)) > -1)
    {
        collection1[index] = item;
    }
}
collection1.AddRange(collection2.Skip(20)); // append the rest

It’s highly likely that the code can be improved even more, if your intentions were a little more clear. There may be a cleaner way if the problem was better understood.