ProjectProgram.cs

class MyClass
{
    public int RowNo { get; set; }
    public int Value { get; set; }
}

public static class LinqEx
{
    public static IEnumerable<T> Project<T>(this IEnumerable<T> lst1,
        IEnumerable<T> lst2, Func<T, object> id, Action<T, T> action)
    {
        if (lst2.Count() > lst1.Count())
            throw new Exception("Projected list's count must be greater than " +
                "or equal to the other list.");
        var sortedList1 = lst1.OrderBy(id).ToList();
        var sortedList2 = lst2.OrderBy(id).ToList();
        for (int i = 0; i < lst2.Count(); i++)
        {
            action(sortedList1[i], sortedList2[i]);
        }
        return sortedList1;
    }
}
public class Example12_4
{
    public static void Main()
    {
        var List1 = new List<MyClass>
        {
            new MyClass { RowNo = 1, Value = 11},
            new MyClass { RowNo = 2, Value = 22 },
            new MyClass { RowNo = 3, Value = 33},
            new MyClass { RowNo = 4, Value = 88 }
        };

        var List2 = new List<MyClass>
        {
            new MyClass { RowNo = 1, Value = 44},
            new MyClass { RowNo = 2, Value = 55},
            new MyClass { RowNo = 3, Value = 66 }
        };


        var lst = List1.Project(List2, x => x.RowNo, (y, z) => { y.Value = z.Value; });
        foreach (var item in lst)
        {
            Console.WriteLine($"{item.RowNo}, {item.Value}");
        }
        }

var List1 = new List
{
new MyClass { RowNo = 1, Value = 11},
new MyClass { RowNo = 2, Value = 22 },
new MyClass { RowNo = 3, Value = 33},
new MyClass { RowNo = 4, Value = 88 }
};

        var List2 = new List<MyClass>
    {
        new MyClass { RowNo = 1, Value = 44},
        new MyClass { RowNo = 3, Value = 66 }
    };

you were right about me doing things "wrongly", I was using a struct to store the values, as I believe it should be